All About Flower Bulbs

When tulips, daffodils and lilies burst into bloom, you’re probably not thinking much about the part of the plant that’s underground: the bulb. Flower bulbs are actually a type of food storage organ, a way that plants stash their homemade nosh to help fuel future growth and flowers. Many plants get lumped under the heading bulbs, including tubers, corms and rhizomes. Knowing a little about different types of bulbs can help you understand how these plants grow —and how you should handle them at planting time.

If you cook at all, you’re probably familiar with the internal structures of an onion. Guess what? Onions are what’s known as true bulbs, tulips are also true bulbs. A true bulb has layers of fleshy tissue that act as the food storage organ. Roots form at the base of the bulb and serve to anchor the bulb in soil and absorb water and nutrients. When you buy bulbs, you’ll often see dried root remnants at the base of the bulb. Examples of true bulbs: onion, garlic, allium, daffodil, tulip, amaryllis, grape hyacinth, Dutch hyacinth, Dutch iris, scilla, lily.

Inside a true bulb is a central shoot that contains layers of leaves and immature flowers. With bulbs planted in your garden, this central shoot forms after flowers fade. This is why it’s important to let leaves of bulbs like tulips and daffodils remain and stay green until they naturally die back. As long as leaves are green, they’re helping to store food that helps form the shoot for next year’s show. Most true bulbs have a protective papery skin (think onion, daffodil, tulip). An exception to this rule are the lilies, including Asiatic and Oriental types.

Lily bulbs lack an outer protective, papery layer. They’re often sold packed in sawdust or peat moss. It’s important to store lily bulbs correctly prior to planting because they have no outer layer that helps protect against moisture loss or temperature fluctuations. Keep lily bulbs cool (below 45 degrees F) but not freezing to help prevent sprouting. If sprouts form prior to planting, handle bulbs carefully. If you break the sprout, the lily won’t flower that year.

A gladiolus grows from a type of bulb known as a corm. A corm is a solid mass of stem tissue — not layers of tissue like a true bulb. They’re usually round and sort of flat. Both buds and roots grow from the base of the corm, which is the part that stores food for the plant. Examples of corms: gladiolus, crocus, freesia, acidanthera (African gladiolus), crocosmia.

Corms typically have an outer papery protective layer. Even so, it’s best to keep them cool in storage (40 to 55 degrees). If shoots appear on corms prior to planting, handle them gingerly. If you break the shoot, you risk losing that year’s flowers. Once a corm is planted, as the plant grows it actually devours all the stored food in the corm, which shrivels and dies. As the plant continues to grow, it forms a new corm on top of the old one.

The canna is a popular tropical plant that’s not hardy where the soil freezes solid. It grows from a type of bulb known as a rhizome, which is located at or just beneath the soil surface. A rhizome is basically an enlarged or fat stem that stores starches and other easy-to-use plant foods.

Typically a rhizome grows horizontally in soil, either just below the soil surface or sometimes deeper underground. Like an above-ground stem, a rhizome has nodes, spots where buds reside. These buds can grow leaves and/or roots. Because of this trait, it only takes one piece of a rhizome to grow an entire plant. This canna rhizome has five nodes or places where sprouts are emerging. Dried root remnants are also visible.

A calla lily is another popular plant that grows from a rhizome. Other examples of rhizomes: canna, bearded iris, ginger, bamboo, lily of the valley. When growing rhizomes that aren’t hardy in your zone, dig and store them over winter. Wait for frost to kill (or at least damage) leaves. Dig up rhizomes and cut off leaves. Let a 1- to 2-inch stem stub remain. Cure the rhizomes in a warm, dry place for several days — until cut surfaces are dry.

The trick with storing rhizomes over winter is not letting them dry out. To store rhizomes, place them in barely damp peat moss and keep them at 50 degrees F for callas; 60 to 70 degrees Fahrenheit for cannas. Check the rhizomes one to two times over winter to make sure they aren’t rotting (too wet) or shriveling (too dry). In spring, many rhizomes in storage start to sprout, like these calla lily rhizomes. Take care not to break these sprouts prior to or during planting, or you'll diminish the flower show.

Another type of bulb is known as a tuber. There are two types of tubers: root tuber and stem tuber. A dahlia grows from a root tuber. Examples of root tubers include dahlia, peony, tuberous begonia and ranunculus. A potato is probably the most famous stem tuber. Examples of stem tubers include potato, caladium, cyclamen and anemone.

A tuber lacks a papery protective covering and has buds on the surface known as eyes. With root tubers like dahlias, the eyes or buds are at the stem base, where it joins the root tuber. Stems and flowers develop from these eyes. As long as you have a tuber with an eye, you can grow an entire plant. The eyes on these dahlia tubers have sprouted to form tiny leaves.