Planting a vegetable garden isn’t just a one-day affair. It’s often done over several weeks, depending on the weather and other factors. There are a variety of ways to determine when to plant what.
Cool Season/Warm Season
At their simplest, vegetables tend to fall into two categories for planting — cool season and warm season. Cool season crops are those that will germinate before the threat of frost has passed. They include greens, such as lettuce, spinach, chard and kale, as well as onions and peas. Many guides recommend that these seeds be planted “as soon as you can work the soil,” which means the frost is out and it is not sopping wet or muddy. Warm season crops include melons, tomatoes, peppers, squash and corn and these should be planted “when all danger of frost is passed.” If you are pressed for time, consider planting most of your cool season crops on a warm day in late April, then all the warm season crops on another day in late May. It should work for most crops.
Many garden books (and all seed packets) recommend planting based on frost dates. The packets will tell you how far before or after your last frost date to plant the crop. Plant potatoes 5 weeks before the last frost date, or tomato starts two weeks after the last frost, they’ll say. One thing to keep in mind, though, is that in cold climates like ours spring is more compressed than in other places. We go from highs in the 30s to highs in the 90s quickly and unpredictably. Melinda Myers in her book Month-by-Month Gardening in Minnesota recommends planting potatoes after April 15 (about three weeks before the last frost, depending on where you live) and setting tomato plants out on May 20 (10 days to two weeks after last frost).
If you want to be even more scientific, you can invest in a soil thermometer and go by the temperature of the soil, since that is often a key factor in germination. Crops such as kale, spinach, argula and radishes will germinate as long as the soil temperature is consistently above 40 degrees. Once it gets above 50, you can plant onions, Swiss chard and turnips. Soil needs to be above 60 degrees to plant beans, beets, cabbage and carrots, and it has to be above 70 to germinate seeds for cucumbers, squash, melon or corn. This website has a helpful chart on when to plant what based on soil temperature.
As our weather has changed, more people are using natural signs (or phenology) for planting their gardens. I mentioned a few in the post on frost dates and here are a few more. When crocus bloom, remove the mulch on your strawberries. When you hear the spring peepers, plant peas. When leaves first emerge on lilacs, plant lettuce, beets, spinach and other cool-weather crops. When lilacs are in full bloom and the barn swallows return, set out your tomato plants and basil. When irises bloom, set out your squash and melon transplants. These are based on hundreds of years of observation and are fairly reliable, if you enjoy observing nature. Keeping records of your own garden will help you develop your own phenology for when to plant what.