By Brent McGhie, Butte County Master Gardener, February 5, 2016.
Not all of these plants are technically bulbs. Both gardeners and plant companies alike tend to group true bulbs along with corms, rhizomes and tubers under the general heading of “bulbs.” A true bulb consists of layers of fleshy leaves, like an onion. Amaryllis, Chalochortus and lilies are true bulbs.
Corms are shaped like bulbs, but are solid (not layered) inside. Crocosmia, gladiolus and Liatris grow from corms.
A tuber is actually a fleshy underground stem and its “eyes” are buds from which new plants can develop. A potato is the most familiar example of a tuber. Tuberous begonias grow from tubers (other begonias form rhizomes) and dahlias develop from tuber-like roots. Rhizomes are actually underground stems. Cannas, callas and tuberose are summer-blooming plants that produce rhizomes.
Bulbs need well-drained soil and will often rot if planted in soggy soil. When planting a bulb, loosen the soil past root depth and then amend it with compost or other organic material before placing the bulb in its hole. Commercial fertilizers formulated specifically for bulbs are also available and are normally worked into the soil beneath the bulb at planting time. There may be some individual variation, but generally most true bulbs, corms and tubers should be planted about three times as deep as they are wide. Rhizomes should be planted just under the soil surface, while tuberous roots (e.g. dahlias) should be covered by three or four inches of soil.
Like most plants, bulbs benefit from regular watering during the growing season. Once they have flowered, the spent blooms can be deadheaded, but don't cut back any green leaves. The plants need these leaves to photosynthesize and to replenish the food lost by the bulb during growth and flowering. Once the leaves have browned off, they can be removed. Over time, most bulbs will multiply and become naturalized. To avoid overcrowding and encourage plentiful blooms, it is a good idea to divide the clumps of bulbs every two or three years.
It's too late this year to plant other summer-blooming bulbs, but if you want to plan for next year, Allium, Anemone and Watsonia can be planted this fall. There are also several native bulbs that should be planted in the fall. Species of Brodiaea, Sisyrinchium (blue-eyed grass) and Triteleia (Ithuriel's spear) all bloom from late spring to early summer. Soaproot (Chlorogalum) blooms appear from May to August. If you do buy native bulbs, be certain you buy from reputable dealers who propagate their bulbs rather than collecting them from the wild. This will help ensure these beautiful wildflowers will be available for others to see for generations to come.