The term “bulb” is widely used in gardening to describe any plant that stores its life cycle in a fleshy, underground organ, including true bulbs, corms, tubers, and rhizomes. Unlike plants grown from seed, these storage structures contain the entire blueprint for the plant, including the next year’s flower bud and food reserves. While most people associate planting these structures with fall or early spring, summer planting is possible. However, only certain types of geophytes—plants that hold their resting period underground—are suitable for warm-weather installation.
Which Bulbs are Ideal for Summer Planting
The bulbs that thrive when planted in the summer are primarily tender or summer-flowering varieties, which are sensitive to frost and need warmth to grow. These bulbs are planted in late spring or early summer to ensure the soil is warm enough to initiate active growth and root development. Warm soil temperatures trigger the growth hormones in these plants, allowing them to establish quickly and bloom later in the same season, often extending the garden’s color through mid-summer and into the fall.
Dahlias (tubers) and Gladiolus (corms) are prime examples that benefit from this warm-weather timing. Other excellent choices include Caladium tubers, grown for their colorful foliage, and Canna Lily rhizomes, which produce tropical-looking flowers. These varieties provide a reliable burst of color because they are not reliant on a cold period to complete their life cycle, unlike spring-flowering counterparts.
A second category of viable summer planting involves pre-sprouted or potted bulbs found at garden centers. These are often spring-blooming varieties, such as certain lilies, that are already actively growing or even flowering in containers. Planting these live plants with established root systems allows for instant color when transplanted into the garden bed. This approach bypasses the need for the bulb to establish roots from scratch and minimizes the risk of rot that dormant bulbs face in hot, moist soil.
Essential Planting Techniques for Warm Weather
Planting bulbs during the summer requires specific attention to soil conditions and watering to counteract high temperatures and prevent the storage organs from rotting. Good drainage is paramount because summer planting often coincides with frequent watering to combat heat stress. It is beneficial to amend heavy clay soil with well-rotted organic matter, such as compost or leaf mold, which improves aeration and ensures that excess moisture drains away effectively.
Immediately after planting the bulb at the recommended depth—typically three times its height—a deep, thorough watering is necessary to settle the soil and encourage initial root growth. For Dahlias and Cannas, consistent moisture is required throughout the growing season, making regular watering necessary in warm weather. The goal is to keep the soil moist but never soggy, as saturated soil can quickly lead to fungal issues that destroy the bulb or tuber.
Applying a thick layer of organic mulch, such as shredded bark or straw, is important in summer. Mulch acts as an insulator, shading the soil surface and helping to regulate the temperature around the newly planted bulb. This protective layer significantly reduces water evaporation from the soil, allowing it to retain the consistent moisture that these actively growing plants need to flourish in the heat.
Why Summer is Too Late for Spring-Flowering Bulbs
Summer is the wrong time to plant dormant spring-flowering bulbs, such as tulips, daffodils, and hyacinths, because of a biological requirement called vernalization. These hardy bulbs require a prolonged period of cold temperatures, generally between 32°F and 45°F, to break their dormancy cycle. This chilling period, which typically occurs naturally underground during winter, signals to the bulb that spring has arrived and it is time to sprout and bloom.
Planting these bulbs in the heat of summer means they miss the necessary cold stratification, disrupting their internal flowering mechanism. Without this cooling period, the bulb will usually fail to produce a flower, or it may produce a bloom with a very short stem, a condition known as “blasting.” Furthermore, a dormant bulb placed into warm, moist summer soil is highly susceptible to rot and fungal diseases before it can establish a root system. The most practical time for these specific bulbs is the fall, which allows for root establishment before the winter chill sets the stage for a spectacular spring display.