May marks the true start of the warm-season planting period for many regions. This month typically signals the end of spring frost threats, allowing for the successful introduction of tender plants. As soil and air temperatures stabilize, conditions become favorable for food crops and ornamental annuals that require consistent warmth. Gardeners transition from starting seeds indoors to setting out established plants, accelerating the timeline toward a summer harvest and vibrant floral displays.
Determining Your Local Planting Safety
Successfully planting tender crops in May relies entirely on local conditions. The most important calculation is the “Last Frost Date” (LFD), which represents the average date after which a freeze event is statistically unlikely. Planting after the LFD minimizes the risk of damage to sensitive seedlings. This date can be found using local agricultural extensions or historical weather data.
Warm-weather seeds and transplants also depend on sufficiently warm soil temperature. For most tender vegetables, the soil needs to be consistently above 50°F before planting to encourage vigorous root growth and germination. Crops like tomatoes and peppers prefer even warmer soil, often waiting until temperatures reach 60°F to 65°F for optimal establishment. Using a soil thermometer provides a more accurate reading than relying on air temperature.
Seedlings purchased from a nursery or started indoors must undergo “hardening off” before being set into the ground. This involves gradually exposing the young plants to outdoor conditions, including wind, direct sun, and cooler temperatures, over seven to fourteen days. Moving transplants directly from a sheltered environment can cause severe shock, leading to stunted growth or death. Hardening off helps plants develop protection against excessive water loss and sunscald.
Warm-Weather Vegetables to Plant
May is the ideal time to establish a summer vegetable garden using both transplants and direct seeding methods. Selecting the correct method for each plant ensures a productive and timely harvest.
Transplants for Quick Harvest
Long-season, heat-loving crops such as tomatoes, peppers, and eggplant are planted as transplants in May. This ensures they have enough time to mature before the end of the growing season. Tomatoes can be selected as determinate varieties, which ripen fruit all at once, or indeterminate varieties, which produce fruit throughout the summer. Setting out healthy seedlings gives these plants a necessary head start in climates with shorter summers.
Peppers and eggplants also benefit from being planted as transplants, as their seeds take considerable time to germinate. These plants require at least six to eight hours of full sun daily. When planting, bury tomato transplants deeper, often up to the first set of true leaves, to encourage a stronger root system to form along the buried stem.
Direct Sowing for Summer Production
Many fast-growing, warm-season vegetables perform best when sown directly into the garden soil after the frost risk has passed. Beans (bush and pole varieties) should be planted directly when the soil has warmed, as they germinate quickly and do not transplant well due to fragile root structures. Corn should also be planted directly, often in short, wide blocks rather than long single rows, to facilitate better wind pollination.
Cucumbers and summer squash, such as zucchini, are prolific producers best started by direct seeding in May. These plants germinate rapidly in warm soil and quickly develop deep taproots easily damaged by transplanting. To maintain a steady supply of greens, succession planting of Swiss chard or heat-tolerant lettuce varieties can also be direct-sown this month.
Annual Flowers and Culinary Herbs
May is the perfect time to introduce colorful annual flowers and tender herbs that thrive in the summer heat. These plants are intolerant of cold temperatures and are best planted as established seedlings for immediate effect.
Annual Flowers
May is the month to plant popular annual flowers for instant color in garden beds, hanging baskets, and containers. Petunias, known for their long-lasting blooms and trailing habit, are excellent choices for cascading over pot edges. Marigolds and zinnias are reliable, sun-loving flowers often planted as transplants to provide immediate visual impact and attract beneficial insects.
Impatiens and coleus are selections for shadier areas, providing season-long color and texture. Coleus is grown specifically for its richly patterned foliage. Most annuals purchased from garden centers should be planted after the last frost date, as a brief cold snap can damage the tender foliage.
Culinary Herbs
Tender culinary herbs should wait until May to be planted outdoors, as they suffer damage from frost. Basil requires consistent warmth and full sun to produce its fragrant leaves. It can be planted in the ground or in containers, which allows for protection if an unexpected cold night occurs.
Cilantro and dill prefer cooler temperatures but can be started in May for an early summer harvest before the heat causes them to “bolt.” Mint, often planted in May, is best kept in a container due to its vigorous, spreading root system that can quickly overtake garden beds.