The summer garden achieves sustained color through the strategic use of underground storage organs, commonly referred to as bulbs. These structures act as pre-packaged energy sources, allowing plants to produce vibrant blooms from mid-summer until the first autumn frost. Understanding the different types and their needs allows gardeners to create continuous flowering displays that form the backbone of a successful late-season landscape.
Understanding the Difference Between True Bulbs, Corms, and Tubers
The term “bulb” is often used broadly to describe any plant that grows from a fleshy, subterranean storage organ, known botanically as a geophyte. True bulbs, such as lilies, are characterized by a layered structure of fleshy, modified leaves surrounding a central embryonic flower bud. Corms, like those of gladiolus, are solid, swollen stem bases that store nutrients, often covered by a thin, papery tunic.
Tubers and rhizomes are further distinctions, representing thickened storage stems or roots without the layered internal structure of a true bulb. Dahlias and tuberous begonias grow from tubers, which feature ‘eyes’ or buds from which new growth emerges. Canna lilies grow from rhizomes, which are horizontal underground stems that spread and send up shoots. All these structures store the starch and nutrients needed to fuel the plant’s summer display.
Essential Summer-Blooming Varieties
Tall Background Plants
Canna lilies are excellent choices for adding height and a tropical feel to the back of a border, with some varieties reaching over six feet tall. Growing from rhizomes, they require full sun and consistently rich, moist soil to support their lush foliage and continuous spikes of red, orange, or yellow flowers. Gladiolus, growing from corms, provide dramatic vertical interest with stately flower spikes, blooming in mid-summer. They are best planted in succession every few weeks for an extended flowering period.
Lilies, including Asiatic and Oriental varieties, are true bulbs that offer a range of bloom times throughout the summer. Asiatic lilies are the earliest to flower, producing non-fragrant, upward-facing blooms in early summer. Oriental lilies flower later and are prized for their intense fragrance and large, classic blossoms. Both types thrive in full sun to light shade and prefer rich, well-drained soil, returning reliably year after year.
Mid-Border Fillers and Container Favorites
Dahlias, which grow from tubers, are the most versatile summer bloomers, offering a diversity of flower shapes, sizes, and colors. Shorter, bedding varieties work well as mid-border fillers, blooming profusely from mid-summer until frost, provided they receive full sun and well-drained soil. Their extensive bloom period makes them favorites for cutting gardens and for sustaining color late in the season.
Tuberous begonias are popular for delivering brilliant color in shaded areas where many sun-loving flowers struggle. These tubers produce large, camellia-like blooms and are ideal for hanging baskets and containers, preferring filtered light and rich, organic soil. Caladiums are also grown from tubers but are sought after for their dramatic, colorful foliage rather than their flowers, providing texture and color in deep shade conditions.
Timing and Technique for Spring Planting
Summer-flowering geophytes are tender, meaning they are intolerant of freezing temperatures and must be planted in spring after the danger of the last frost has passed. The general rule is to wait until the soil temperature consistently reaches 55°F to 60°F, which typically occurs a few weeks after the final expected frost date. Planting too early risks the storage organs rotting in cold, wet soil.
Soil preparation is crucial, as these structures require excellent drainage to prevent rot. Amending the planting area with compost or other organic matter improves both drainage and nutrient content. A common planting guideline is to set the bulb, corm, or tuber at a depth approximately two to three times its vertical height, ensuring the growing point or ‘eye’ is oriented upward. For a head start, slow-starting varieties like tuberous begonias can be started indoors in trays of moist medium six to eight weeks before the outdoor planting date.
End-of-Season Care and Winter Storage
In colder climates, most summer-flowering geophytes must be dug up and stored indoors for the winter to protect them from freezing. The signal to begin this process is typically the first light frost, which causes the foliage of plants like dahlias and cannas to yellow or blacken. This dieback cues the plant to transfer its remaining energy into the underground storage organ.
After the foliage is cut back, the storage organs should be carefully lifted from the soil and allowed to dry or “cure” in a warm, dark, and well-ventilated area for several days. This curing process toughens the outer skin, reducing the risk of rot during dormancy. Once cured, they must be stored in a cool, dark location where temperatures remain consistently above freezing, ideally between 40°F and 50°F. Storing them in a breathable medium like peat moss, wood shavings, or vermiculite helps maintain a stable moisture level, preventing the organs from shriveling until they are ready to be replanted the following spring.