It's tempting when the weather's mild to think about putting out tomatoes and peppers, and sowing seeds of squash and cucumbers, especially when the garden centers are full of plants. It's still too soon for us for the really warm-weather loving plants, based on the last 'average' frost date of April 15.
But, there's a progression of sowing as the soil temperatures gradually warm. First, the really cold-hardy plants (garden peas, snow peas, sugar snap peas, fava beans, and onion plants), then as spring gets underway (but frost might still be a possibility), planting spinach, potatoes, carrots, beets, chard, more greens, and lettuce. Winter squash seed can be planted now, getting a jump on the long season they need, but basically the seeds wait until the soil warms up enough to trigger germination.
It's always an interesting progression, swapping out bed space, checking soil temperatures, and watching the amount of rain and whether a cold snap might be in store. This year, I seem to have an awful lot of space allocated to garlic, onions, and their kin. We quite enjoy all of them, but definitely I'm not ordering so many onion plants next year! Clearly fresh 'scallions' and young garlic will be on the menu to make room for some of the warm-season vegetables.