By Julie Christensen
Most perennials, shrubs and trees aren’t affected by freeze and frost dates, but if you’re planting annuals or vegetables, these are details you need to know. Many vegetables, including tomatoes, green beans, squash, cucumbers, pumpkins, peppers and corn, don’t tolerate even a light frost. Annual flowers, such as petunias, nasturtiums and morning glories, among others, can’t tolerate cold temperatures either. Plant them out too early and they’ll likely be stunted or killed. Conversely, these crops are usually the first to be killed in the fall by cold weather.
A light frost is defined as temperatures between 32 and 28 degrees Fahrenheit. A hard frost is defined as temperatures below 28 degrees. Most leafy vegetables, peas, broccoli, Brussels sprouts, leeks and carrots can tolerate hard frosts. This means that you can plant these crops earlier in the spring and they’ll grow for longer in the fall.
Finding Your Last Frost Date
But how do you find out the average last freeze or frost date for your area? One option is to consult your local nursery. Most nursery workers know these dates for the area you live in. You can also ask that green thumb neighbor, especially one that’s lived in the area for years.
Failing these simple suggestions, it’s time to head to the Internet. The best place to go is the National Climactic Data Center. Here, you’ll see a tab listing all the states. Click on your state and the site will direct you to a page listing data for numerous research stations within the state. Find the one nearest you. Now, you’ll see that each station offers data for the following temperatures – 36, 32, and 28 degrees. For most crops, the one you’ll want to pay attention to is 32 degrees. You’ll notice three dates listed under Spring and Fall. If you want to play it safe, use the 10 percent probability date. This means there is only a 10% chance that the last frost will be later than that date. If you are prepared to cover your plants in event of frost, you can use the 50 or 90 percent dates and plant earlier.
Another great resource is the Mother Earth News Garden Planner. Punch in your zip code and the site automatically keeps track of your frost-free dates and sends you alerts on when to plant. You can also manually change the dates to accommodate microclimates in your yard. The Garden Planner simplifies laying out a garden too, by offering layout ideas and automatically spacing plants appropriately. You’ll end up with a blueprint that shows exactly how much to plant, when and where.
Knowing the last freeze and frost dates is a useful tool in planning your garden, but it doesn’t tell you everything you need to know. First, these dates don’t apply to perennials and shrubs. When choosing shrubs and perennials, you need to pay attention to their U.S.D.A. plant hardiness zones, which are usually listed on plant tags. Hardiness zones are based on the minimum cold temperatures in a certain area and help you determine which plants will thrive in your yard.
Additionally, most yards have microclimates — pockets that are warmer, colder or windier than a surrounding area. Places in your garden near the house or a concrete patio usually warm up sooner. Locations with a southern exposure stay warmer than those on the north side of the house. Pay attention to these areas and make notes from year to year about when to plant based on the conditions in your yard.
Duping Mother Nature
So, you know that your last frost date is May 30 and your first frost date in the fall is September 15. That doesn’t leave a very large window for growing long-season crops, such as tomatoes and pumpkins. Fortunately, there are several things you can do to extend the season. Black plastic mulch stretched over the soil can raise the soil temperature by as much as 10 degrees, allowing you to plant two or three weeks earlier. Then, cover planted crops with floating row covers. This agricultural fabric lets water and sunlight through, but it keeps plants warmer. Various weights are available; choose one suited to your climate. Water-filled cloches are a boon to the northern gardener. With these, you can plant tomatoes, squash, peppers and other heat-loving plants as much as three weeks early.
Julie Christensen learned about gardening on her grandfather’s farm and mother’s vegetable garden in southern Idaho. Today, she lives and gardens on the high plains of Colorado. When she’s not digging in the dirt, Julie writes about food, education, parenting and gardening.