By the end of summer we often can recognize the need to divide and/or move perennial plants. Signs to look for are diminished bloom, overcrowded plants, a change in available light, or a decrease in plant vigor.
For many perennials, autumn planting may not provide sufficient time for roots to reestablish in our northern climate, but some plants are best divided and reset during the cooler, moister autumn period. There are also plants that should not be moved in the fall.
Let’s look at the no-move plants first. Many of these plants are in their prime bloom season. Their job at this time of year is to flower and produce seed, not to concentrate on root growth. Grasses and chrysanthemums are among the ones to avoid transplanting at all costs during the fall. Spring is best for them. Roses should not be moved after the Fourth of July. Avoid transplanting early spring bloomers. Always refer to a northern climate reference such as those of the University of Minnesota for the optimum time.
The exceptions are some of our most beautiful and yet the toughest and most dependable of perennials. Three that can be safely divided in autumn include hostas, day lilies, most lilies, and peonies. In fact, peonies should be divided in September even though they may not bloom next year. A hosta can outgrow its space quite readily or you may just want to spread the joy around; now is a good time. True lilies may have multiplied and are producing smaller blooms. Diminished blooms scapes and diminished blooms per scape are clues in day lilies. Division can bring them back to optimum performance.
Whenever you divide or move a perennial, make sure the new spot is appropriate for light and soil. For most perennials prepare the soil with lots of organic matter to help make nutrients available, to create better tilth, and to create a hospitable soil web. Allow room for future growth when transplanting.
Water the plant well the day before. Dig the parent plant carefully. If it has a long taproot, dig deeply and then attempt to duplicate a similar hole — with a bulb planter that fits on an electric drill, for instance. Then, gently supporting the plant, place it in the hole and carefully add amended soil until the plant is at the same depth as previously. For fibrous-rooted plants dig the plant and look for natural divisions and separate them carefully. Shake out the roots and place the plant in a prepared hole over a cone of soil and fill to the appropriate depth. With tuberous roots such as those of daylilies, less care is needed. Dig the whole plant and separate out sections for replanting or simply cut with a sharp blade a section of the parent. Plant in a prepared spot. Fill in the void of the old plant with amended soil. Hostas can be divided this way also. Peonies should be cut back to the ground, dug and carefully separated into sections with at least three eyes (growing points) and then replanted, with only an inch of soil covering the eyes. In all cases water well and firm the soil; if the soil settles, add more so that it remains at the previous planting depth.
Cut back the foliage at least halfway. Mulch the plant, avoiding the stem. In the fall wait to mulch until after the ground freezes and then mulch well. With plants that form rosettes, mulch around the crown under the leaves and very lightly on top. Water well until freeze-up. Mark the location and make a map of the changes and make note of changes to make in the spring.
More information on perennials is available from U of M Extension. Use this link: www1.extension.umn.edu/garden. For help with gardening questions, call 444-7916, leaving your name, number, and question. A local master gardener will call to assist you.
