Summer flowering bulbs produce vibrant blooms from late spring through the end of summer. The term “bulb” is used broadly to include true bulbs (like lilies), corms (Gladiolus), tubers (Dahlias), and rhizomes (Canna Lilies), all of which are underground storage organs for the plant’s energy. These summer-blooming varieties are generally not winter-hardy, unlike spring-blooming counterparts such as tulips and daffodils, and must be planted in the spring.
The Critical Timing Based on Climate
The most important factor for planting summer flowering bulbs is avoiding cold soil and frost, as most of these varieties are classified as tender. Tender bulbs cannot survive freezing temperatures and will rot or be damaged if planted too early into cold, wet ground. The definitive starting point for planting is the last expected frost date for your specific region.
A general rule of thumb is to wait until two to three weeks after this final frost date has passed to ensure the soil has adequately warmed up. Ideally, the soil temperature should reach at least 60°F (15.5°C) before planting varieties like Dahlias, Gladiolus, or Canna Lilies directly into the garden. Planting generally occurs from mid-spring until the end of May, depending on the local climate.
Planting in staggered batches every two weeks, particularly for Gladiolus, can help extend the flowering season throughout the summer. In regions with extremely short growing seasons, it is possible to start tubers and rhizomes indoors in pots several weeks before the last frost to get a head start. This technique allows the plants to develop roots and foliage in a protected environment, ready for transplanting outside once the weather is reliably warm.
Preparing the Ideal Planting Site
Selecting a suitable location that meets the specific environmental needs of summer bulbs is important before planting. Most summer flowering bulbs prefer full sun, meaning at least six hours of direct sunlight per day, though some, like tuberous begonias, prefer partial shade. Adequate drainage is required, as all bulbs are highly susceptible to rotting in soggy, waterlogged soil.
To ensure proper drainage, especially in areas with heavy clay soil, amend the planting site with organic matter. Incorporating well-rotted compost, leaf mold, or aged manure improves soil structure by creating more air pockets and allowing water to pass through more freely. Adding sharp sand or horticultural grit can further enhance the porosity of dense soil mixtures.
The optimal soil mixture should be well-drained but still capable of retaining some moisture and nutrients to support vigorous summer growth. Prepare the ground by digging it over thoroughly and removing any perennial weeds. This preparation creates a fertile and hospitable environment for new root development.
Proper Planting Techniques
Planting requires attention to depth, spacing, and orientation. A general guideline is to plant most bulbs at a depth of two to three times their height or diameter. For example, a bulb that measures two inches tall should rest approximately four to six inches below the soil surface.
Bulbs should typically be separated by a distance of at least twice the width of the bulb to allow for air circulation and growth. Proper orientation involves placing the bulb with the pointed end, which contains the developing shoot, facing upwards toward the sky. Some exceptions exist, such as Dahlias and Begonias, which are often planted just beneath the surface or with the crown slightly exposed.
When dealing with corms, such as Gladiolus, the flat end is the base where the roots emerge, and the pointed end should face up. For tubers, like Dahlias, the growth buds, or “eyes,” should be positioned facing upward or sideways. Planting at the correct depth is important because planting too shallowly can expose the bulb to damage, while planting too deeply may hinder its ability to flower.
Essential Care in the First Few Weeks
Immediately following planting, the first watering is a necessary step to establish the bulbs in their new location. A deep, thorough watering settles the soil firmly around the bulb and eliminates any potential air pockets that could dry out new root growth. This initial moisture is vital for stimulating the dormant bulb to break dormancy and begin root development.
While excessive moisture can lead to rot, a balanced approach to watering should be maintained in the early weeks. After the initial soak, subsequent watering should only occur when the top inch of soil feels dry to the touch, preventing the ground from becoming waterlogged. Applying a balanced, slow-release fertilizer or a dose of high-potassium feed at the time of planting can provide the necessary nutrients to support early shoot growth.
The tender nature of these bulbs means that a late, unexpected frost could still damage emerging shoots, so having a protective cover like horticultural fleece on hand is advisable. New shoots are particularly vulnerable to common pests, especially slugs and snails. Monitoring the area and applying protective measures against these garden pests is beneficial before the plants are fully established.