Bulbs 101
“Bulbs” is a catch-all term for several kinds of plants with specialized underground structures that store food and energy for growth. In other words: bulbs (and bulb-like plants) are nature’s built-in “battery pack.”
There are bulbs for most seasons, and many of our favorite perennials fall into this category. Because we usually plant bulbs for their flowers, it helps to remember one key timing rule: most bulbs are planted a season before they bloom. That’s why fall and winter are the most popular planting windows for spring color.
Always follow depth instructions on the label, and feed at the right time. Early growth benefits from higher nitrogen. Once flower buds set and toward the end of the growth cycle, a balanced fertilizer (e.g., 10-10-10) helps support bloom and replenish the bulb for next year.
After flowering, let leaves remain as long as they’re green—this is how the bulb “recharges” for the next season. Some bulbs (like tulips) also need a period of cold before planting to mimic natural winter conditions (many gardeners refrigerate them—never freeze).
Bulb Types
Botanically, a bulb is a modified underground stem surrounded by fleshy, scale-like leaves that store food for the developing shoots inside. In everyday gardening, people also lump in other storage structures—like corms, rhizomes, tubers, and tuberous roots—because they’re planted and cared for in similar ways.
Here’s how these “bulb types” differ so you can tell them apart:
True Bulb
A true bulb is an underground leaf bud surrounded by fleshy “scales,” with a small stem plate at the bottom. The easiest example is an onion: the layers we eat are the fleshy leaves, and the flattened base (where roots attach) is the stem plate.
Tulips and daffodils are also true bulbs. Most produce “offsets” (baby bulbs) that can be divided from the mother bulb.
Corm
A corm is a swollen underground stem base. It can look like a bulb from the outside (often with a papery covering and roots at the base), but it’s solid inside—made up of stem tissue rather than layered scales.
After bloom, the original corm is often “used up,” and new corms form on top or alongside it. Gladiolus, sparaxis, and freesia grow from corms.
Rhizome
Rhizomes are horizontal stems that grow just below the soil surface. They may resemble roots, but they have nodes—tiny growth points where leaves and shoots emerge.
Bearded iris and calla lily are classic rhizome growers. Many grasses and ferns also spread by rhizomes.
Tuber
A tuber is a thickened underground stem that stores energy. Tubers form on or at the ends of underground stems and can grow fully or partially below ground.
Tuberous begonias, cyclamen, and potatoes grow from tubers.
Tuberous Root
Tuberous roots are swollen roots (not stems). They typically grow in clusters, and new shoots form from buds located near the old stem area at the top.
Dahlias and sweet potatoes grow from tuberous roots.