Fresh vegetables straight from the garden beat grocery-store vegetables every time. Most vegetables are in season only during certain times of the year, because that's simply when they grow the best. While it's possible to get tomatoes during the dead of winter, those tomatoes probably traveled thousands of miles and up to several weeks to get to your grocery store--but they certainly won't taste as good as those grown locally.
Some vegetables need cool temperatures to grow and then to ripen. They don't tolerate the heat well and growth slows down or the opposite happens. The warmer temperatures cause the plant to bolt or flower and then set seed. This happens with lettuces, spinach, kale, chard and other leafy greens. When vegetables flower the leaves begin to taste bitter and become tough.
Cool-season veggies include peas, lettuces, leafy greens, and the cruciferous family of vegetables including: cabbage, broccoli, cauliflower and brussels sprouts. Root crops of beets, carrots, parsnips, turnips and rutabagas do best in the cool season as well.
Some cool-season veggies can have two seasons. The first are those planted in early spring that are ready before the long days of summer. The second harvest is planted in late summer about 8 weeks before the average first frost. The days are getting shorter and temperatures cooler. These veggies include peas, lettuces and root crops.
Many vegetables need the warm, long days of summer to ripen and soil temperatures above 60 degrees to germinate. These include tomatoes, corn, beans, onions, peppers and eggplants. The early harvest varieties will ripen in areas with shorter summers. Some of these (like tomatoes) will continue to ripen if brought inside before a frost; and others, like corn, will not.
Hot House Vegetables
Many vegetables can be grown in a hot house, or indoors. Their natural preferences should be maintained. In other words, if growing lettuces, keep the temperature cooler and the growing lights off for a longer period of time than say tomatoes. Additionally many vegetables like tomatoes and cucumbers require pollination to set fruit. Inside a greenhouse or your house you have to do the work of the bees by gently brushing the flowers with a soft brush to spread the pollen from blossom to blossom.
Expanding the Seasons
It's possible to get a head start on the growing season by starting seedlings indoors or in a protected area outside. Cool-season veggies can be started 4-6 weeks before the average date of the first frost and then set outside to finish growing in very early spring.
Warm-season veggies can be started before the ground is warm enough for planting seeds by starting them in pots that are set on black plastic and then bringing them inside at night. The plastic warms up the soil which hastens germination. Bringing the plants inside keeps them protected from cooler night temperatures.