Your Last Spring Frost Date Tells You When it's Safe to Plant Outside Again
It seems that when the minutes of daylight start to increase in winter, so does the yearning to get outside in the garden again. But don't let a couple of warm days in early spring fool you into setting out your homegrown seedlings or new plant babies from the garden center too soon; a cold snap could wither them overnight. That's why you'll often come across the advice to wait until after your last frost date to add any new plants to your yard. So when is that, exactly? The short answer is: It depends. But you can get a pretty good idea of when it will be based on when that date has occurred in past years in your region. Plus you can plant certain vegetables and flowers outside even before the last frost. Here's what you need to know about your last frost-free date so you can avoid any frozen plants.
Average Frost Dates
A “frost” date really means when temperatures fall to 32°F or lower, which is cold enough to damage leaves or kill tender plants. The “growing season” is essentially the time between when the last freeze happens in spring and the first time temps get to freezing later in the year, known as the first fall frost date. Those events don't happen on the exact same days each year, of course, but you can get a pretty good idea of the time frame based on the average dates they have occurred in the past.
To figure out when you'll likely see the last of ice this winter, take a glance at the map above. It's compiled from 30 years of weather data collected by NOAA National Centers for Environmental Information. For example, if you live in southern Illinois, you’ll probably be able to plant outside during the first part of April, but if you’re farther north in Illinois, you should wait till the second half of the month to be safe. You can also get the map information personalized for your Zip code through the National Gardening Association's online database.
With these guidelines, you’ll have a window for planting outside. However, spring weather can often shift quickly, so make your local weather forecasters your BFFs. They'll warn you about any sudden temperature drops coming your way. If that happens, make sure to bring in any tender plants in containers, or you can cover newly planted veggies or annuals with an old cotton sheet until the weather warms again.
What You Can Plant Outside Before the Last Frost Date
While you’re impatiently waiting to get your tomato seedlings in the ground, go ahead and start growing cool-season vegetables and flowers. These unsung heroes of the garden are cold-hardy crops you can grow as the temperatures are still chilly in spring. You can plant them again in late summer for fall color and harvests.
As long as your soil has thawed out enough to dig in, you can sow most cool-season vegetables right in the garden rather than starting them indoors first. These include all those leafy greens you know you should eat, like Swiss chard, kale, spinach, and pak choi. Radishes also grow quickly in spring and can tolerate frost well. Certain annual flowers like pansies and snapdragons that often appear at garden centers very early in the spring also can take some freezing temperatures and keep on blooming.
Bare-root trees (especially fruit trees hardy in your area) can be planted before your last frost date, too. Because they haven't started to actively grow new leaves yet, they won't be harmed by a little below-freezing weather.