By Alison Collin, Inyo-Mono Master Gardener Volunteer
Perennial plants often experience dormant periods during their life cycles. They generally have stems that support buds, which will develop into stems and leaves, and roots that grow and multiply when conditions are favorable. However, some plants have adapted to produce storage systems that enable them to survive excessive heat or drought, or severe cold and damaging frost. Stored food allows these plants to grow rapidly when conditions improve.
The timing of dormancy varies by plant. For example, daffodils are dormant during winter so they can grow quickly at the end of that season, while amaryllis are dormant during hot, dry summer months.
Bulbs, rhizomes, corms, and tubers are all types of storage systems. They differ in structure, and understanding these differences helps gardeners know which should be planted deeply and which should be planted at or just below the soil surface.
Bulbs
Bulbs are swollen, underground stems with a basal plate of compressed tissue. Fleshy scales (modified leaves) and a central flower bud are attached at the top, while roots grow from the bottom. Onions are a familiar example, where the layered leaf scales and root attachments are easy to see.
Daffodils, tulips, lilies, and amaryllis are also bulbs. They are usually perennial and produce new bulbs along the sides of the base plate, eventually forming clumps. These young bulbs can be separated and grown on.
Corms
Corms are solid, swollen stems. Unlike bulbs, their leaf scales are reduced to a dry, leaf-like covering. New corms are produced on top of the old one, which then dies. Small cormels form around the sides of the parent corm.
Gladiolus and crocus are examples of corms. In some species, corms can grow quite large, such as garden cyclamen (Cyclamen coum and Cyclamen neapolitanum), which can reach several inches across.
Rhizomes
Rhizomes are swollen, fleshy, underground stems that are often mistakenly called “roots.” Like all stems, they have true roots attached to the underside for water and nutrient uptake, and buds (or “eyes”) on the upper surface that produce new growth.
In rhizomes, the food-storage stem is elongated and grows horizontally, often on or near the soil surface. Examples include ginger and large-flowering bearded iris.
Tubers
True tubers are swollen underground stems with growth buds on their surface and roots growing from the bottom. Potatoes are the most familiar example. Tubers can be cut into pieces, and as long as each piece contains at least one growth bud (eye), it can produce a new plant. Jerusalem artichokes are also tubers.
There is an important distinction between true tubers and plants with tuberous roots. Dahlias are an example of plants with tuberous roots. Although their roots resemble potatoes, they do not have growth buds on the root surface and will not grow if cut into pieces. In these plants, growth buds are located at the base of the stem above the roots.
The links in the text provide illustrations of these parts if you're not familiar with the plants discussed.