Defining terms: Bulbs aren’t always, well, bulbs

When you shop for garden bulbs, you may immediately notice some variations on the underground-storage theme. The minor but key differences are worth knowing because they affect not only what sort of plants the bulbs produce but also how to divide them to get more plants. Here’s a rundown of some of the plant structures that gardeners call bulbs:

• True bulbs: Bulbs are composed of concentric fleshy scales, or layers, which are actually modified leaves. Yes, just like an onion. At the base of a true bulb is a basal plate, the place from which the roots grow. Examples of true bulbs include allium (flowering onion), amaryllis, daffodil, hyacinth, and tulip. Interestingly, when you cut a bulb in half, you can see the future plant parts — stem, leaves, flowers. Offsets that you can pry off and plant may appear. In some cases, notably with lilies, you can grow new plants from individual scales.
• Corms: These structures resemble true bulbs somewhat, with a growing point on top and a basal plate at the bottom. However, they don’t have the clasping modified leaves; technically, corms are swollen underground stems. Also, they use up their store of food in one growing season — though new little cormels (baby corms) often appear atop the old one to carry on. Examples include autumn crocus, crocus, crocosmia, freesia, and gladiolus.
• Rhizomes: Rhizomes are thickened stems that grow horizontally; the roots grow down from the underside. The tip tends to have a primary growing point, though productive side-buds are common, and you can cut them off and plant the buds individually. Examples of rhizomes include agapanthus, bearded iris and many other irises, canna, and lily-of-the-valley.
• Tubers: Tubers are swollen stem bases. Roots grow from their sides as well as their bases, and you may see multiple productive buds. A classic example of a tuber is the potato. Other examples include most anemone, tuberous begonia, caladium, cyclamen, dahlia, and gloxinia. The way to divide tubers varies by the type — dahlias, cyclamen, and gloxinia involve different methods. Generally, you want to cut the tuber so that each piece contains a bud.
• Tuberous roots: These structures are in fact roots, not modified stems. Fibrous roots, like fingers, radiate outward from a central point. You can easily divide these roots to get more plants, making sure, of course, that each piece has at least one bud on it. Examples include alstromeria, clivia, eremurus, and ranunculus. Many gardeners discuss the differences between hardy bulbs and tender bulbs. Unfortunately, making the distinction isn’t easy, because a tender bulb in one climate (like Zone 4) may be a hardy one in Zone 7. Generally, hardy bulbs can survive wintering in the ground without too much trouble, whereas tender bulbs have to be dug up and stored. Your local nursery can help you determine which bulbs are considered hardy in your area.