Garden Variety: Divide your perennials for better performance
Hostas, daylilies, sedums, daisies and other perennial flowers and ornamental grasses can outgrow their space and exhibit reduced performance over time. Digging plants up, dividing them and replanting smaller portions is the best way to alleviate these problems. Fall is often touted as the best time to perform this work, but spring is also ideal for most species.
Reasons to divide
Landscape beds that are full of plants are ideal for aesthetics and weed control, but overcrowded plants compete for water and nutrients and bloom less. Airflow is also reduced around crowded plants, leading to leaf surfaces staying wet for longer periods of time. Wet leaves offer greater potential for spread of bacterial and fungal plant pathogens. Dividing plants creates more space and improves air circulation.
Some perennials die out in the center over time. Sedums such as “Autumn Joy” and most ornamental grasses are notorious for dying out this way. When dividing perennials that are experiencing this problem, the dead portions of the crown can be removed and discarded, resulting in a healthier, more uniform appearance.
Division is an easy way to make more plants. If there is a bare spot in the landscape but you have overgrown daisies in another area, divide the overgrown daisies and plant the divisions in the bare area. Sometimes other gardeners are interested in trading divisions, or a neighbor or friend might be happy to have your discarded extra daylilies or maiden grass.
Divisions should not be sold, except by individuals and businesses who have obtained the necessary licenses to do so. Besides the potential for spreading unwanted pests with the plants, there are plant patents and trademarks that protect some varieties.
Use a regular shovel or one with a long blade, sometimes called a sharpshooter, to dig up the entire plant. If the plant is growing in a clump, such as a hosta, the mass can simply be cut in half, quarters, or more sections depending on the size of the clump and desired size of the division. In most cases, a shovel is the easiest way to make the cuts. In some cases, a serrated bread knife also works.
If the plant spreads by underground roots or grows in some other habit than clumps, portions of the plant may be removed without digging up the entire mass. Black-eyed susans, lily-of-the-valley and Solomon’s seal are all examples of plants with spreading growth habits that can be divided in this fashion.
If the center of the plant is dying out, remove dead or damaged portions of the crown as you chop.
Keep divisions large enough to still have a viable plant with some root and shoot growth. This will vary with plant species but a 1-gallon nursery pot could be used as a size guide, with divisions being large enough to fill the pot at a minimum.
Replant the clumps/ portions of the plant that were removed. Water the newly planted plants and mulch around them to reduce moisture and temperature fluctuations around the roots.
What to divide and when
Mums, asters, sedums and other fall bloomers should only be divided in spring.
Hostas, coneflowers, veronicas, catmint, salvias, black-eyed susans, yarrows, lambs-ear and ornamental grasses can all be divided in spring or fall.
Daylilies can be divided throughout the growing season. The most ideal time is after they have finished blooming for the year.
Peonies should only be divided in the fall.
All of these plants typically benefit from division every three to five years.
Division may be unnecessary for some species, but plants may still benefit from being lifted and reset every few years. This is especially true when using heavy layers of mulch.
— Jennifer Smith works in regulatory horticulture and has worked as a horticulturist for various government entities. She has experience in landscape design and maintenance and as an educator.