Many seed packets say to wait until “all danger of frost has past” before you plant. It’s hard to know when there will be a frost or when the last frost has happened for any given year. Don’t rely entirely on “frost-free” dates that are published by the USDA. The USDA charts and local Extension Service seasonal predictions are based on averages from the past 5 to 20 years of temperatures at or below 32 degrees. But frost can still happen at a temperature at 38 degrees, or even higher. To understand how that is possible, you have to understand how those records are created. First, all official temperatures are observed at 6 feet off the ground, not at the ground level where newly planted seedlings are located. The rest is all about thermodynamics. Simply put, cool air is denser (and therefore heavier) than warm air. During the day, the ground and plants absorbed the warmth from the sun. During the evening and night, they slowly radiate that warmth into the air just above the ground. The warmer temperature of that air causes it to rise and cooler air settles down to take its place (called inversion). If the air is moist and there is little or no wind, the coldest air remains close the ground, condenses and forms frost on the plants near ground level. Low lying pockets are the most vulnerable to this radiation frost. Your yard could be quite different from what the weather report is predicting depending on your elevation, relationship to a body of water (which can moderate air temperature), and any surrounding vegetation.
Use frost dates as a guide, but watch the daily weather when first planting out new seedlings or seeds. Frost is more likely on a full moon or when the sky is clear of clouds and there is no wind. If frost is expected and the plants are already in the ground, protect them with a floating row cover (Remay is one brand), hot caps, or even a few lawn chairs with an old bed sheet. Not all cold temperatures cause frost. Cold temperatures (above freezing) can cause a plant to stunt even if there is no noticeable damage. Frost will cause plant cells to burst as it starts to thaw. If you wake early one morning and find an unexpected frost, all may not be lost. Before the sun rises and directly shines on the plants, water the plants overhead with a sprinkler. Water, even cold water from the hose, can ‘melt’ the frost before causing too much damage. If you have a hose-end attachment that is designed to add fertilizer while watering, adding liquid kelp can provide additional help.