Know Your Last Frost Date

The last frost date for Colorado Springs is May 15. That’s the date I tell anyone who asks and the date I base most of my gardening decisions around. There is only a 10 percent chance that Colorado Springs will have temperatures below freezing past that date.

Do you know your Last Frost Date? If you don’t, find out today. Go to the NOAA National Climactic Data Center site to look up historical climate information for where you live.

You may ask why the Last Frost Date is so important. It’s important because, like me, you should base most of your early-season gardening decisions around it.

It’s extremely important for gardeners new to an area to know as they begin gardening. Colorado has human transplants from California, Texas, Florida and many other dissimilar places and gardening here is different from almost any other region of the United States. A gardener moving from Colorado to any place else will encounter differences too.

Snow on April 17 covering the apricot tree.

Not knowing your date could prove costly. New plants are beginning to appear in the big box home improvement stores, in major retail stores, and even in local nurseries. Just because a plant is offered for sale by a local merchant doesn’t mean it is ready to be planted in your garden. I’ve lost track of the number of new gardeners I’ve talked to who planted tomatoes in their Colorado garden in April, only to have a late-season snowstorm kill everything.

It’s not as crazy as it sounds. I lived in an area of California where the last frost date was April 10; my father grew up where it was March 3. Even if I knew what the date meant those many years ago, it didn’t enter into my garden planning. All I knew was that you plant tomatoes in early spring. By moving to Colorado and not learning about the different climate, it would be easy to assume that it’s okay to plant in spring too, especially when all of the stores are selling the plants. That may be one reason why new Colorado gardeners make the same fatal mistake.

If you are planting seeds you need to know the date too. The seed packet may say something like “plant indoors 2-4 weeks before your last frost date.” That’s a little hard to determine if you don’t know what it is. Assuming the date is in early spring is not good; you need to know exactly when it is.

Squash seeds from Territorial Seed Co. referencing starting seeds 2-4 weeks before last frost date.

You also need to understand the different statistics you’ll find on the NOAA site or that local Extension offices will provide. When you ask for the LastFrost Date you’ll be presented with a few dates with 90 percent, 50 percent or 10 percent attached to them. All of the information is based on historical climatological information compiled by the U.S. government (other countries offer similar data). The 50 percent date means that historically 50 percent of days below freezing occurred before that date and 50 percent of the days below freezing occurred after that date; this is the Average Frost Date (May 4 in Colorado Springs).

I prefer to focus on the 10 percent date. It means that historically 90 percent of days below freezing occurred before that date and only 10 percent occurred after. It’s important to note that there is still a 10 percent chance that you may have freezing days after your Last Frost Date. If your seed packet says something like, “Sow in the ground after all danger of frost has passed”, you definitely want to wait until after your Last Frost Date and maybe even a week or two later so you’re closer to the zero percent date.

Melon seeds from Baker Creek Seed Co. specifically saying sow two weeks after last frost date.

This is nothing new from me, I’ve written about it before in my posting “Know Your Important Garden Dates.” Of the entire calendar if you only know one date, the Last Frost Date is the one to know.

Don’t be tempted to buy those lovely, green, inviting plants that are displayed at a box store too early. Just because the sun is shining and the days are warm (we set a new record high temperature two days ago) you may be asking for trouble if you do. The nights will still get cold and could devastate your new garden.

Like USDA Plant Hardiness Zones (see my blog “Understanding USDA Plant Hardiness Zones“), where you live may differ slightly from the location that supplies the official data. The official Colorado Springs climate data location is our airport, a common spot for most cities. I live about 15 miles north and almost 1,500 feet higher than the airport. While I use the official Last Frost Date of May 15 as a guideline for my gardening decisions, if a young plant is very susceptible to frost damage I add a few weeks to my planting date.

That’s why typically I won’t plant tomatoes, peppers, or pumpkins until Memorial Day weekend at the end of May. Those few weeks past the official Last Frost Date help ensure the plants won’t be damaged by temperatures that may affect my specific garden location. And I’m always looking at the long-range weather forecast whenever I plant too.

Be aware that you can plant before the Last Frost Date. Cool season plants will survive and often do better in the cooler temperatures. With a cold frame, mini greenhouse, or cloche, you can get a head start on warm season plants by planting early. I’ll be covering those topics in the days ahead. For now, know your Last Frost Date and use it to your advantage.


One response to this post.

  1. I’ve meant to post about something like this on my webpage and you gave me an idea. Cheers.


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