May signifies the transition from early spring growth to the full exuberance of summer. This month is defined by a dual purpose: celebrating established perennial and bulb displays reaching their peak, while also providing the optimal window for planting annuals for summer-long color. Warmer soil temperatures and extended daylight hours propel both existing plants and new installations into rapid development. This seasonal shift encourages gardeners to move their focus from preparation to active cultivation, setting the stage for the coming months of vibrant bloom.
Established Blooms of Late Spring
The May landscape is characterized by spectacular, mature displays of plants that have been rooted for years, completing their seasonal cycle with a burst of late-spring energy. Among the most noticeable are Lilacs (Syringa vulgaris), whose dense, cone-shaped flower clusters release a powerful, sweet fragrance that permeates the air. These hardy shrubs are often found in shades of purple, lavender, and white, marking the season’s zenith before the heat arrives.
The garden’s structure is also defined by the Bearded Iris (Iris germanica), known for its complex, showy flowers with three upright “standards” and three drooping “falls.” Irises span a broad spectrum of colors, with varieties like Siberian and Japanese Irises extending the bloom time into late May and early June. Their rhizomes, or thickened stems, store the energy necessary for this colorful display.
Peonies (Paeonia) close out the month, showcasing large, often fragrant, blooms in shades of pink, white, and deep burgundy. These herbaceous perennials offer a structured elegance with their heavy, multi-layered flowers that require deep, well-drained soil to thrive. Complementing these are the late-season bulbs, specifically the Ornamental Alliums, which send up striking, globe-shaped flower heads on tall, slender stalks. These purple or white spheres provide a distinct texture and continue the bulb season after the earlier flush of tulips has faded.
Annuals for May Planting
As the perennials finish their show, May becomes the prime planting window for annuals, which are installed now to provide continuous color until the autumn frost. These tender plants are purchased as young starts and are selected for their ability to thrive in the summer heat and their capacity for prolific, season-long flowering.
Among the most reliable choices are Petunias, particularly the newer, heat-tolerant series like Supertunia, known for their self-cleaning habit and trailing growth perfect for hanging baskets and containers. Their trumpet-shaped flowers require full sun to produce the dense, continuous flower mass. Another sun-loving, heat-tolerant option is the Marigold (Tagetes), which produces cheerful, pom-pom or daisy-like flowers in yellows, oranges, and reds. French Marigolds are compact and often used for edging, while African Marigolds grow taller, providing vertical interest in bedding displays.
For areas receiving less direct sun, the Impatiens hybrid SunPatiens offers a reliable alternative, delivering vibrant color in partial shade to full sun conditions. Meanwhile, Lantana is a top choice for hot, dry locations, featuring clusters of tiny flowers that attract butterflies and exhibit exceptional drought resistance once established. Planting these annuals in May ensures they have ample time to develop robust root systems before the most intense summer temperatures arrive.
Essential Care Considerations for May
Gardening activities in May must be timed precisely to avoid late-season weather hazards and ensure newly planted specimens survive the transition. The single most important factor before planting any tender annual is confirming the local “last frost date,” which represents the typical point when the risk of a freeze has passed. Planting before this date can result in immediate loss for sensitive plants like petunias or impatiens. Gardeners must consult regional data to determine the safest time for ground installation.
New plants, particularly annuals started indoors or in greenhouses, need a gradual transition process known as “hardening off” to prepare them for the elements. This involves slowly acclimating the plants to outdoor conditions—direct sun, wind, and cooler night temperatures—over a period of seven to ten days. Skipping this step can cause severe transplant shock, resulting in stunted growth or sun-scalded leaves.
Once the hardened-off plants are in the ground, they require specific attention to root development. Initial watering must be thorough to settle the soil around the roots and eliminate air pockets. A balanced, diluted liquid fertilizer can be applied after planting to encourage rapid root establishment and vegetative growth, helping the annuals achieve their full, continuous blooming potential over the summer season.