Bulbs 101

wine red daylilies in sidewalk planter in Monrovia, CABulbs are actually several kinds of plants with specialized roots or stem bases that store nutrients and energy for the plants growth.

There are bulbs for most seasons. Many of the perennial plants we admire most are bulbs or related plants. When most of us think about bulbs we are planting them for the flowers that they produce. Most bulbs need to be planted a season before they bloom, so the most popular time for planting bulbs is in fall and winter so we can enjoy their blooms in spring.

Different bulbs are planted at different depths. Check the planting instructions for those you have chosen and use a bulb Fertilizer at the right stages of growth to maximize bloom. During the bulbs early growth use a high nitrogen fertilizer. A complete fertilizer (ie. 10-10-10) is best when flowers have set and at end of the growth period. This will also help the bulb prepare for next year. The bulb will refresh & manufacture its nutrients best if the leaves are left growing on the plant after the bloom is finished. Some bulbs like Tulips need to be put in a freezer before planting to simulate their natural conditions.

Bulb Types

A bulb by definition is “a modified underground stem, such as that of the onion or tulip, surrounded by scale-like modified leaves and containing stored food for the undeveloped shoots of the new plant enclosed within.” Another definition is “an underground stem or root such as a corm, rhizome, or tuber.”

Here’s how bulbs differ so you can tell them apart:

True bulb diagram of an onionTrue Bulb

This is a modified underground leaf bud surrounded by fleshy leaves and with a small stem at the lower end. Picture an onion. We eat the fleshy leaves, but that base with the stringy, dried roots that we cut off, that’s the small stem, compacted. Other bulbs in this category are tulips and daffodils. Most true bulbs produce offsets. They can be divided and separated from the mother bulb.


Corm Bulbs split in halfIs a swollen underground stem base. These resemble bulbs but are different because they are made up of all stem cells. Visually, you might not be able to distinguish this difference. They have the same papery leaves coving the outside, and roots at the base. Also, like a bulb, they are a food storage vessel for the plant.  After bloom the old corm is used up, but new ones have grown on top or at the sides to take its place.  Gladioli, sparaxis, and freesia grow from corms.

two Calla Lily flowers Rhizome stemmedRhizome

Rhizomes are horizontal stems that grow below the ground near the surface. They look like roots, but actually have scale-like leaves and leaf buds at nodes. Examples are bearded irises and calla lilies. Other examples of rhizome growers are some grasses and ferns.


Tuberous rooted Cyclamen BulbsA thickened stem that runs underground. At the ends of the stolons grows a tuber or a storage chamber that is short, thick, and round. It grows totally or partially underground. Tuberous begonias, cyclamen, and potatoes grow from tubers.

Sweet potatoes pulled out of groundTuberous Root

Growing underground, this differs from a tuber in that it’s a swollen root rather than a thickened stem. Tuberous roots grow in clusters and have growth buds on top in the old stem portion, from which spring the plants. Dahlias and sweet potatoes grow from tuberous roots.