The term “bulb” is commonly used in gardening to describe any plant that stores its entire life cycle in a fleshy, underground structure called a geophyte. This storage organ, which can be a true bulb, corm, tuber, or rhizome, allows the plant to survive harsh conditions by holding nutrients and water. Fall-planted geophytes like tulips require a winter chilling period to bloom in spring. Spring-planted geophytes are tender, subtropical varieties that will bloom throughout the summer and into the autumn. These tender plants cannot survive a freeze, making the warmer spring months the only time they can be safely planted to ensure vibrant summer and fall color.
Categorizing Spring-Planted Bulbs
Spring-planted geophytes include a diverse collection of specialized storage organs, most of which are categorized as tender and must be lifted before winter in cold climates. True bulbs, such as Lilies, are generally hardy and are an exception, often planted in spring for summer flowering. The majority of spring-planted varieties, however, fall into the categories of corms, tubers, or rhizomes.
Gladiolus grow from a corm, which is an enlarged, solid stem base that stores food. They are known for their tall, sword-like foliage and dramatic flower spikes that bloom through the summer in full sun. Dahlias develop from tuberous roots, which are thick, fleshy structures attached to the main stem. They provide prolific blooms from mid-summer until the first hard frost and require full sun exposure and well-draining soil to thrive.
Tuberous Begonias grow from a flattened, saucer-shaped tuber. They are unique because they perform best in partial shade, providing vivid color in areas where other flowers struggle. They offer a long blooming season and are often utilized in containers and hanging baskets. Canna Lilies grow from rhizomes, which are horizontal underground stems, producing tropical foliage and bright, dramatic flowers in full sun starting in mid-summer.
Determining the Right Planting Time
The timing for planting these geophytes hinges entirely on avoiding cold soil and frost, as they will rot if chilled or waterlogged. Gardeners must wait until all danger of the last expected spring frost has passed for their specific region. This timing ensures the emerging foliage will not be killed back by a late cold snap.
The soil temperature is the most accurate indicator for planting safety. The ground must reliably warm to a minimum of 55 to 60 degrees Fahrenheit before planting can begin. Planting earlier than this threshold risks the geophyte rotting in the cool, damp soil before it can sprout. For gardeners eager to get a head start, certain varieties like Dahlias and Begonias can be started indoors in containers several weeks before the outdoor planting date, then transplanted once conditions are safe.
Step-by-Step Planting Guide
The planting site requires the soil to be loosened to a depth of at least 10 to 12 inches and amended with compost to ensure sharp drainage. Excellent drainage is paramount to success, as most spring-planted geophytes will quickly rot in soggy conditions. The general rule for planting depth is to bury the geophyte two to three times its own height, but specific types have important exceptions.
Gladiolus corms should be planted 3 to 6 inches deep, with the pointed end facing upward. Dahlia tubers should be planted much shallower, just deep enough so the crown—where the tuber meets the stem—is right at or slightly below the soil line. Tuberous Begonias are typically placed with the concave or indented side, where the sprouts emerge, facing up and barely covered with a half-inch layer of soil. After planting, the area should be watered thoroughly to settle the soil, but avoid overwatering, especially before growth is visible.
Seasonal Care and Overwintering
Maintaining these summer bloomers involves consistent attention to watering, staking, and feeding throughout the growing season. Dahlias and Gladiolus often require staking to prevent the stems from snapping in wind or rain due to their height and heavy blooms. Pinching back the main stem of Dahlias early in the season encourages a bushier plant structure and increases the number of blooms.
Watering needs increase once the plants are actively growing and blooming, though the soil should always be allowed to dry slightly between waterings. Applying a balanced, low-nitrogen fertilizer, such as a 5-10-10 formulation, monthly can support the heavy flowering production of Dahlias and Canna Lilies.
Overwintering Tender Geophytes
The tender nature of these plants means they must be lifted and stored before the ground freezes in regions colder than USDA Zone 8. The overwintering process begins after the first light frost has blackened the foliage, signaling the end of the growing season.
The plants (including Dahlias, Begonias, Gladiolus, and Canna) should be carefully dug up, and the soil gently brushed off. They must then be cured—dried in a cool, dark, well-ventilated area. The curing period ranges from a few days for Dahlias to a few weeks for Gladiolus, allowing the outer layers to harden. Finally, the geophytes are stored in a cool, dark, and dry location, ideally between 40 and 50 degrees Fahrenheit. They should be nestled in a medium like peat moss or vermiculite to prevent shriveling or rotting until they can be replanted the following spring.