Answer: It is possible to start warm season plants such as tomatoes and pumpkins indoors ahead of time to try to harvest earlier; counting back from the average last spring frost date I would allow about 6 to 8 weeks for tomatoes and peppers and only about 2 weeks for squash because the plants get big so fast. You can plant them outdoors once the weather has settled and the soil has warmed. You can also start both of these outdoors after frost and once the soil has warmed (soil at 70 degrees). Carrots however are best planted in the garden where they are to grow. They can be planted outside from early spring to mid summer.
If you are wondering about late summer crops, check the "days to maturity" for the variety you are growing and see if you can plant successively for more than one crop, or plant both an early and a later maturing variety of the same crop. This is commonly done with beans and many gardeners plant both an early and a later tomato.
Planning for a fall garden is a bit tricky. For some plantings, such as peas and beans, you need to calculate backwards from the average first fall frost date and add about a week or so (to allow for the slowing of growth as the season progresses) to the maturity date. In addition, the more cold tolerant vegetables such as cole crops, carrots, greens, and radishes can sometimes be kept going long after frost by use of a cold frame or other form of protection.
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