Question: How often should we divide our iris plants and also what about dividing Lily-of-the-valley clumps. How should big should the clump be when divided? Erica, Wilkinsburg, PA
Answer: Erica, when iris has been in one place more than three years, the plants should be dug up and divided. Buds for next year are already beginning to form. If dividing is postponed until later you will stop this development. If you want bloom next year, divisions with several rhizomes (fleshy roots) should be made. In two years, clumps which grow from these will again need dividing.
Disease cannot be eliminated unless you clean off all the old corky tissue and plant single or twin rhizomes. Some will not flower next year, but dividing will not again be necessary for about four years. Improve the soil with plenty of organic matter and bonemeal plus 3 pounds of 4-8-12 fertilizer per 100 square feet. When the bed is thoroughly worked and firmed, set the rhizomes 9 inches apart. Cover them but firm the soil against the roots underneath the rhizome. Dust or spray against borer in both new and old plantings.
Lily-Of-The-Valley
Divide lily-of-the-valley. When neglected, the soil becomes filled with starved roots causing flowering to cease. When dug up, the roots are like a piece of sod. Separate them by shaking them loose, then pick out bundles with six or more pips (roots with buds). Gather the stringy roots in a ball and plant. The soil should be well supplied with food materials so the plants may remain in one place a long time without dividing again.
Pyrethrum, like lily-of-the-valley, is another neglected plant. Plants should be divided now and the soil improved. If you are not familiar with the newer pyrethrums, you are missing something. They bloom from May through June and make excellent cut flowers that last well.
With the possible exceptions of hydrangeas and rose-of-sharon, few summer-flowering shrubs or trees are seen in gardens. The two tree lilacs – Syringa japonica, Japanese tree lilac, and S. pekinensis, which is somewhat smaller – provide spectacular bloom in July. They are somewhat coarse, however, and they need space.
Golden rain tree, Koelreuteria paniculata, is a more refined tree. The yellow wistaria-like flowers are a pleasing sight in July. It is pest-free, too. The chaste tree, Vitex latifolia (V. macrophylla), is a large shrub that needs to be cut back each spring. In most places it will kill back. This shrub has large spikes of dark blue flowers in late July and August which should be removed as soon as they fade to encourage more bloom. The oak-leaf hydrangea, Hydrangea quercifolia, will grace any planting. The large white panicles show up to best advantage in shade and the plant does best in such a location.
Chrysanthemums
With the exception of the cushion mums and so-called azaleaniums, which get along with very little attention, chrysanthemums require the best care you can give them for top results in flower production. Now that the plants have plenty of foliage they can absorb extra feeding and water. Use one of the concentrated soluble liquid fertilizers applications. But don’t overdo the feeding. Follow the manufacturer’s directions. Overfeeding can bring on disease.
Chrysanthemum varieties that flower in October are generally given their final pinching this month. That is, the tips of all the stems are removed to farther increase the branching. If there are many plants, this arduous job can be greatly expedited by using a hedge shears, so long as the shearing is confined to the tips. In addition to feeding and watering, keep the foliage free from insects and diseases. Spray or dust regularly. I find that the all-purpose material is a very good remedy when used every week.
Dahlias
Dahlias, too, are increasing in size this month (July). Many avid gardeners set out plants from pots at the beginning of July. This is a difficult month for the large-flowered types. The dwarf varieties, will flower in late July if planted in June. These need no care if the soil was well prepared before planting. Not much feeding is done until August. Keep the soil stirred around the plants. An application of ground limestone around each plant will assist in encouraging roots and faster growth. If a dry spell occurs, water will be needed.
