There are four types of bulb plants, according to the Gardener's Network: true bulbs, corms, tuberous roots and rhizomes. All are geophytes, whose growth and reproduction point is underground.
True bulbs include grape hyacinth, hyacinth, allium, amaryllis and tulip. Corms include snow crocus, Elephant's Ear, freesia and gladiolus. Tuberous begonia, caladium and lily of the valley are rhizomes, which have root-like underground stems instead of rounded bulbs. Dahlias and tiger lilies are tuberous roots.
All bulb plants must endure freezing temperatures for approximately three months before they begin a new growth spurt. According to Sharon S. Bale, Extension Floriculture Specialist at the University of Kentucky, Cooperative Extension, iris need nearly 20 weeks of cold while true hyacinth need only 11. Grape hyacinth and many other bulb plants need 14 to 15 weeks of cold to flower.
The new growth cycle begins when the bulb pushes spikes of green through the soil. These spikes become spiked leaves. The bulb stores energy and grows larger as temperatures rise.
Optimum bulb diameter varies from nearly four inches for amaryllis to a half inch for grape hyacinth. Golf-ball size gladiolus dwarf the marble-sized crocus.
Tiger lilies, one of the rhizomes, can form a bundle four feet or more in diameter and 18 inches deep, not counting feeder root length, if not regularly divided.
Once they reach optimum diameter, true bulbs, rhizomes and tuberous roots will form a new bulblet on the side. Corms form from the top.
Dividing Bulb Plants
Bulb plant beds require little care once established, but they must be divided periodically or they will stop flowering or forming new bulbs. Do this in late spring or early summer, after all blooms are gone.
Divide the bed by carefully digging under the soil three or four inches from the nearest visible bulb top or flower stem with your fingers or a spoon. Work slowly to avoid cutting into any bulbs. Feel for the main bulb, corm, rhizome or tuberous root bundle. Dig around and underneath it two to six inches deep. Carefully separate the main bulb and side bulbs (bulblets) once out of the soil.
Wrap bulbs without rootlets in a paper towel. Keep them damp, in a cool but not cold place, until new rootlets form. Rootlets serve as feeders and as anchors in the soil. Without rootlets, the bulbs, corms, tuberous roots and rhizomes may not bloom when transplanted.
Store them all in a cool, dry place or they may rot. Plant in late fall, water well and wait until spring. If you live in a warm climate, place your bulbs in the refrigerator for 12 weeks before planting.
Lop off flower heads as soon as they open, forcing the plant to spend its energy on growth of the bulb, corm, tuberous root or rhizome and formation of new ones. This practice is known as dead-heading.
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