When most gardeners talk about “bulbs,” we are generally referring to a group of plants known as geophytes. Simply put, geophytes are plants with underground storage organs. These underground organs contain food and energy for the plant’s life cycle. Geophytes tend to be herbaceous, which means that they go through a cycle of putting out leaves, blooming, dying back to the ground, and going dormant until the next growing season.
There are actually five types of geophytes:
- True bulbs
- tuberous roots
The different structures are important to gardeners because they affect how the bulb is planted, how it grows, and how it reproduces. Here’s what you need to know about the different types and how they grow.
A narcissus bulb has a flat basal plate, sprouting top, and papery outer covering.
True bulbs are rounded, with a flat bottom (called a “basal plate”) that produces roots, and a top that produces the stem. They grow vertically and must be planted right-side up. Bulbs have layers (think about onions), and if you slice a true bulb right down the middle, you’ll find an immature flower and leaves tucked in there, surrounded by layers of food.
Bulbs can be subclassified into tunicate (with a dry papery outer covering) and nontunicate (scaly). Nontunicate bulbs are more fragile when transplanting.
True bulbs reproduce by creating smaller bulbs (called “offsets” or “bulblets”) attached to the parent bulb. These can be separated and planted to produce more plants.
Examples of true bulbs include:
- Lily (lilium sp.)
Gladioli are examples of corms.
On the outside, corms look a lot like bulbs. The difference is that they are actually the fattened base of the stem itself. A corm doesn’t grow in layers or have an embryonic bud at the center – it’s purely a solid-textured food supply for the stem above. As the plant grows and blooms, it consumes the food in the corm, and the corm shrivels and dies. Meanwhile, it has produced new, baby corms (called “cormels”) next to it, which sprout next year and can be transplanted to propagate the plant.
Examples of corms include:
This Bearded Iris shows the horizontal rhizome with roots and growth nodes.
Rhizomes are actually fat underground stems that grow horizontally. They creep along just under the soil surface and sprout stems and leaves upward, and roots downward, all along the length of them. Unlike bulbs and corms, which are more like self-contained units, rhizomes have many growing points and can be propagated by cutting them into sections.
Examples of rhizomes include:
- Calla Lily
- Bearded Iris
Each of the “eyes” on a potato are capable of growing roots and leaves.
The word “tuber” is sometimes used as a catch-all for any plants that don’t fit into the other categories. True tubers are swollen underground stems that have eyes (think about potatoes) where plants and roots will sprout. They don’t have a basal plate – instead, roots grow all along the bottom and sides, with plant growing points along the top surface. They also may be propagated by cutting them into pieces.
Examples of tubers include:
- Water Lily
Dahlias are examples of tuberous roots, usually growing in clumps.
Tuberous roots look very similar to tubers but are actually swollen roots, not stems. They don’t have eyes. Instead, tuberous roots often sprout at one end (called the “crown”) and grow in clumps, so a large flowering plant may have multiple tuberous roots radiating out from a sprouting “crown.” The plant sprouts from the base of the old stem, not from the root. Propagating tuberous roots is trickier, because many of them will only sprout if the divided section contains adequate crown tissue.
Examples of tuberous roots include:
- Sweet Potato
- Anemone (some types)
- Tuberous Begonia
Botanically speaking, geophytes can be sub-classified in great detail. For example, tuberous begonias are actually subclassified as an “enlarged hypocotyl,” because the enlarged part technically isn’t a root, it’s the area just above the root. The location of growth nodes and other details of the plant anatomy allow for near-endless categorizing and become more important if you get into the business of plant cultivation and development.
For gardeners interested more in growing and enjoying these plants, the major differences between the five main types—bulb, corm, rhizome, tuber, and tuberous root—are the most important.
Tuberous begonias are another colorful example of an “enlarged hypocotyl.”