Summer-flowering bulbs, which include a variety of structures like true bulbs, tubers, corms, and rhizomes, are generally tender and originate from warmer climates. Unlike spring bulbs that require a cold winter period, summer bulbs cannot withstand freezing temperatures and must be planted in the spring for a vibrant summer display. Getting the timing right ensures the plant emerges into favorable conditions, maximizing its growth period and the potential for abundant blooms.
Identifying Summer Bulbs
The term “summer bulb” is a broad category encompassing several different types of underground storage organs that share an intolerance for frost. These structures serve as food and water reserves, allowing the plant to survive dormancy and push out vigorous growth when conditions are right. True bulbs, such as lilies, have layers of fleshy scales, while corms, like those of the gladiolus, are solid, swollen stems. Tubers (Dahlias and Begonias) are thickened underground stems, and rhizomes (Cannas and Calla Lilies) are root-like stems that grow horizontally beneath the soil surface. These structures are typically non-hardy, meaning they must be lifted and stored indoors for the winter in colder regions to prevent freezing and rotting.
Determining the Optimal Outdoor Planting Window
The window for planting summer bulbs outdoors is directly governed by two specific environmental conditions: the local average last frost date and the soil temperature. Planting should only occur after the last expected spring frost to ensure the tender plant tissue is not damaged by a late cold snap. You can find this date by checking your specific Hardiness Zone data.
Beyond air temperature, the soil must be sufficiently warm to trigger root growth and prevent the bulb from sitting dormant and rotting. The ideal soil temperature for most summer bulbs to begin active growth is consistently between 55°F and 60°F. Planting in cold, wet soil, even if the air is warm, can lead to the bulb becoming susceptible to fungal diseases and rot.
A general guideline is to wait approximately one to two weeks after the average last frost date before planting directly into the garden soil. This delay allows the sun and warmer air to penetrate and raise the temperature of the soil to the necessary range. Monitoring the actual soil temperature with a thermometer is a more reliable method than relying on the calendar alone.
Starting Bulbs Indoors
Gardeners who wish to achieve earlier blooms or who live in regions with short summers can employ the technique of starting summer bulbs indoors, often called “pre-sprouting.” This method allows the plant to bypass the waiting period for naturally warm outdoor soil, effectively extending the growing season. The process typically involves potting the dormant structures into small containers four to six weeks before the anticipated outdoor planting date.
Specific bulbs, such as Dahlias and Begonias, respond well to this indoor head start. Place them in a well-draining potting mix with the growing point facing upward in a warm location, ideally with bottom heat and bright light. Providing warmth (above 60°F) encourages rapid root and shoot formation. Once the threat of frost has passed and the soil outdoors has warmed, the sprouted plants can be transplanted into the garden, reducing the time until they produce flowers.
Essential Planting Depth and Immediate Care
Once the outdoor timing is correct, the physical act of planting requires attention to depth and initial watering to ensure successful establishment. A general rule for planting depth is to bury the bulb, tuber, or corm at a depth of two to three times its height. For example, a two-inch corm should be planted four to six inches deep, which provides stability and protection.
Proper spacing is also important, ensuring that the mature plants will not overcrowd one another and compete for light and nutrients. After placing the bulb in the hole with the growing point or “nose” facing upward, cover it with soil, and water thoroughly to settle the earth and eliminate air pockets. Following this initial soaking, watering should be minimal until the first shoots break the soil surface. Overwatering in the early stage is a leading cause of rot in newly planted summer bulbs.