You should know two very important weather dates for your area if you want to grow vegetables successfully:
✓ The average date of the last frost in spring
✓ The average date of the first frost in fall
These frost dates tell you several important things:
✓ When to plant: Cool-season vegetables are generally planted 4 to 6 weeks before the last spring frost. Fall planting of cool-season vegetables is less dependent on frost dates, but it’s usually done 8 to 12 weeks before the first fall frost. Warm-season vegetables are planted after the last spring frost or in late summer in warm areas for a fall harvest.
✓ When to protect warm-season vegetables: Frosts kill warm-season vegetables. So the closer you plant to the last frost of spring, the more important it is to protect plants. And as the fall frost gets closer, so does the end of your summer vegetable season — unless, of course, you protect your plants
✓ The length of your growing season: Your growing season is the number of days between the average date of the last frost in spring and the average date of the first frost in fall. The length of the growing season can range from less than 100 days in northern or cold winter climates to 365 days in frost-free southern climes. Many warm-season vegetables need long, warm growing seasons to properly mature, so they’re difficult, if not impossible, to grow where growing seasons are short.
How are you to know whether your growing season is long enough? If you check mail-order seed catalogs or even individual seed packets, each variety will have the number of days to harvest or days to maturity (usually posted in parentheses next to the variety name). This number tells you how many days it takes for that vegetable to grow from seed (or transplant) to harvest. If your growing season is only 100 days long and you want to grow a melon or other warm-season vegetable that takes 120 frost-free days to mature, you have a problem. The plant will probably be killed by frost before the fruit is mature. In areas with short growing seasons, it’s usually best to go with early ripening varieties (which have the shortest number of days to harvest).
However, you also can find many effective ways to extend your growing season, such as starting seeds indoors or planting under floating row covers (blanketlike materials that drape over plants, creating warm, greenhouselike conditions underneath).
There you have it; now you know why frost dates are so important. But how do you find out dates for your area? Easy. Ask a local nursery worker or contact your local Cooperative Extension office (look in the phone book under county government).
Frost dates are important, but you also have to take them with a grain of salt. After all, these dates are averages, meaning that half the time the frost will actually come earlier than the average date and half the time it will occur later. You also should know that frost dates are usually given for large areas, such as your city or county. If you live in a cold spot in the bottom of a valley, frosts may come days earlier in fall and days later in spring. Similarly, if you live in a warm spot or you garden in a microclimate, your frost may come later in fall and stop earlier in spring. You’re sure to find out all about your area as you become a more seasoned vegetable gardener and unearth the nuances of your own yard. One thing you’ll discover for sure is that you can’t predict the weather.
Listening to your evening weather forecast is one of the best ways to find out whether frosts are expected in your area. But you also can do a little predicting yourself by going outside late in the evening and checking conditions. If the fall or early spring sky is clear and full of stars, and the wind is still, conditions are right for a frost. If you need to protect plants, do so at that time.
Source of Information : vegetable gardening for dummies