Some of the most common questions we hear this time of year have to do with dividing perennials, so the timing of this months’ column is perfect for talking all things division.
The three main reasons we divide perennials is to control, rejuvenate and multiply. Some plants grow quickly and need to be divided to keep their size under control. Overcrowded plants must compete for water and nutrients and dividing them will help rejuvenate them. Division is also an inexpensive way to increase the number of plants in your garden.
In general, it’s best to divide spring- and early summer-blooming perennials in the fall and fall bloomers in the spring. Mid- and late-summer bloomers can be divided in either spring or fall. Plants that are divided in the fall need about four to six weeks to become established before the ground freezes, so September through early October is best.
Most perennials should be divided every three to five years but some perennials, such as shasta daisies, asters and mums, benefit from more frequent divisions. Some other perennials, such as peonies, balloon flower, butterfly weed and false indigo, rarely if ever need dividing. Perennials have some tell-tales signs that it’s time to divide them. When the flowers become smaller than normal or decrease in numbers, the center of the plant hollows out and looks dead or the foliage at the base of the plants becomes sparse and poor, it’s probably time to get out the shovel and start digging.
It’s best to wait for a cloudy day rather than a sunny one to keep your divisions from drying out. Water a day or two in advance if the garden is dry. When dividing in the fall, prune the plant down to 6-8 inches from the ground to make it easier to see what you are doing and to cut down on moisture loss. Start at the drip line and dig a trench around the perennial, then angle your spade down and under the clump until you can lever it out of the hole. Shake or hose off the loose soil so that you can see what you are doing and to help loosen tangled roots.
Different types of root systems require different techniques. Plants with spreading root systems such as coneflowers, bee balm and asters can usually be pulled apart by hand or cut apart with a knife. Divide these types of plants into clumps of three to five vigorous shoots each. Discard small or weak woody divisions.
Clumping perennials include hosta, daylilies, astilbe and many ornamental grasses. These plants are easier to divide by cutting through them with a heavy, sharp knife or pruning saw. Make sure each division has at least one to three eyes or buds. Bearded iris are examples of rhizomes. Iris can be divided summer through early fall by cutting rhizomes into sections at least a few inches long and with one fan of leaves.
Some perennials are small woody shrubs and should not be divided. The most common plant in this category is lavender.
Never allow your divisions to dry out. Keep a bucket of water close by to moisten the roots as needed until you plant them. Remove broken or damaged roots by trimming with your pruners or a sharp knife and replant your new divisions at the same depth they were growing originally. Water well and mulch after a few hard frosts to prevent heaving the first season.