All areas of the United States, except those which never see the temperature fall below 32F, have a date in the spring which is the average date of the last frost. This date, often in March, April or May tells farmers and backyard gardeners when it is safe to plant certain crops.
The date is bit misleading. First, it’s an average date meaning there are years when the frosts stop occurring weeks earlier or even a week or two later. I find the dates used are often on the later side which isn’t necessarily a bad thing.
Just because the average date of frost has passed doesn’t mean you should go rushing out to plant your warm weather crops like peppers, tomatoes and cucumbers.
Cool and damp weather, a staple of spring in New England can take hold for much of May and while temperatures don’t go below freezing, it’s cool enough for plants to basically linger or even die.
A better measure of when to put out your tomatoes and other warm weather vegetables is to look at soil temperature. For example, tomatoes, according to a 2001 study conducted by horticulturalists H. G. Taber and R. Gansemer at Iowa State University, require a soil temperature above 60 degrees Fahrenheit for ideal shoot growth and flower development. If you have not had a series of warm days and mild night, chances are your soil is too cool.
Beans, which you should start outside, will rot in the ground if the soil is not warm. I like to plant my beans when the soil reaches at least 70F. They may germinate earlier, but you are risking it if it’s too cool or wet.
The chart below shows the optimal temperatures for most of the more common vegetables you likely grow. They are divided into warm and cool crops or those which can handle a light frost and those which will die if exposed to freezing temperatures.
You can purchase a simple soil thermometer for under 15 dollars. Here’s a link to a reliable vendor. I get many of my supplies from Johnny’s in Maine.
I find that pushing the warmer vegetables into the ground too early can cause problems with disease. If I plant a week or two later, the later planted tomatoes or other crops always catch up anyway. I usually wait until around May 20th to May 25th to put out my tomatoes based on the longer range forecast. I don’t a cool wet period of weather in the days just after I plant. I live on the line of Zone 6A/6B according the USDA plant zone chart. You can check your own location by clicking here.
If your indoor plants are simply getting too big and must go in the ground you can use certain products that keep them a bit warmer. You can also simply take a gallon milk jug and cut the bottom off then place it over your seedlings. This will create a mini greenhouse. Be sure you remove the cap or your plants will get too hot and burn. If you are really diligent you could put the cap back on as the sun sets in the evening, but be sure you remove it again in the morning.
I use a black landscaping plastic mulch around my warm weather crops which I find heats up the soil significantly. Rolls of this mulch are widely available. There is red one reportedly good for tomatoes, but I have not had experience using this.
I’ll be giving more garden tips on Twitter @growingwisdom.