May represents the transition from tentative early spring preparation to the main growing season for most gardeners. This month marks the point when the threat of frost typically recedes enough to allow tender, heat-loving plants to move into the garden. Successful May planting depends entirely on a localized assessment of climate and geography. Understanding these local conditions determines whether you are setting out transplants, sowing seeds directly into the soil, or doing a mixture of both.
Assessing Your Local Planting Window
The most significant factor in determining your May planting schedule is the date of your last expected spring frost. This date represents the point after which the probability of a temperature dip to 32°F or below falls to a low percentage (often 10%). For many regions, this date falls sometime in early to mid-May, though it can extend into June in colder climates. Planting tender vegetables before this date risks catastrophic damage, as these plants cannot tolerate freezing temperatures.
Air temperature alone is not sufficient for planting; the soil temperature must also be warm enough for seeds to germinate and for transplants to establish roots effectively. Cold soil can stunt growth and hinder the absorption of nutrients, delaying the harvest. Warm-season crops like peppers and tomatoes require a soil temperature consistently in the 60°F to 65°F range for optimal growth. Using a soil thermometer to confirm the temperature at a depth of a few inches is a simple, actionable step that reduces the risk of early planting failure.
Warm-Weather Vegetable Staples
May is the designated time for transplanting the main summer crops that were started indoors several weeks earlier. These tender vegetables, which include tomatoes, peppers, and eggplants, thrive in the heat and cannot handle any frost. While the last frost date guides the general timing, the plants themselves must be properly transitioned to the harsh outdoor environment. This transition is known as “hardening off.”
Hardening off is a gradual two-week process that acclimates indoor-grown seedlings to outdoor conditions, including direct sun, wind, and fluctuating temperatures. Without this step, tender plants suffer from transplant shock, manifesting as wilting, leaf discoloration, or severely stunted growth. The process causes the plant to thicken its cell walls and reduce its internal water content, making the tissue firmer and more resilient.
The process begins by placing transplants in a shady, protected area, such as a covered porch, for only a few hours. Over the next seven to fourteen days, the duration outside is increased daily, slowly exposing the plants to more direct sunlight.
It is also helpful to reduce the frequency of watering during this period, but you must prevent the plants from wilting completely. Once the plants have successfully spent 24 hours outside, they are ready to be set into the garden soil.
Tomatoes can be planted deeply, as they sprout additional roots along the buried stem, leading to a stronger root system. Peppers and eggplants are sensitive to cool nights and should not be transplanted until nighttime temperatures remain reliably above 50°F (ideally above 60°F). Insufficient heat will cause the plants to stall and delay fruit production for weeks, even if they survive.
Direct Sowing Quick Crops and Ornamentals
Once the soil has warmed sufficiently, May is the ideal time to direct-sow many crops that do not tolerate root disturbance or grow quickly. Vegetables that are intolerant of transplanting, such as bush beans, cucumbers, and summer squash (like zucchini and pumpkins), should have their seeds placed directly into the garden bed. These crops germinate rapidly in warm, moist soil and quickly develop extensive root systems, which are easily damaged if moved.
Other heat-loving vegetables, including sweet corn, thrive when sown directly into soil requiring temperatures of at least 60°F. For a continuous harvest, quick-maturing crops like radishes and certain varieties of lettuce can be succession planted every few weeks by sowing small batches repeatedly.
Annual flowers and herbs can also be direct-sown in May to provide color and attract beneficial insects. Flowers like zinnias, marigolds, and sunflowers are easy to grow from seed placed directly in the garden after the last frost. They germinate quickly and offer vibrant color until the first frost.
Heat-loving herbs, particularly basil and cilantro, should be sown directly now for summer use. Basil, in particular, will flourish only when the weather is consistently warm and the soil temperature is high, making May the perfect time to start the seeds.