Many vegetables, the "hardy" varieties, do well in early spring. They can even stand a frost.
Several of our vendors, including Gales Meadow Farm, will have many hardy vegetable varieties at the market this weekend: kale, lettuce, collards, broccoli, peas, onions, beets, and more. At market you'll find many varieties of most of these, and some pots of mixed varieties. Any of them can be planted right now or as soon as your garden is ready.
These veggies need good soil and a spot that gets sun for at least 6-8 hours a day. Most of them will do well in pots on a sunny deck or parking strip. so you can have fresh homegrown vegetables even if you don't have a sunny garden. For hardy spring veggies, a light dressing of complete organic fertilizer mixed into the top 2-3 inches of the bed should be good for the whole season. If the soil is clay or sandy, a generous dose of compost applied before planting and mixed into the soil will help.
It's good to do your transplanting in the evening or on a cloudy day. Water the pots before you remove the vegetables, and as soon as you have finished planting, water the newly transplanted vegetables well to settle them into the ground and establish good contact between the soil and the roots.
Many vendors' pots of veggie starts have more plants than garden store six-packs. The pots may look crowded, but the plants don't mind! You need to be gentle as you separate them, but they are not terribly delicate. Gently take the whole block of potting soil out of the pot. Plant each one as you peel it from the soil; don't let the roots have a chance to get dry.
You can start picking lettuce, collards, chard, and kale leaves and beet greens in a few weeks; the peas will be ready before long; the onions can be harvested young or left to mature in August.