Archive for the ‘Weather’ Category

Understanding AHS Heat Zones

What are AHS Plant Heat Zones? They’re the counterpart to USDA Plant Hardiness Zones. USDA Plant Hardiness Zones are based on average cold temperatures of winter and the AHS Plant Heat Zones are based on average hot temperatures of summer. Hardiness Zones help gardeners determine if a plant can survive their region’s winter extreme temperatures while Heat Zones help determine if a plant can survive their summer extreme temperatures.

The American Horticultural Society coordinated with the same people who helped develop the USDA Hardiness Zone map to develop a Heat Zone map based on temperature data from the National Climactic Data Center. Daily high temperatures from National Weather Service stations throughout the United States were compiled for the years 1974 through  1995. In 1997 they produced a national map representing their findings.

The result is a map that color codes the country into 12 zones that indicate the average number of days when the temperature is above 86F degrees (30C). These are “heat days”. Zone 1 has an average of less than one day per year above 86 degrees while Zone 12 has an average of more than 210 days above 86 degrees.

Why 86F (30C) degrees? That is the point that many plants begin to experience distress and potential damage from sustained heat. Above this point plants can drop blossoms, drop leaves, fade in color, reduce fruit development, and possibly die. Some plants won’t die right away but will be stressed for so long that each year they perform less productively than the year before.

Many plants will wilt in heat, but will recover once temperatures fall. Sustained heat can have a serious physiological impact on some plants and triggers a lingering decline to ultimate death. Knowing how a plant will handle hot days is the reason for the AHS Heat Zone Map.

Most gardeners are familiar with the USDA Hardiness Zone system and look for the number on a plant tag when selecting new plantings. I live in Zone 5 and always make sure new perennial plants are at least hardy down to -20F degrees that the zone represents. I prefer plants hardy to Zone 4 for the occasional extremely low temperatures we get that approach -30F in winter.

I’m in AHS Plant Heat Zone 5. That represents 30 to 45 days above 86F degrees. I prefer to select plants for at least Zone 6 for the recent hot summers we’ve had; Zone 6 allows for 45 to 60 days above 86F. This year we’re definitely encroaching on Zone 6 heat days.

Plant growers and distributors that include AHS Plant Heat Zones on tags will list both zone ranges. You’ll now find a listing like “3-9, 6-1”. That means the plant is suitable for USDA Hardiness Zones 3 through 9 and is suitable for AHS Heat Zones 6 through 1. Many plant catalogs are also including this information in their plant descriptions.

For me, an ideal plant would be something like “4-9, 7-1”. That means it can handle the cold of the Hardiness Zone 4 and the heat of Heat Zone 7. My garden is well within both ranges and the plant should do well.

There are some limitations with the AHS Heat Zone map. Because it is relatively new and unknown, there aren’t many resources available to make it easy for you to identify your zone. You have to try and determine exactly where your city falls within the zones on the national map. Apparently the AHS had a tool for determining exact locations, but the zone finder application is nowhere to be found now. I haven’t been able to find any other source for finding Heat Zones by zip code like the USDA map has.

You can look at the map at:

At gardening I found a breakdown of the Heat Zones by state, which was a little easier to read. You can find it here:

Over 15,000 plants have been coded for heat tolerance. As more plants are coded and more companies begin listing both USDA and AHS zones on plant information, you can expect more gardeners to become familiar and comfortable with the conversion to a two-zone system.

For many of us we choose our plants, put them in our gardens, and then see how they do. For various reasons some plants do well while others struggle. Using both zone maps for selecting plants can help us put in plants that will not only grow well, but will thrive.

If some of your plants didn’t do well in summer it may be because they weren’t able to tolerate your garden’s hot days. That may be an indication that they’re inappropriate for your region. Understanding and using the AHS Heat Zones can help prevent similar problems in the future.


Sow Cool Season Plants in Summer for a Fall Crop

Two harvests in the same growing season are easy to accomplish. Many gardeners sow and plant in spring, spend summer tending to their crops, harvest in early fall, and then wrap it all up and wait until spring to repeat the same process. With little effort you can double your garden’s output with sow, tend, sow, harvest, tend, and then harvest again. Then you can spend winter satisfied that your garden played double duty to produce all it could.

Grow chard to harvest in fall

Grow chard to harvest in fall

In late summer you sow seeds for cool season plants that will grow as temperatures begin to decline. They’ll be ready to harvest after the first frosts have appeared and long after the last tomatoes were pulled from the vines. There is no need to create new gardens; you use the same beds you do now.

Cool season crops are primarily the ones that provide leaves, stems, and roots for harvest. These crops include:

Arugula, beets, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, celery, chard, collards, endive, fennel, kale, kohrabi, leeks, lettuce, mustard greens, onions, peas, parsnips, potatoes, radishes, spinach, turnips

Peas, especially, snow peas do well in cold

Peas, especially, snow peas do well in cold

Many gardeners grow these plants in spring and hope for a slow start to summer so the plants will mature and be ready for harvest before high heat causes them to bolt and their taste turns bitter. We take advantage of their natural ability to tolerate low temperatures by planting early.

We can also use that ability to plant late. The mature plant tolerates the low temperatures of fall, there is no risk of bolting, and many people believe many of these crops taste superior after exposure to frost.

Those of us who sow in early spring and hope the soil has warmed enough to initiate germination are ready for a percentage of our seeds to never grow. Those of us who sow in late summer are pleased that the warm soil provides speedy germination with little seed loss.

Early seed and plant care is different when you plant late. In spring there is less need for extra watering; lower temperatures mean less evaporation and there is always a chance of rain or late snow. In summer, the heat requires more gardener attention to maintaining moist soil conditions for seeds and young plants; you may need to mist or water the plants two or three times a day in harsh sun.

Temperatures above 80F (27C) will cause broccoli and spinach to bolt quickly. Sowing in a shady spot or setting up a row cover can reduce this concern.

Once the plants have a few sets of leaves the need for watering becomes less than in late summer. While early plants need more water to combat the increasing summer heat, later plants require less water as the decreasing temperatures bring comfort. The plants are less stressed in fall. They grow in the conditions they like best; they’re called cool season plants for a reason.

As long as the day temperatures remain about 10 to 15 degrees above freezing (40F – 50F, 4C – 10C) you can expect the crops to continue growing and producing. When the day temps remain below 40F (4C) the plants may begin to suffer. Careful harvesting will still produce results. The center of the plant may still have new, tasty leaves while the exterior leaves look frazzled.

Hard freezes, cold days, and icy conditions will adversely affect most cool season crops and will spell the end for your second harvest, but you can delay winter by mulching heavily with straw and using a season extender like cold frames, cloches, or plastic tunnels. I use my hoophouse system to harvest well into November and even December.

Broccoli and beets ready for overnight protection

Broccoli and beets ready for overnight protection

Some crops like cabbage, kale, and spinach can do well, even in snow. With heavy mulching, beets, leeks, and parsnips can often be overwintered and harvested in spring (I’ve done this).

Use your entire vegetable garden for fall planting. If you have a cool-season spring bed for broccoli, spinach, or lettuce and then let that bed remain filled with bolty, straggly, dried plants throughout the summer, rip those plants out and sow again for a fall crop.

Your tomatoes, peppers, and melons will decline in cooling weather. Anticipate their decline and sow seeds among those plants, in the same beds. When the first frost zaps your tomato plant, cut it out and let the cabbage and broccoli growing nearby overtake that space.

Root crops won’t be as big as spring plantings, but may be tastier. Try growing small, thumb-sized carrot varieties or harvest them young before the ground freezes. Beet roots will be harvested when they’re just a few inches big, but the beet leaves can be harvested continually.

Beet leaves are edible and tasty

Beet leaves are edible and tasty

Garden pests can be less of a concern for a fall garden. Many insect pests are less active, if not gone, in fall. The weeds will spend all summer attacking aren’t active in fall. Even deer seem to be scarce as they make their way to find a winter bed.

While growing a second crop in the same season sounds like extra work, it doesn’t need to be. Summer garden beds should be cleaned up before winter so insects don’t have a place to overwinter. That clean up works well to prepare the beds for fall crops.

You’ll spend a few more days in the garden watering and harvesting, but is that really a bad thing? Fall gardening allows you to do more of what you like and for me that’s a good thing.

Wildfire Mitigation for Homes

The enormous power of wildfires can be devastating. While much of the Rocky Mountain region confronts an historic fire season, tragic lessons are being learned. Many of us living in urban neighborhoods used to think we were safe when grasslands and forests burned near us, but the firestorm that engulfed West Colorado Springs proved those assumptions wrong.

Crack fire crews were ready, air tankers had dropped tons of fire retardant slurry, national experts put a good defensive plan in place, and sudden, erratic, unanticipated 65 mile per hour winds made all of that irrelevant. Entire neighborhoods were erased in minutes. A firestorm overwhelmed all preparations and incinerated hundreds of houses. These weren’t houses sitting solitary in a forest. They were homes sitting side by side along wide streets with sidewalks, playgrounds, and fenced backyards.

I was in Colorado Springs the afternoon of June 26, 2012, and felt sickened by the sight of flames cresting the ridge line that was perceived by all of us as a critical border between the city and the threat beyond. I wasn’t overly worried because I knew the fire crews were ready based on the many updates we were following on the news channels. Upon arriving home, 20 miles east of the danger, I told my wife the fire was worse. It was a sorrowful understatement.

The best-trained, professional, defensive fire teams in the world can be defeated when Mother Nature adds enormous destructive energy to an already devastating force of nature, but those events aren’t common. The Colorado Springs fire is being described by career firefighters as”epic”, with growth patterns and expansive actions previously unseen.

This tragedy has many of us reviewing our own homes and neighborhoods with an eye toward the threat of fire. The idea of “wildfire mitigation” was previously unknown or ignored by many homeowners, but now is the discussion topic at the dinner table.

A wildfire is an uncontrolled burning of grasslands and woodlands, or prairies and forests. The large majority of urban settings are still safe from wildfires, but houses and neighborhoods that border zones of bone-dry vegetation should be aware of practices to reduce the fire threat. Wildfire mitigation for a homeowner involves taking actions to lessen or eliminate the potential damage from a wildfire.

A primary factor in determining a home’s ability to survive a wildfire is the “defensible space” around it. This defensible space is the area of vegetation around a building that can either hinder or fuel a fire. Gardeners are uniquely qualified in determining the appropriateness of such vegetation.

A house is more likely to resist a wildfire if overgrown grass, dried brush, and overhanging trees are thinned or removed from the immediate vicinity of the building. With no or little fuel, a wildfire’s progress can be slowed when it approaches.

Extending a clear space around a structure provides firefighters room to work as they fight flames, keeping a structure fire or a wildfire from moving to other structures or to surrounding woodlands. Giving the trained defenders a defensible space can make the difference between success and failure.

When viewing the area around your home and analyzing the defensible space, think horizontally and vertically. The horizontal space runs across the ground and encompasses low vegetation that could be potential fire fuel. The vertical space runs from the ground to the top of bushes and trees that might ignite. Vegetation that provides both high horizontal and vertical fuel potential poses the biggest threat; thick stands of brush and tightly-packed trees can be hazardous.

Mitigation of wildfire for homes involves disrupting the natural continuity of these horizontal and vertical fuel sources. Thinning large shrubs and trees so there is at least 10 feet between crowns reduces the potential of wildfire moving from one plant to another. Removing low branches and smaller plants under a tree removes these “ladder fuels” that can transform a low, grass fire into a high, tree fire.

A few years ago I was fortunate to receive forestry training as part of our Master Gardener program. It included education on wildfire mitigation and creating defensible zones around houses. I garden using many of those concepts. I’ve worked to prune lower branches off trees near the house to a height of about 10 feet. I don’t plant shrubs near trees. I keep the grass within 100 feet of the house no higher than six inches. Dead trees and branches are removed quickly. No logs or wood are stored within five feet of the house and nothing is stored under the deck.

Even with my education and awareness a wildfire mitigation analysis shows deficiencies and potential hazards in my landscape. I was aware of some of them, overlooked others, and discovered new concerns.

My gardening activities focused on my backyard. Many earlier problems with potential wildfire fuels were corrected. It now offers a substantial defensive space and is maintained well. Little needs to be done there.

A good defensible space

The front of the house has the road and gravel driveway as fire barriers and the old Ponderosa Pine is pruned up to about 15 feet; it is not threatened by a slow-moving, low fire. But there are Aspens and shrubs that abut the exterior walls. They are thick and not pruned as well as they should be. We like the way they look but in a wildfire situation they pose a danger to our home. This is the first place in our landscape where a decision needs to be made between aesthetics and safety. It’s difficult sacrificing landscape plants, but it may be necessary to mitigate fire danger.

Obvious fire mitigation concerns

A similar situation exists on the north side of our house. Open pasture leads to the lawn, a thick stand of Aspens grows about 20 feet from the structure, overgrown bushes rest against the house, and a lone Ponderosa Pine rises within 10 feet of the deck and house. Though the lower branches are removed, the tree poses a serious threat. If it were to catch fire from a wind-blown ember, it threatens both the wood deck and the house. It is now a priority for removal.

Close growth is the main concern

The worst situation is on the south side. Our neighbor’s thick brush and numerous trees flow into a space filled with pine trees on our property that grow right up to the house. Their branches intermingle. There is nothing to stop flames from spreading between them and to our roof.

Serious concerns in this zone

The first defensible zone around a house should extend at least 15 feet around it. All flammable vegetation should be removed from this zone for maximum fire prevention. The second zone extends to at least 75 feet. Within this zone the continuity and arrangement of vegetation should be adjusted to reduce fuel potential. The south side of my home breaks all the rules of creating a wildfire-defensible space.

Many people move to the country or into the forest because they enjoy the scenery and wish to be engulfed by the plants and trees. No one expects that their home will be annihilated by a wildfire. We are now confronted by this obvious possibility.

In the early days of the Waldo Canyon fire as it threatened Colorado Springs, news crews and commentators highlighted the structures that remained immune to the widening fire lines. These homes were in the heart of the forest but they had obvious tree-free zones extending well beyond their walls. The grasses caught fire as the onslought approached but they were easy to extinguish. These homeowners who practiced serious wildfire mitigation practices saw their houses spared.

Creating defensible zones as part of wildfire mitigation works. It needs to exist on a large scale to be most effective. And an entire neighborhood needs to be involved. You can do what you can to reduce fire threat in your landscape but a neighbor’s recklessness can still spell disaster.

I’m working with our homeowner’s association, of which I’m a board member, to address this issue in our community. Already we’re discussing plans for teams to help neighbors remove dead trees and brush if they’re not able to do it on their own.

Many fire stations offer help in fire mitigation. If they have the resources they’ll be happy to examine your landscape and identify problems. Believe me, it is beneficial for them to have defensible space around the homes they protect.

Mother Nature always has the last word. Even the best-defended home can be lost in a firestorm, as we saw this week. In a typical wildfire, wise mitigation practices can prevent loss. Educating yourself becomes critical when confronted by sustained drought as much of the U.S. is facing. High winds  and catastrophic low humidity increase danger. When the fire approaches it is too late to prepare your landscape. Think and act in advance to protect your home.

For more information read these fact sheets from Colorado State University:

Creating Wildfire-Defensible Zones, no. 6.302
Fire-Resistant Landscaping, no. 6.303
Forest Home Fire Safety, no 6.304
Firewise Plant Materials, no. 6.305

Microclimates in the Garden

Microclimates give gardeners an opportunity to grow plants that might not normally be successful in their landscape. Microclimates can also spell doom for plants that should grow well. As in the real estate game, in gardening it’s all about location, location, location.

Some spots show obviously different reactions to weather

A microclimate is a geographical area with a different climate than the larger area surrounding it. The effect of localized weather varies in microclimates. Some microclimates cover miles of territory, but for our garden discussion we’re talking about garden spots that may only be a few feet wide. Every season offers the opportunity to find microclimates in your garden (see my articles: “Look for Microclimates in the Garden“, Jan 13, 2011; and “Microclimates in Summer“, July 8, 2011). Spring is a good time to try and identify some of the microclimates in your garden that you may not notice at other times.

Gardening is a long-term activity and by noting changes in your garden throughout the year you can use your time and effort to provide the best environment for your plants. Microclimates exist everywhere but until you notice their effects on you and your plants you may be wasting valuable resources.

Some microclimates are obvious. Maybe there’s a spot in your yard that is always high and dry; snow melts first in that spot, grass dries out and needs more water in that spot, or maybe every shrub you plant dies in that spot. Pausing for a little analysis may reveal it is always sunny, windy, and exposed. That’s a microclimate.

On the other end of the spectrum think about the garden bed that is always wet and cold. Maybe its a low spot under some trees that never sees sun and never dries out; maybe it’s the spot that always has the last pile of snow in spring. That’s a microclimate.

These two microclimates may exist mere feet from each other but to plants they may as well be separated by miles. A plant that can grow in one of them can’t grow in the other. It’s up to you the gardener to recognize the differences and act accordingly.

My wife and I have noticed an interesting microclimate in our yard. We have three stands of Aspens that are each only separated by about 50 or 60 feet (15-18 meters). The stand in the front yard always buds out first. Two or three weeks later as those trees have full leaves, the second stand begins to show signs of green. Another two or three weeks go by before the third stand finally begins to leaf out.

Trees on right have leaves, new growth in the middle, no leaves on the left

An analysis of the landscape reveals a minor grade in the landscape; the last Aspen stand to grow is in the lowest elevation. The upper stand also gets a little more sunshine; the lower stand is shaded by trees and the house late in the day. It’s also likely that the lower stand is more exposed to wind, common in the spring; the two upper stands have some wind protection from nearby structures and other trees.

No leaves and noticeable terrain difference on the left

Recognizing this microclimate is important to my gardening and plant selection. And I’ve made mistakes by not analyzing it earlier. Initially I planted Irises under the trees in the lower stand. They get full sun in early spring before the trees leaf out and I thought that would be enough for their success. The problem comes in autumn. As you might guess, the upper Aspen stands drop their leaves first and this lower stand is the last to lose leaves in fall. That means that the Irises are still shaded when they should be getting sun to nourish their rhizomes. I never get the flowers I should because the Iris rhizomes aren’t nourished by sunlight as well as they should be.

This microclimate analysis changed my approach to plant selection. First, I planted new lavender plants near the upper Aspen stand. The slightly improved sun exposure helped those lavender plants grow better than others in the back yard. This year I’ve planted Coral Bells (Heuchera) in that lower spot. Coral Bells like sun when it’s cold and some shade when it’s hot, exactly what the location offers.

Without a microclimate analysis I might spend years fertilizing and babying the Irises and other plants that were never really suited for that garden bed. Now I can focus on the proper plants for better success.

Another microclimate, that I created, is evident in my stone patio. I wanted a spot to support the growing requirements of lavender and other high desert plants. I chose a location that gets full sun and build a raised, stone patio. It is now the spot that warms quickly in spring and where snow melts first. The lavender, sedum, thyme, and iceplants are thriving in a spot designed for them.

Warm stone helps surrounding plants

I chose a nice location with full sun for my vegetable garden. But even in this select area microclimates exist. The raised beds are built on a slope and the upper beds are a few feet higher in elevation than the lower. The upper beds have more exposure to wind and receive late-day shade before the others. Last year tomato plants in the lower beds did better than tomato plants just a few feet away; the wind and shade made a difference even though it is barely noticeable to me. The plants obviously noticed.

Look at how your garden wakes up in spring. Note the different growth rates of plants just a few feet apart. Spot elevation, sun, and wind variations in your landscape. Take a few minutes to analyze why some plants do well while others struggle.

By thinking about microclimates in your garden and acting to match plants with the best growing environment you can improve your garden’s success. We often don’t think much about a minor difference in temperature, sun, or shade, but to tender plants it can mean the difference between life and death.

Before you put new plants in a garden bed and before you sow seeds notice the year-round characteristics of that spot of ground. It may be perfect for young plants in the spring but does it offer the best environment and microclimate later in the plant’s life cycle? The best gardening practices look into the future. It may sound difficult but it begins by thinking about how little differences in localized climate exists from one section of your garden to another. By spotting these changes you can use microclimates to your advantage.

When the Last Frost Date Isn’t the Last Frost Date

The United States abounds with record high temperatures this month and spring arrived early in many places. I’ve been enjoying days that are 15 and even 20 degrees above normal for weeks; unusual is an understatement. At a time of year we normally cringe at the thought of more, freezing overnight temperatures, sunscreen and shade are now on my mind. This is a year when the projected official last frost date won’t even be close. Maybe.

Late spring frost on apricot blossoms

The last frost date for me is still more than two weeks away. That is, the official 90% last frost date is the 15th of May for my area. My garden is at a higher elevation than the city proper so I always add a few days to be safe, but it’s close. As I’ve written before (Mar 18, 2011, “Know Your Last Frost Date“; Mar 29, 2011, “What to Plant Before the Last Frost“; May 13, 2011, “What is the Last Frost Date?“), the “90% last frost date” is the one most often referred to by seed companies, the USDA, NOAA, and anyone else that throws out a date as the last one for gardeners to be concerned about.

Last year I got my last frost on May 15; the model proved true. This year the actual last frost in my garden should be different. In fact, I think the last frost happened yesterday.

Last year's snow in May

Last Frost Dates are determined by taking historical temperature data and finding a symbolic date that meets certain criteria. The “90% last frost date” is the date for a certain area when 90 percent of historical last frosts have already occurred. The “Average Last Frost Date” or “50% last frost date” is the point on the calendar when half of the historical last frosts have already happened. The third type of last frost date is the “10% last frost date”; by this date 10 percent of historical last frosts were final. The key to all of these dates is that they’re determined by using historical dates, days that occurred in the past.

For my garden zone, 10 percent of the last frost dates in years past happened by April 24, and 50 percent happened by May 4. Could this be another one of those kind of years?

As far back as I can remember, in my 15 years living here, the last frost happened in May. I always advise local gardeners to wait until late May to plant warm season plants. Many gardeners new to the area learn the hard way about planting too soon in our high-mountain, weather-crazy area.

This year is one of the ones that cause serious gardeners like me to start biting our nails and tearing out our hair. A look at the long-range weather forecast shows we’ll be well above freezing for the next 10 days. That puts us very close to the 90% last frost date with no worries. Historically, we can point to previous years like this one when no frost occurred in May.

Then again I can point to years when we had frosts in June. A month is a long time in the weather world. Something could be brewing off the coast of northern Alaska right now that won’t be revealed until two or three weeks from now. A blast of Arctic air might descend on my garden when I least expect it. Or not.

The last frost date is just a spot on the calendar for garden planning. It’s up to individual gardeners to determine what they do with it. With any mathematical model, some points of data lie well outside the norm. For a date to be considered average (technically the mean), half of statistical points happened before and half happened after. It’s nice that we gardeners have the 90% and 10% last frost dates at our disposal to develop a more precise planting plan.

As I look at my calendar, every day without frost puts me closer to the mathematical point when future frost isn’t likely to happen. I’ve passed the 10 percent point and should breeze past the 50 percent point with no problem. The 90 percent point looks very promising.

Does that mean I can relax and plant with no frost concerns? That’s a good question and one for which I wish I had the answer. Whatever I choose to do I have to be ready for the consequences… or rewards. Knowing how last frost dates are determined and using that knowledge will help me make an informed decision when it comes to planting.

I’ve just about convinced myself that this is one of those years when the last frost occurs early. Indications are good that warm days and nights ahead will help my plants. The ground is already warmer than it normally is at this point of the year. The weather trends seem positive.

But I’m not a gambler and I’m not ready to bet everything on a statistical probability. I’ll plant tomatoes this week almost a month ahead of normal, but I’ll enclose them in plastic season extenders. I’ll put some beans in the ground, but only in the beds with protective covering. I’ll delay planting squash and pumpkins until I’m absolutely sure the calendar coast is clear.

Usually I wouldn’t be considering such measures at the end of April, but this year is different. Sunny days are too inviting to sit back and wait for historical events to happen. Part of being a gardener is trying new things in the garden and by a small measure I’ll roll the dice a little. We all experience missteps and minor failures in gardening so I’m ready for the worst, but if my conjecture about the last frost date holds true I’ll be able to get a nice jump on the growing season.

I know my last frost date, all three of them actually. I know what they mean. I have experienced the ups and downs of weather’s effect on my garden. Also I’m anxious to get my garden growing. This year my last frost date won’t be an actual last frost date. That’s what I’ve determined based on the data at hand. Or at least that’s what I’m betting on.

Extreme Temperature Changes and Gardening

Did you ever plan a vacation to Florida and get off the plane in North Dakota? Would you want to? I once left Hawaii where it was 82F degrees (28C) and sunny and landed hours later in Montana where it was -10F (-23C) degrees and snowing. It’s an experience I don’t recommend repeating.

Late winter often provides some of us similar extremes and we don’t have to leave home to experience them. Residents along the Front Range of the Rocky Mountains are in the middle of such an event and much of the Eastern U.S. will follow soon.

A bird caught in a sudden spring storm

Yesterday we enjoyed a high temperature near 70F degrees (21C). Today will be about 45F (7C) and tomorrow we’ll be lucky to reach 30F (-1C) for a daily high. The low temperature tonight will reach 10F (-12C). Along with an 80-degree temperature difference in just over a day, we’ll also receive inches of snow.

I knew about the forecast temperature in Montana when I left Hawaii and was able to put warm clothes on in the airplane. I can do the same tonight and tomorrow for this storm. But my plants are like the vacationers to Florida. They’re prepared for sunny, warm weather and are about to be surprised by an unexpected arrival in Minot, North Dakota.

This is the time of year that can lead to damage and death of many garden plants. During the gradual climate changes of fall and early winter, plants prepare for cold conditions and change their growth cycles accordingly. In late winter and early spring as weather and soil warms, many plants come back to life and begin growing again. When a sudden burst of arctic cold interrupts that cycle they can suffer.

As a concerned and conscientious gardener it’s my job to try to reduce that suffering. By taking a few simple precautions I can limit plant damage when faced with a huge temperature swing.

It’s important to note that not all plants need protecting when conditions like these threaten. I encourage gardeners to make a regular survey of their gardens. That’s why I’m often wandering through the garden at this time of year to check on plant growth. Many sections of my yard are still buried in snow and have no new growth. Some shaded spots still have frozen ground. It’s only the areas that have started to thaw and where the plants have responded that need attention.

And even in those areas not every plant is a concern. There are perennial plants like some of my daisies and penstemons that shrug off the cold. The strawberries are peeking above the mulch but can handle freezing conditions. Many groundcovers don’t care how cold they get. A lot of plants stay partially green even through cold winters and are ready to grow rapidly when spring finally arrives. A sudden 80-degree temperature shift may slow them down but it won’t kill them. I’m not worried about those plants.

The daisies can handle cold and snow

It’s the bushes and small trees that concern me most. Buds begin to open and grow on the warm days and they’re not designed to handle extreme cold. If they grow too soon in the season and get hit by harsh cold, damaged buds affect branch, flower, and fruit growth. I’ve had an entire apricot tree lose its blossoms in an April freeze.

Frozen apricot blossoms

A tarp or blanket thrown over the plant can head off disaster. By shielding the plant and holding in some of the heat from the warm day, a simple covering can prevent serious damage. You may not be able to protect the entire bush or tree, but anything saved is good. You can’t cover larger bushes and trees and they will probably lose buds in severe temperature changes, but their size and girth should provide some natural protection.

Small and young perennial plants are most susceptible. I lost 10 lavender plants about this time last year. They were planted in early fall and made it through the winter fine. I kept them covered with protective straw through the worst months. A fellow master gardener and I checked their new growth after early-season, warm weather and confirmed they were alive, supple, and doing well. A devastating cold surge a week later changed all that.

A frozen, young lavender

Simple milk jug coverings can cover and save small plants. The protective umbrella holds some of the soil and plant heat in, usually enough to counter the sudden cold. If they’re sturdy or if you provide short supports, a tarp thrown over a grouping of plants can save them all. The concept is just to give them some protection from the cold.

Plastic protection for young growth

A close look at the forecast can save you some effort and worry. Snow is a great insulator and actually provides protection. Snow does not get much colder than 32F degrees (0C). That’s warmer than our current average night temperatures so any buds that are growing can handle 32 degrees. A blanket of snow provides a relatively constant temperature that can be well above the extreme low temperature of outside air.

So that means that if the forecast calls for inches of snow before the severe cold temperatures, a natural blanket will help the plants and less covering may be needed. If the forecast has the cold temperatures coming before the snow, that’s when you have to take protective action.

By focusing on plants you know are growing and that you know are susceptible to cold damage you can center your protective efforts. By analyzing the type of weather expected you can decide how much action is necessary.

In the end, how many of your plants survive drastic and severe temperature changes may be up to you. I’ll take another look this afternoon before temperatures plummet to make a final evaluation. My tarps and milk jugs are ready.

Changes to the USDA Plant Hardiness Zone Map

Yesterday, the U.S. Department of Agriculture released a new version of the Plant Hardiness Zone Map. This is the first revision of the map since 1990. The new version includes 13 zones, two more than before. Much of the United States is now one half-zone warmer.

The old map

The new map reflects an increase in the average temperature of the U.S. in recent decades. The 1990 map used average annual temperatures from 1974 to 1986 while the new map uses information from 1976 to 2005.

For the first time, the influence of factors like elevation, prevailing winds, large bodies of water, terrain, and city heat island effects are included in calculations. I’ve written previously about how these aspects can affect specific regions, cities, and gardens. New algorithms and more sophisticated mapping methods now account for those effects. The Associated Press points out that American cities that include St. Louis, Des Moines, Honolulu, and Fairbanks, Alaska, are now in warmer Plant Hardiness Zones.

The map changes mean that, generally, the coldest winter day isn’t as cold as in the past. Relatively mild winters of the last two decades now allow some plants to grow in areas previously considered off limits.

You can expect that seed packets and gardening advisers will soon change and use the new data for recommending what plants can grow where. Burpee will include the new map on next year’s seed packets. Gardening catalogs will identify which plants are suitable for the warmer zones.

It’s important that I point out that the Plant Hardiness Map should only be used as a guide for planting.

Climate can still vary dramatically within zones, even the new ones. While some mountain areas are now in a cooler zone, a review of the new zone map still has my neighborhood in the same zone as before, 5b.  This doesn’t adequately account for the terrain and altitude differences that my neighbors and I encounter. My garden is fully 1,000 feet higher than the city 15 miles away, also zone 5b. I’m surrounded by forest. I have inches of snow and ice still on the ground from our first major winter storm well over a month ago; the city lost its snow covering the first day after the storm.

Every gardener should learn about the individual characteristics of their region and garden. Knowing your Plant Hardiness Zone is important. It will help you choose seeds and plants, but it is only one part of gardening planning.

Get to know your garden. Just because the USDA says you’re in a new zone it doesn’t mean everything has changed. The Hardiness Zone Map is based on averages, average low temperatures. The averages may be warmer, but extreme low temperatures can still be possible. Caution should still be practiced when choosing plants that are barely suitable for your zone, old or new.

The new map is more accurate than previous versions and gardeners have a new tool as we confront climate change. Like the rest of our gardening tools it is appropriate only for its intended purpose.

If you haven’t had success with a particular plant, a new zone number probably won’t change that. If you want to seek out new plants a new zone number may give you new options, but it is only a guideline.

I accept the new map with anticipation. Many gardeners in many places will try new things and that is a great idea. Entire regions are more accurately assessed. Yet even though most of us feel like our neighborhood is warmer, experience is a harsh steward. I’ll continue to recommend that gardeners, particularly in challenging zones like mine, choose plants that are best suited for success.

It only takes one or two days of historical low temperatures to wipe out plantings that should expect to survive the new zone identifications. If you live in an area of surprise temperature extremes, it’s better to opt for plants a zone lower than yours — planting 4b plants when you live in 5b — than trying to recover from winter kill.

Of course, microclimates, cold frames, hoop houses, and other season extenders will all provide the extra protection that you may need for plants challenged by temperature. If you’re going to try new plants that may be questionable consider one of these aids. Check out some of my other articles on those subjects for more assistance.

For now enjoy better information and access. The new USDA Plant Hardiness Zone Map has an improved online interface. Easier than in the past, you only need to enter your zip code for an instant zone report. A click of your mouse will bring up your color-coded state map.

Try it at: