Wait Until the Last Frost Date
Thanks to careful data collection during the past 100 years, most U.S. locations have records from which average annual last and first frost dates can be determined. Although an area's last average frost date indicates when the air temperature may dip below freezing for the last time in spring, the soil temperature may be slightly warmer. Most seeds need specific soil temperatures to germinate. So if you want to start vegetable plants from seeds, then either find their specific minimum temperature for germination on the seed packets or start the seeds indoors and transplant the resulting seedlings in the garden after air and soil temperatures warm.
In southern states, such as those in U.S. Department of Agriculture plant hardiness zones 7 and above, the last frost tends to be about April 15 while the Midwest and northern states in USDA zones 3 through 6 are generally not frost-free until closer to Mother's Day in May. Last frost dates vary with altitude as well as climate, however. So consult your county's Cooperative Extension Service to get the most accurate information for your location.
Choose Cool-Season Crops
Vegetables that can be grown as annual plants can be divided into cool-season and warm-season crops. Cool-season crops can germinate and grow when temperatures are below 70 degrees Fahrenheit. In USDA zones 3 through 7, these are among the kinds of vegetables that can be grown from seeds or transplanted as seedlings in early April:
Cruciferous crops such as cabbage (Brassica oleracea var. capitata), broccoli and cauliflower (Brassica oleracea var. botrytis). Root crops such as beet (Beta vulgaris), carrot (Daucus carota) and radish (Raphanus sativus). Leafy greens such as lettuce (Lactuca sativa), Swiss chard (Beta vulgaris Cicla Group) and spinach (Spinacia oleracea). Herbs such as cilantro (Coriandrum sativum) and chives (Allium schoenoprasum) that withstand cool nights. Chives also can be grown as a perennial in USDA zones 4 through 8.
Avoid Warm-Season Crops
Warm-season crops are not safe to plant in April except in USDA zones 8 and above because late-spring frosts and snows are likely to cause damage until May. Warm-season vegetables include:
Tomato (Lycopersicon esculentum), which is perennial in USDA zones 10 through 11. Pepper (Capsicum annuum), which is perennial in USDA zones 9 through 11. Cucumber (Cucumis sativus).
Use Successive Planting
Planting cool-season crops in early April can allow you to grow plants successively. For example, several rows of lettuce can be sown in early April, and another few rows can be sown several weeks later. That planting technique allows for a continual harvest during the growing season.
Additionally, a cool-season crop such as radish or spinach that is quick to mature can be grown in April and harvested before June. Then a warm-season crop can be planted in its place, and that crop can be replaced by another cool-season crop in fall.