No First Plants ‘til Last Frost
As the temperature drops, there’s is a moment when the water that is in the air becomes solid. That moment usually happens at 32 degrees and is referred to as frost. (1) Frost covers your cars, your windows, and your plants, collecting overnight in the late fall and early spring around Richmond. On a microscopic level, the water inside the cells of your plants freeze, expand, and explode, destroying fruit and, if cold enough, killing the whole plant. Entire harvests can be lost to planting too early or harvesting too late. So knowing when that moment is coming is crucial.
For Richmond, the average last frost date (LFD) is April 6. (2)
But that’s just the short answer. Of course, LFD is different every year, and as Earth’s climate continues to fundamentally change, LFDs will continue to come extremely early or extremely late. Both the earliest and latest LFDs of the last 100 years came in the last decade, two months apart. (3)
For tender plants like tomatoes, 32 is enough to kill them, that’s referred to as a light frost. For underground potatoes, and hardy collard greens, they’ll survive to the mid 20s or lower. Sturdier plants and trees are not destroyed by frost, but frost will stop them from growing. (1)
Frost severity is dependent on the landscape too. Cold air collects in valleys, or it can be moved along quickly in windy areas. Your plants might be fine while the ones on the next block aren’t. (1)
If you are planting in containers, having them on top of the asphalt can keep them alive through a light frost. If you are planting into raised beds or directly into the earth, you have more things to be careful about.
Cold temperatures also freeze water that is present in soil. As that water expands while becoming ice, pockets are opened up in the soil. After it melts, rain can later fall into that pocket, freeze at night, and expand further. This cycle of freezing and thawing can create dramatic erosion issues. (4) This issues are made worst by tilling. And so breaking up the soil too early, when there are still risks of freeze thaw cycles, can cost the land its health and structure. Waiting to plant just a few weeks and leaving the wintered-over cover crops a little longer can prove super helpful to the harvest.
That said, people gotta eat. So people around Richmond are eager to start their crops. Well right now, while frost is still a risk, it’s best to start your plants indoors by a window on with some good glass or plastic protection outside. Peas planted early can be hardened to withstand a frost. (5) If you start lettuce, spinach, and beets right now, a light frost might even make them taste sweeter. (6) If conditions are right, you may be able to get away with starting kale, lettuce, radish, spinach, cabbage, and carrots early. (7)
But most plants “won't show fast, healthy growth until the middle of April, when soil temperatures rise to a point where the soil food web becomes active – about 45°F, 4 inches deep.” (8) Wait until mid-May to start seeds straight into the ground in Richmond. That’s when you can get your melons, cucumbers, okra, peanuts, and beans going. (9)
For now, wait on the summer veggies. There will be thyme. (10)
_________________________
1: https://www.nationalgeographic.org/encyclopedia/frost/
2: My own calculations of the average from the last 100 years, based on the data available at https://www.weather.gov/wrh/Climate?wfo=akq but the average for the last 10 years is April 2.
3: https://www.weather.gov/wrh/Climate?wfo=akq
4: https://glisa.umich.edu/resources-tools/climate-impacts/freeze-thaw-cycles/
5: https://hgic.clemson.edu/early-spring-vegetables/
6: https://www.thespruce.com/cool-weather-vegetables-1402171
7: https://www.burpee.com/vegetables/early-spring-vegetables/
8: https://www.growveg.com/guides/which-vegetables-are-best-for-early-planting/
Red Russian kale covered in ice crystals after a late night frost. Photo by Steven Casanova.
9: https://www.southernexposure.com/sowing-summer-crops-straight-into-the-garden/
10: Shouts out to V, she got thyme.