Once you've chosen a site and decided which vegetables it can accommodate, you'll need to consider climate and length of growing season before making your final choice of which vegetables to plant.
To calculate the length of your growing season, count the number of days between the average last-frost date in spring and the first-frost date in fall (your Cooperative Extension Office or a local nursery can give you these dates).
Last-frost dates let you know when it's safe to set out tender vegetable plants or sow seeds; first-frost dates tell you when you'll probably have to provide protection for tender kinds late in the season. Aim to select vegetables that can mature and bear a good crop in the interval between these two dates. For example, if the seed packet says a certain variety of winter squash requires 120 days from seed to harvest but your growing season lasts only 100 days, look instead for a variety adapted to short growing seasons; or plan to use season-extending techniques.
Warm-season vegetables may need protection from frost when they are first planted in spring, then again as temperatures begin to dip in fall. In spring, individual plants can be protected with various plastic or paper caps known as hotcaps. Also available is a special plastic hotcap for tomatoes; it consists of water-filled cylinders that trap heat effectively.
Floating row covers made of polyethylene, polyester, or polypropylene are one of the most useful tools for protecting plants from cold temperatures (and from certain insect pests, as well). Sold in rolls, these fabriclike covers can be laid directly over seeded beds or plants or propped on stakes; they serve as miniature greenhouses. They are extremely lightweight, transmit 80 to 95% of the sunlight that strikes them, and allow both water and air to pass through. Burying the cover's edges in the soil will seal out insect pests, though any already on the plants may proliferate (remove covers when plants begin to bloom to admit pollinating insects).