The other major factor determining when to plant hardy vegetable seed is soil temperature. Most hardy vegetables will germinate at a soil temperature of 50 degrees. A few, like lettuce, spinach and radish, will germinate at 45 degrees.
Soils that have lots of organic matter warm up sooner. Soils that are loose and open because they have been spaded or cultivated recently also warm faster. Raised grow box soils warm the fastest because they are usually a third or more organic matter.
I usually plant started plants of cabbage, broccoli, Brussels sprouts, kale and other vegetables in the cabbage family by late March. You can also plant seed of these vegetables at about the same time, but they won’t be ready to harvest until about six weeks later than the transplants.
Although you can also find transplants of other hardy vegetables, I prefer to plant virtually all of them from seed in the middle of March. I plant lettuce, spinach and radish seed in early March because they will germinate at lower temperatures. I make two or more sowings of lettuce, spinach and radish about four weeks after the first planting to extend the harvest period.
I plant seeds of beans and sweet corn in mid-April. I plant transplants of tomatoes and peppers in late April to early May. I use larger plants up to gallon size to speed up the harvest date.
I sometimes plant smaller transplants of eggplant, squash, cucumber and melon at about the same time. I often start seeds of these vegetables in early April, placing three seeds under a 1-gallon milk jug with the bottom removed to warm the soil. After seeds sprout, I remove the jugs in the daytime and replace them at night for frost protection for about a month.
Remember that average last frost date means half the time frost occurs after that date. So be ready to protect tender plants with blankets if frost warnings occur.
I have a seven-page leaflet on vegetable gardening. Send me an email if you would like a free copy.