May marks the transition from cool-season planting to the main summer growing season. For many regions, the average last frost date has passed or is imminent, signaling that the soil is finally warm enough to support heat-loving plants. This shift allows gardeners to focus on sowing the seeds that will yield a bountiful harvest of summer produce.
Seeds for Direct Sowing Outdoors
May is the ideal time to sow the seeds of large, warm-weather crops directly into the garden bed, as these plants generally dislike the root disturbance caused by transplanting. Success with these vegetables hinges on waiting until the soil temperature reaches a consistent warmth, typically between 60°F and 70°F. This warmth encourages rapid germination and growth. Planting earlier in cold soil often leads to seed rot or stunted seedlings that struggle to catch up.
Bush and pole beans are excellent candidates for direct sowing, requiring soil temperatures of at least 60°F for reliable sprouting. Bush beans should be spaced about three to four inches apart, while pole beans need six to ten inches of space and a sturdy vertical support structure to climb. Sweet corn also performs best when sown directly into the ground, ideally in blocks of multiple short rows rather than one long row, which significantly improves wind pollination.
Cucurbits
Cucurbits, including cucumbers, melons, and summer squash, are highly sensitive to cold and should only be sown once the soil is reliably warm. Plant seeds one inch deep in hills or groups, spacing the hills several feet apart to accommodate their sprawling or vining habit. Zucchini plants need substantial room, often three to four feet between hills, while cucumbers can be trellised to save space. Consistent warmth and moisture are necessary to prevent bitterness in the fruit, which is a common issue when plants are stressed.
Tender Crops to Start Indoors in May
Even though May signifies the end of frost, certain long-season, heat-loving crops can still be started indoors this month. This is useful for a late-season harvest or if the initial spring planting window was missed. These seeds will produce transplants that can be moved to the garden in June or July.
Late-season tomatoes, peppers, and eggplant benefit from an indoor start to guarantee they have enough time to fruit before the first autumn frost. These seeds require significant warmth for germination, ideally a soil temperature between 75°F and 85°F, often achieved using a specialized heat mat. Pepper seeds, in particular, can take up to three weeks to germinate without adequate bottom heat.
Tender herbs, such as basil, can also be started indoors in May for a guaranteed supply of young plants ready for transplanting. Although basil can be direct-sown in warm soil, starting pots inside ensures a quick, lush crop. Once the seedlings have developed two sets of true leaves, they must be placed under strong grow lights or on a very sunny windowsill to prevent them from becoming leggy and weak.
Root Vegetables and Leafy Greens
May is a suitable time for succession planting, a technique that involves staggering plantings of the same crop to ensure a continuous harvest rather than one large flush. Root crops like carrots, beets, and radishes should be sown directly into deeply worked, stone-free soil to encourage straight, uniform growth.
Carrot and beet seeds are small and should be sown thinly to minimize the need for extensive thinning later, which can disturb adjacent seedlings. If necessary, thin carrots to about two to three inches apart and beets to four inches apart to allow the roots to swell properly. Radishes, with their quick maturity time of often less than 30 days, are perfect for intercropping or filling small gaps while slower crops establish themselves.
Leafy greens can still be successfully sown in May, provided the gardener selects heat-tolerant varieties to avoid premature bolting. Bolting occurs when the plant focuses energy on producing flowers and seeds, making the leaves bitter. Swiss chard and certain cultivars of heat-tolerant kale are excellent choices that resist bolting longer than traditional lettuce or spinach. Consistent and deep watering is important for all greens during this period, as dry soil can accelerate bolting.
Annual Flowers and Flavoring Herbs
May is the perfect time to directly sow the seeds of warm-weather annual flowers and many popular culinary herbs straight into the prepared garden soil. These plants contribute significantly to the garden’s ecosystem, attracting beneficial insects and providing fresh flavorings for the kitchen.
Annual flowers such as zinnias, sunflowers, cosmos, and marigolds germinate quickly in the warm May soil and thrive in full sun. Zinnias and cosmos are beneficial for attracting pollinators like bees and butterflies. Marigolds are known for their dual purpose, as their strong scent can help deter certain garden pests.
Many herbs can be started from seed this month to ensure a fresh supply for summer cooking. Dill, cilantro, and basil are easily sown directly into the ground or in containers. Cilantro tends to bolt quickly in the increasing heat, so successive, small sowings every few weeks are recommended to extend the harvest window. Basil, which requires warm soil, will germinate rapidly now and can be harvested lightly once the seedlings have established several sets of leaves.