Gardeners dutifully buy and plant more and more Dutch bulbs every autumn. In wet climates, many of the bulbs would rot in humid summer conditions. But in our sunny, dry climate, bulbs will often return, and even improve year after year, forming dense clumps of color from a single plant. For these naturalized bulbs, home gardeners must think ahead and anticipate that the compact bulb dropped into the earth represents an investment that can pay dividends for years. These groupings of bulbs can be divided, much like any other perennials.
To ensure that bulbs will flower year after year, try to site them in an optimum microclimate. Daffodils tolerate more shade and like lots of moisture. However, crocuses, tulips and alliums like lots of sun and heat. Like any perennial, bulbs do best in amended soils, in sites that are not too extreme in sun exposure, dryness or traffic. If your bulbs are not blooming prolifically after a couple of years, they may need more nutrition. Add compost or commercial dry-bulb fertilizers when the bulbs are dormant. Or give the leaves a light feeding with half-strength fertilizer such as Peter's Blossom Booster after flowering is done, but before the foliage begins to turn brown.
Sometimes, bulbs will grow so close together that they can't reach a sufficient size to produce flowers. Overgrown clumps have a very dense growth of narrow foliage and very sparse bloom. These bulbs are good candidates for division. Ideally, dig bulbs in midsummer when they are completely dormant. Next, plant the bulbs in fresh soil.
Seasoned gardeners have discovered that most bulbs do quite well if moved right after blooming. They can be divided much like any other perennial, replanted promptly and watered in. Some miniature bulbs like reticulate irises and snowdrops will bloom with no transplant shock the spring following division, provided they are not kept out of the ground too long. Daffodils and tulips are nearly as forgiving. Transplanting "in the green" as this is called, is the best way to avoid slicing dormant bulbs, and to make sure you don't leave any bulbs behind. In a year or two following division your planting will look like new.
For more information, see the following Colorado State University Extension fact sheet(s).
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Updated Tuesday, November 19, 2013