May is the transition month in the gardening calendar for temperate regions, shifting from cool spring conditions to the primary growing season. This period allows gardeners to move beyond frost-tolerant greens and introduce the heat-loving species that will form the bulk of summer and fall harvests. Success hinges on understanding this change, as soil and air temperatures finally stabilize enough to support tender growth.
Understanding Local Climate and Frost Dates
The concept of “May planting” is highly relative and depends entirely on a gardener’s specific geographical location and microclimate. The defining factor is the Last Expected Frost Date (LEFD), the average date historical weather data suggests the final spring frost will occur. Planting any tender vegetable or flower before this date risks severe damage if temperatures dip below 32°F.
Gardeners should use resources like the USDA Hardiness Zone map or local agricultural extension offices to find their specific LEFD. While air temperature is important for preventing frost damage, soil temperature is the variable for direct-sown seeds. Warm-season seeds will not germinate effectively if the soil is too cold, regardless of how warm the air may feel.
For instance, corn requires a minimum three-day average soil temperature of 50°F to begin germination, though uniform emergence is best achieved closer to 60-70°F. Planting into cold, wet soil can cause cell rupture and non-viability in seeds. Testing the soil temperature at planting depth provides a much more accurate assessment of readiness than simply observing the calendar.
Warm-Season Vegetables for May Sowing
May is the traditional time for setting out warm-season vegetable transplants, which were started indoors six to eight weeks earlier. Tomatoes, peppers, and eggplants are examples of these tender fruiting plants that must be moved out only after the danger of frost has passed. Before planting, these seedlings must undergo hardening off, which gradually acclimates them to harsher outdoor elements like direct sunlight and wind over seven to ten days.
The hardening off procedure involves first moving transplants to a shady, protected outdoor location for a few hours each day. Duration and exposure to sun and wind are then incrementally increased. This process encourages the development of a thicker cuticle on the leaves and slows the plant’s growth rate, preparing it for the garden environment. Tender transplants are susceptible to stunting or leaf scorch if planted without this transition.
Many other warm-season vegetables, particularly those with delicate root systems, are best started by direct sowing seeds once the soil has adequately warmed. Cucurbits, including summer squash, zucchini, cucumbers, and melons, suffer from transplant shock when their roots are disturbed. Bush beans and pole beans also prefer direct sowing, as they germinate quickly in warm soil and thrive on consistent summer heat.
For gardeners in regions with a late May LEFD, this month offers a final opportunity to plant fast-maturing cool-season crops. Radishes and leaf lettuce can be sown early in the month and harvested before the intense summer heat causes them to bolt, a process where the plant prematurely sends up a seed stalk. This strategy maximizes the productivity of the garden space during the seasonal transition.
Annual Flowers and Companion Herbs
May is the prime time to introduce annual flowers that provide continuous color and support for pollinators throughout the summer months. Most popular annuals are frost-sensitive and are typically purchased as transplants to be planted after the LEFD. This group includes vibrant species such as petunias, zinnias, and marigolds, which contribute aesthetic appeal and beneficial insect attraction.
Zinnias are favored for their ease of establishment and ability to draw in butterflies, while marigolds are often interplanted with vegetables for their mild pest-repelling properties. Planting these flowers from nursery starts ensures instant color and a longer flowering season. The warmth of May soil helps these annuals quickly root and begin their summer display.
Heat-loving herbs are also planted now, with basil being the most recognized example that requires warm air and soil temperatures above 50°F. Rosemary, a perennial herb often treated as an annual in colder climates, can also be safely transplanted in May. In contrast, cool-season herbs like dill and cilantro can be direct-sown early for a quick harvest, though they will rapidly bolt once the full summer heat arrives.