Basil plants for sale in March in Northern Virginia
When planting in the spring, you want to know two things. The first is which of your plants/seeds are cool season versus warm season. Cool season plants can take a cold nip while warm season plants have to be planted after the danger of frost has passed and the night temperatures are constantly in the mid-fifties. Now, while it is cool and there is a high chance of frost, we can direct sow cool season plants such as spinach, lettuce, cilantro, kale, etc. If we get a light frost at night they would not be affected. However, if I were to plant a basil or tomato while frost is highly likely, the low temperatures would destroy them so I will wait until it gets warmer.
As time goes by the chance of frost diminishes. Traditionally, I start my warm season plants around Mother’s Day as a marker for when there should be no chance of frost with temperatures in the mid-fifties. However, this year, spring seems to be coming early. For gardeners, there is that almost gravitational pull to get outside and start planting now.
Tomato plants for sale in March in Northern Virginia
So keep in mind the second aspect: what is your level of risk? Are you willing to start warm season plants when there is a 50 percent chance of frost? Are you willing to “get a head start” on tomatoes knowing you may have to rush out and cover them when frost is predicted that night?
Frost is predicted when air temperatures reach 32 degrees but may also occur when air temperatures are just above freezing due to open exposure to the clear night sky. Frost can be a “light freeze” at 29 to 32 degrees, which usually kills tender plants. Using the 1991-2020 climate normals from the National Oceanic and Atmospheric Administration, the National Gardening Association provides a useful chart outlining probabilities of cold temperatures by dates.
By entering my zip code in Northern Virginia, March 29 is the 50/50 point where there is a fifty-fifty chance of frost occurring. Some gardeners look for this point to get a head start knowing they may have to cover their plants. I am more interested in being safe than sorry — I have no desire to rush out and cover my plants so I am looking for a number lower than 50. Also, the National Gardening Association recommends using 36 degrees as the marker because frost can form at 32 degrees and then fall down into your garden even though it is 36 degrees on the ground. Using this logic, the 50/50 mark is April 9.
On April 10, I have a 10 percent chance of 32 degree temperatures. Ten days later, I have a 10 percent chance of 36 degree temperatures. As time goes by it, the 10 percent chance of frost will diminish to zero (until autumn). Because I am a low risk taker (or possibly too lazy to go outside and cover my plants), I am shooting for less than 10 percent to near zero. Therefore, I could start my basil and tomatoes at the end of April. This is earlier than my traditional Mother’s Day but keep in mind these numbers come from averages from several years ago and there is nothing to say that a freak freeze won’t occur at the end of April. So I could also be lazy and keep the Mother’s Day marker so I don’t have to change anything. Like I said, it is personal preference, you pick what you are most comfortable with or what fits your schedule.
This is a complicated topic for my gardening column. Frost, or at least chill, was an important component of the climate here when orchard production was still an important industry. It is still an important consideration for home gardening. However, I also write for the Canyon News, which is in Beverly Hills, which lacks frost, and gets such minimal chill that only a few cultivars of apple are sometimes productive there.