No matter how big or small your garden is, for successful vegetable gardening it is crucial to know the difference between cool-season and warm-season vegetables, as well as the approximate last frost date in the spring, and the first frost date in the late summer/early fall in your area.
Cool-season crops are the first ones to plant in the garden year. This can be anytime from several weeks to a couple of months before the last frost date. These early vegetables cannot only withstand cold temperatures, they need them to germinate, grow, set fruit and mature. Some winter-hardy vegetables, which are the toughest of the cool-season vegetables, such as kale or Brussels sprouts, even benefit from a light frost, as it converts the starches into sugar and improves the taste.
It is important to plant cool-season crops early enough in the spring so they can complete their full cycle up to harvest before the temperatures get too warm. While some cool-season vegetables can withstand hot weather and will still grow, their quality becomes inferior. For example, radishes turn fibrous and unpleasantly sharp and pungent in hot summer weather.
Many cool-season crops can be sown in the early spring and again in the fall but keep in mind that they must be planted early enough to reach maturity before the onset of cold weather that will kill most of them, with the exception of winter-hardy vegetables.
Cool-season vegetables are generally grown directly from seed in the garden, either as soon as the soil can be worked in the spring, or until the soil and air have reached certain minimum temperatures that are usually indicated on the seed package.
Warm-season crops require higher soil and air temperatures; they are always planted after the last frost date. In Pennsylvania, warm-season crops are usually started from seed indoors and transplanted into the garden as soon as the soil and air are warm enough. The growing season in our area is too short to allow for some of the warm-season crops to be directly sown in the garden soil.
For example watermelons take about three months from seed to harvest, and the only way to beat the calendar is to start them indoors.
Unlike cool-season vegetables, warm-season vegetables have only one growing cycle, ranging from late spring, after the last frost date, to late summer. Daytime temperatures may still be warm enough, but drop so much at nighttime that the weather is not suitable for warm-season crops any longer. For example, cucumbers turn bitter when the nights are too cold.
While the requirements of cool-season and warm-season vegetables determine the basic planting times, gardeners have several few tweaks at their disposal to work around them. For example, to warm the soil faster in the spring, you can cover it with black plastic, which may allow you to plant warm-season crops a bit earlier than what the calendar indicates, provided the air temperature is warm enough. There are other so-called season extenders like high tunnels.
At the other end of the spectrum, however, in warm weather, leafy vegetables like salad greens and spinach rapidly produce flowers and seeds, a type of survival strategy to make sure the next generation of plants is present before the plant succumbs to temperatures that are too warm for it. This is called bolting. One way to work around this is to choose bolt-resistant varieties, which are also indicated in the seed catalog or on the seed package.
To make sure you have the required space in your garden at the time when you need it, it is highly recommended that you plan your garden on a piece of paper, complete with space requirements and planting times so that when it is time to transplant the winter squash seedling into the ground one week after the last frost, you will have the proper spot for it.
The Vegetable Planting and Transplanting Guide at: http://extension.psu.edu/gardening/fact-sheets/vegetable-gardening/vegetable-guide lists the common vegetables and the best time to plant them.
Common cool-season vegetables: asparagus, beets, broccoli, Brussels sprouts, chives, cabbage, carrots, cauliflower, Swiss chard, kale, leek, lettuce, onion, parsnips, peas, radishes, spinach, and turnips.
Common warm-season vegetables: beans, corn, cucumbers, eggplant, melons, peppers, zucchini and summer squash, pumpkin and winter squash, sweet potato, tomato, watermelon.