Most bulbs propagate by multiplication, producing small bulblets beside the mother bulb. Some do this more prolifically than others. Tulip bulbs (Tulipa spp.), for example, are often slow to produce offspring, while bluebell or squill (Scilla spp.) multiplies both by offsets and by seed to produce large clumps that quickly spread over a wide area. Bulbs multiply fastest when they are growing in prime conditions.
Many bulbs grown in American gardens originated from the gritty, rocky mountain slopes of Europe. Therefore, they do best when planted in soil with excellent drainage and in conditions that match their native habitat. They often come from areas that have cold winters and hot, dry summers. Therefore, they often don’t multiply well in areas that get summer irrigation. Fertilize bulbs in fall or as their tips emerge in spring with a low-nitrogen fertilizer such as composted manure or with a product formulated for bulbs.
Snowdrops (Galanthus spp.) quickly multiply to form pretty clumps of 6-inch green foliage and nodding white blooms that appear in late winter or early spring. They thrive in U.S. Department of Agriculture zones 3 through 9. Daffodils (Narcissus spp.), which come in varieties that bloom in early, mid- and late spring, produce offsets at different rates, so look for those labeled “perennializing” or “naturalizing” for the fastest multipliers. Narcissus grow in USDA zones 3 through 8.
Stately white calla lilies (Zantedeschia aethiopica) bloom in early summer on tall stalks and multiply quickly in suitable conditions. Unlike most bulbs, calla lilies like regular water and will grow in ponds. They are perennials in USDA zones 8 through 10. Camas (Camassia spp.), a native bulb used as food by Native Americans, also prefers regular moisture and multiplies both by seed and by offsets. It thrives in USDA zones 3 through 10.
Kaffir lily (Schizostylis spp.) blooms in late summer and fall, with coral to pink blossoms atop irislike foliage. It quickly spreads to form clumps and thrives in USDA zones 7 through 9. Fall crocus (Colchicum spp.) puts up wide, strappy green leaves in spring, which die back over the summer. Then, in fall, small, usually purple, crocus-like blooms emerge without any foliage. Fall crocus grows in USDA zones 5 through 9.