Vegetables differ not only in size, color, shape, and taste, but in their preferred growing conditions, too. Understanding when to plant vegetables depends on your climate and conditions -- and the vegetable itself.
Plants are classified by the coldest temperature they can endure, using a system developed by the United States Department of Agriculture. Regions of North America have been divided into Zones based on the lowest recorded temperatures, from the coldest, Zone 1, to the warmest, Zone 11.
For example, plants hardy to Zone 6 survive where winter temperatures drop to minus 10 degrees F, while those hardy to the warmer Zone 8 die long before it gets that cold. In Zones colder than their hardiness Zone, these plants must be brought indoors over winter or treated as annuals and replaced each year.
Soil conditions and sun, shade, and wind exposure all influence a plant's ability to overwinter. Plants rated for a range of hardiness Zones can usually survive winter in the coldest region as well as tolerate the summer heat of the warmest one. To find your garden Zone, see the USDA Hardiness Zone Map.
A plant tag or seed packet will list a plant's Zone unless the plant is typically grown as an annual, meaning it grows from seed, bears fruit, and dies in one year. Tomatoes, peppers, and beans -- in fact, most vegetables -- are annuals.
A microclimate is the climate in a small area that is different from the climate around it. This small area may be warmer or colder than the nearby area. Consider locating tender plants in microclimates that are warmer than the rest of your yard.
Houses and other buildings, plus paved surfaces, such as patios, driveways, and sidewalks, can create microclimates by absorbing heat during the day and radiating it into the landscape at night. The south side of a building is usually the warmest. The west side is also warm.
Balconies and rooftops have unique microclimates because they're above the ground. They may often escape frosts that kill plants at ground level. However, cold, dry winds may offset any heat gain.
Fences, walls, and large rocks can protect plants from wind and radiate heat.
Cool-season vegetables grow best when temperatures range between 40 degrees F and 75 degrees F. In most areas, they can be planted two to four weeks before the last spring frost. These crops often are those that develop edible roots, stems, leaves, or buds, such as potatoes, broccoli, and spinach.
Cool-season vegetables are unique in that their seeds germinate best in cool soil. They are usually planted as soon as the soil can be worked in spring. Avoid planting in soggy soil that is still full of moisture from snow or spring rains. Wait until the soil dries and can be cultivated.
The root systems of cool-season plants are shallower and the plants themselves are smaller than warm-season vegetable plants. They stop producing in early summer when temperatures reach 80 degrees F.
In warmer regions, plant cool-season vegetables as early as possible in late winter or early spring, and plant seeds or transplants again in late fall to harvest in winter.
A few cold-hardy vegetables, such as carrots, parsnips, and garlic, can survive throughout winter in some regions when insulated under a blanket of snow. Look for vegetables labeled "frost-hardy" to know which ones will tolerate prolonged freezing temperatures. Some varieties note better frost tolerance. For example, 'Coronado Crown' broccoli tolerates frost better than many other types.
Purchase a soil thermometer to help you know when to plant cool-season vegetables.
At a soil temperature of 40 degrees F, plant arugula, fava beans, kale, lettuces, parsnips, peas, radicchio, radishes, and spinach.
At a soil temperature of 50 degrees F, plant Chinese cabbage, leeks, onions, Swiss chard, and turnips.
At a soil temperature of 60 degrees F, plant beets, broccoli, Brussels sprouts, cabbage, carrots, and cauliflower.
Other cool-season vegetables include asparagus, celery, collards, garlic, kohlrabi, potatoes, rhubarb, and rutabagas.
Warm-season vegetables, such as tomatoes, peppers, corn, and okra, developed in tropical climates. They grow edible fruits instead of edible roots, stems, leaves, or buds, as cool-season crops do.
These tender crops are killed by frost and won't perform well if temperatures drop below 50 degrees F. Don't bother to plant before the soil and air temperatures have warmed up in spring or early summer because the seeds and plants simply won't grow. Wait until about two weeks after the average frost date for your region to plant warm-season crops.
You can encourage many warm-season crops to slowly continue growing into fall by protecting them from frost with row covers, cold frames, and other season-extending devices.
Warm-season crops can be planted indoors. An early start inside gives them a jump on the growing season, but remember to slowly acclimate them to outdoor life by placing them in shade instead of full sun, and allowing them to adjust in short periods to outdoor temperatures.