Start too soon and you’ll lose your crop to lingering spring frosts. Too late and you’ll gamble with winterkill before you can harvest.
When, then, is the right time to put plants in the ground?
That depends on your location; soil type and temperature; microclimates; and plant selection, said Shawn Olsen, an agriculture professor for Utah State University.
“One of the most underutilized tools in gardening is the soil thermometer,” Olsen said. “Plant your cool weather crops when the soil warms to 35 or 40 degrees. Go with your warm weather crops when it gets up to 55 or 60 degrees.”
Also pay attention to the variability of maturity dates listed on seed packets and plants, he said. “Many radishes, for example, mature in 30 days.”
Microclimates play a large role, Olsen said.
“In this area, it makes a huge difference if you’re planting on the top of a slope, the middle or on the bottom, because cold air tends to go down,” he said.
Anything that is heat-absorbing or gives off infrared radiation at night is useful. That means planting alongside a house, stone walls or outbuildings.
“Generally speaking, the south side of a building is warmer; the north side cooler,” Olsen said. “Learn to take advantage of that.”
Loose, sandy soil with a sunny exposure will dry early, he said, while “wet, packed soil takes longer. Your plants will just sit there.”
Have some season-extending tools available – cold frames, frost blankets, grow lights, high or low tunnels, row covers or a hobby greenhouse, said Lewis Jett, an extension horticulturist with West Virginia University.
“You can get a two-week buffer with planting aids,” he said. “Some give you as many as three to eight weeks.”
Raised beds or anything that warms the soil, like mulch, is going to be helpful, he said. “If a person is trying to be early, having some sort of a mulch down is critical to the crop – especially warm-season crops like melons or tomatoes.”
It also pays to know your USDA plant hardiness zone.
“Look to your state extension service calendars,” Jett said. “They’ll give you the dates of the average early frost and the average late frost. A good time to start planting is right after that spring date.”
Learn to distinguish between cool-season and warm-season plants.
Cabbage, broccoli, onions, peas, radishes, spinach and turnips are typical cool-season crops. These hardy plants will tolerate light frosts, prefer temperatures in the 50- to 60-degree range and lose some of their quality in the heat. They can be planted again in mid- to late summer for a fall harvest.
Tomatoes, cantaloupe, watermelon, eggplant and pumpkins, on the other hand, are tender plants craving warmth, or readings at least 15 degrees higher than the cool season varieties. Start them early in a greenhouse or indoors, transplanting them after nighttime temperatures moderate.
“The easiest way to get things growing is to put them under fluorescent lights in a PVC pipe network covered by greenhouse plastic,” Olsen said. “Sunny windows generally don’t have enough energy to grow plants.”