Issue 2, April 30, 2010

Where Have All the Flowers Gone?

While most plants are self-sufficient and able to "do their thing," that is flower without much assistance, once in a while that isn't the case. Dahlias, tulips, and iris are some examples of plants that often have flowering difficulties, leaving the consumer to wonder what happened.

Iris (Iris sp. and cvs.) are typically hardy, long-lived perennials that need a minimum of care. Bearded types grow from thick underground stems called rhizomes. The rhizome serves as a storage organ for "food" produced by the leaves. Only one blooming stalk is produced once from each rhizome. If the plants are not divided on a regular basis, they soon become overcrowded and flowering is reduced. It isn't difficult to determine if a planting is overgrown; the clump will be rather large with a tangled mess of rhizomes, many of which are dried up and no longer productive. The general guideline is to divide a clump every 3 to 5 years, after flowering, in midsummer. Dig up the entire clump and loosen away any soil. Cut the rhizomes apart with a sharp knife. Discard the old rhizome, keeping only young, healthy ones. Each division should have a fan of leaves. To reduce weight, trim the fan back by 2/3. Replant the rhizomes so they are at or just below the soil surface and 12 inches apart. The rhizomes should never be completely buried. Iris generally needs little additional fertilizer; however 1 to 2 pounds of a complete fertilizer per 100 square feet can be applied in the spring and again after flowering to boost growth.

Of all the spring bulbs, perhaps none is more recognized universally as the Tulip (Tulipa sp. and cvs.). Unfortunately, they are not as long-lived and productive as Narcissus over time. Some of the species tulips, T. gregii and T. kaufmanniana for example, are better at returning year after year. Bulbs used in commercial plantings are usually discarded after one season, but not so with those in home landscapes. In the home garden, the first year is spectacular, the second year okay, and by the third year the bulbs dwindle away. Even a new planting can appear spotty with bulbs that never grew and/or flowered. Some things to consider are as follows:

  • The larger and showier the flower, the less chance for return in succeeding years.
  • Buy quality bulbs -- you shouldn't skimp here; bulbs are not that expensive. Invest in the best you can buy. Avoid bulbs that are "on-sale," "end of season," show signs of rot or soft areas, those that are missing the tunic (the outer brown, papery covering)
  • Plant as soon as possible in the fall. This allows them to develop roots and get established. A depth of 8 inches should suffice; planting deeper, especially in clay soil is the kiss of death.
  • Choose a site that is well-drained; avoid sites with heavy clay or where water collects over the winter.
  • After flowering, allow the foliage to "ripen," that is, turn yellow; this allows the bulbs to build up food reserves. Even if this is done, it does not guarantee re-flowering.
  • Fertilize at planting time with a general all-purpose fertilizer.
  • Invest in new bulbs each year. As stated above, they are relatively inexpensive and well worth the cost for a reliable display.

Dahlias (Dahlia cvs.) are grown for their summer blooms that range in size from 2 inches to dinner-plate size flowers 12 inches or more. Sometimes flowering is reduced, especially from tubers stored from the previous year. Assuming the site is not the issue (dahlias are full sun plants and perform poorly with shade), or weather conditions (a particularly cool season can hamper flowering); possible culprits include over-grown clumps of tubers and/or lack of fertility.

As dahlias grow, they produce a clump of tubers in the soil. At the end of the season, most gardeners dig up the clump and store it for the winter (see image). If the entire clump is planted the next spring, the result is often an abundance of healthy foliage and little flowering. To avoid this, the mature clump should be divided in the fall or spring so that you wind up with many tubers, each with one or two eyes as shown in the second image. They can actually be split apart so that you're planting just one tuber. If you proceed with this method, as you are splitting off the tubers, it is recommended to discard the original tuber and save the newer ones.

Another factor is that dahlias are heavy feeders and need moisture. I suggest applying some type of fertilizer pre-plant when you're working up the soil in the spring and then applying fertilizer, either granular or soluble 2x, again during the summer. Dahlias also require ample moisture and benefit from mulch placed around the base of the plants. --Jim Schmidt, Extension Specialist, Horticulture

Jim Schmidt

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