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Understanding Bulbs
and Bulblike Plants
A Closer Look at Bulbs
The main difference between bulbs and other growing plants is their specialized roots.
Bulbous plants—true bulbs, corms, rhizomes, tubers, and tuberous roots—store water and nutrients in swollen structures partially buried or beneath the soil. These unique root systems help them survive periods of drought.
Trees, shrubs, grasses, and flowers, by contrast, generally have permanent roots that extend down into the soil from a trunk or stalk. Whenever they face heat and drought, they wilt or require extra care to sustain them, while bulbs go dormant and wait for conditions to improve.
Let’s look at the different types of bulbs and how to identify them by shape.
There are five broad horticultural categories of bulbous plants: true bulbs, corms, rhizomes, tubers, and tuberous roots.
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- True Bulb—True bulbs, such as this tulip, have a platelike base of modified stem tissue with attached, scalelike storage leaves and a central growth bud. They are often complete plants, with an embryonic bloom, stem, and leaves. Bulbs have concentric rings when they are cut parallel with their base plate. They regenerate fully each year, often forming small offsets called “bulblets” along their basal plate. They sometimes form seedlike “bulbils” to either the site of their former flowers or, for some lilies, at their axils—where their stems and leaves join.
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- Corm—Corms, such as this crocus, have a swollen, underground stem atop a platelike base with one or more growth buds, usually bearing a flaky, dry, skinlike covering that preserves moisture. Corms appear solid, lacking visible structures inside. They are depleted and wither as the plants grow, and a new corm forms atop the old. Offspring—called “cormels”—form around the base of the new corm, taking 2–4 years to flower if detached and raised separately.
- Rhizome—Rhizomes, such as this windflower, have a modified and thickened stem that lies flat on the soil surface, are partially buried beneath, or live completely underground. They bear scalelike leaves with latent buds at each axil. Rhizomes branch outward to form new plants, each with independent roots and growth buds, as the parent root withers and dries.
- Tuber—Depending on species, tubers like the tuberous begonia shown are either modified stems or roots. They lack internal structures, and have tiny surface scales bearing “eyes” (growth buds) along their surface. Tubers usually appear solid when cut. Like corms, many species of tuber shrivel as the plants grow and the nutrients stored within the tuber are consumed; simultaneously, they multiply by division to form new tubers. Others renew and enlarge their tuber each season.
- Tuberous Root—Bulbous plants with tuberous roots, such as this Persian buttercup, have engorged, fibrous, modified roots surround a crown—the junction of the stem with the root—and bear growth buds. True roots emerge from the tuberous root, and stems, leaves, and flowers sprout from the crown. Over time, multiple crowns emerge from the expanding mass of tuberous roots, creating offspring plants.
- Tropical bulbs—Tropical bulbs, such as this florist’s amaryllis, frequently are evergreen foliage plants that flower seasonally, but rarely rely on their unique ability to go dormant. Many tropicals grown as foliage plants or for their showy flowers are either true bulbs or are part of one of the other categories of bulbous plants.
Life Cycle of Bulbs
Most true bulbs and bulbous plants spend part to most of the year inactive, in a dormantlike state. This resting period begins at the onset of dry weather. Their foliage yellows, withers, then dies. After a month or two, the bulb rests in the soil, awaiting rain to trigger new root development and sprouting of foliage and flower spikes.
For evergreen bulbs, dormancy is merely a period of slowed growth. Some tropicals provide year-round, lush-green foliage.
Here’s a typical bulb life cycle.
The onset of rain—or irrigation—causes a stirring within the bulb. As soil moisture penetrates the plant’s tissue, it begins to put down roots.
In cold-winter areas, freezing may interrupt this root growth, which begins again as temperatures moderate. Then, when weather conditions are ideal, a foliage shoot emerges from the bulb, breaking the soil.
In a remarkably short time—sometimes only a few weeks—leaves, stems, buds, and blooms appear. This growth spurt is fueled by the nutrients stored within the bulb. As the bulb is depleted, it either withers entirely, as in the case of corms and some tubers, or shrinks in size. Simultaneously, new offsets are formed, and seeds or bulbils form.
Following bloom, foliage growth continues and supplies nutrients stored for the next year’s growth cycle. Offsets swell in size, and they become independent from their parent bulbs. When the cycle is complete, the bulb rests until new rains fall.
Your Bulb Planting Region
United States Bulb Planting Regions
How to Use The Bulb Planting Regions Map
Find your region and click on Read More (below) to find your U.S.D.A. Plant Hardiness Zone and the best time to plant spring-blooming bulbs in your home’s landscape.
A. New England—USDA Plant Hardiness Zones 3, 4, 5. Optimum planting time: September 15–November 15.
B. Mid-Atlantic Coast—USDA Plant Hardiness Zones 6, 7, 8. Optimum planting time: October 1–November 30.
C. Appalachia—USDA Plant Hardiness Zones 5, 6. Optimum planting time: September 15–November 30.
D. South Atlantic Coast—USDA Plant Hardiness Zones 7, 8. Optimum planting time: October 15–December 15.
E. Great Lakes—USDA Plant Hardiness Zones 5, 6, 7. Optimum planting time: September 15–November 15.
F. East Central—USDA Plant Hardiness Zones 5, 6, 7. Optimum planting time: October 1–November 30.
G. South Central—USDA Plant Hardiness Zones 7, 8. Optimum planting time: October 15–December 15.
H. Tropical—USDA Zone 10. Optimum planting time: December 1–January 15.
I. Gulf Coast—USDA Plant Hardiness Zones 8, 9. Optimum planting time: October 1–December 31.
J. North Central—USDA Plant Hardiness Zones 3, 4, 5. Optimum planting time: September 15–November 1.
K. Central Great Plains—USDA Plant Hardiness Zones 5, 6. Optimum planting time: September 30–November 30.
L. Rocky Mountains—USDA Plant Hardiness Zones 2, 3, 4. Optimum planting time: September 15–November 1.
M. Arid West—USDA Plant Hardiness Zones 5, 6, 7. Optimum planting time: September 30–November 30.
N. North Pacific Coast—USDA Zone 8. Optimum planting time: October 1–November 30.
O. South Pacific Coast—USDA Plant Hardiness Zones 9, 10. Optimum planting time: August 15–December 15 (baboon flower, flame freesia, freesia, bearded iris, corn lily, ornamental onion, squill, wandflower, wood sorrel); August 15–January 31 (bluebell, Persian buttercup (ranunculus), daffodil, grape hyacinth, Dutch iris, lily, windflower).
P. Alaska—USDA Plant Hardiness Zones 1–7. Optimum planting time: September 15–October 15.
Bulb Planting Regions map courtesy of Netherlands Flower Bulb Information Center.
U.S.D.A. Plant Hardiness Around the World
The United States Department of Agriculture (U.S.D.A.) divides North America and other areas of the world into 11 plant hardiness zones [see: Plant Hardiness and Climate].
The U.S.D.A. Hardiness Zones, based on long-term extreme minimum annual temperatures, serve as a general guide to growing conditions.
Many plants, including bulbs, are rated as to the zones for which they are best adapted—for example, zones 4–7—and these are usually listed on plant labels and in plant encyclopedias [See: Bulbs].