The general rule of thumb is that it is best to divide spring and early summer perennials in the fall, and late season bloomers in the early spring. Spring blooming perennials can also be divided immediately after flowering. Dividing a perennial outside of its flowering season means that all of the energy of the plant can go into root and leaf growth. Most perennials are not temperamental and, if properly divided, will fare well whether they are divided in spring or fall.
Rapidly spreading perennials can be kept under control by dividing yearly or in alternate years. Artemisia ‘Valerie Finnis’ is a good example of a beautiful addition to a perennial border that will, however, need taming and management from the gardener. Some ornamental grasses simply get so large and tough to divide that it is best to split them frequently when they are still small and manageable. Most perennials can be left several years before they are divided.
When you are planting edging for your perennial bed or designing with medium to large sized swaths of perennials, a cost efficient way to fill your border is to buy fewer plants than you actually need and fill in the empty gaps with inexpensive annuals or bulbs until your perennials increase in size and can be divided.
Listed below are some common problems that occur when a plant is left alone for too long:
If your soil is dry, water your bed a few days beforehand to make the soil easier to work with.
It is always best to divide perennials on a cloudy day. Never leave divisions exposed in the hot sun. Keep them in the shade until replanted. Avoid letting them dry out.
Before dividing, cut the foliage down to 6 to 8 inches. This will cut down on moisture loss and you will be able to see what you are doing. If you are dividing early in the season this is not applicable.
Dig around the plant with a spade or a fork, leaving ample space for a good sized root ball, and lift the clump out of the ground.There are several possible methods for dividing perennials:
Cut divisions so that they are the size of a quart or a gallon perennial. Each division should have 3 to 5 vigorous shoots and a good root system. The ultimate size of your cutting depends on the size of your garden and how quickly you would like the new divisions to grow. If you have a large garden, larger divisions will keep everything in scale. For smaller spaces, small clusters of new plants will do the trick.
Throw away old or diseased roots. Remember that you will not be able to salvage the entire perennial. The goal is to end up with between two to four healthy divisions per plant.
Replant perennial at the same level that they were growing before. Coral bells, Solomon seal and peonies should be planted slightly below soil level.
Spreading root systems have slender matted roots. These can usually be pulled apart by hand or cut apart with a knife. Asters, bee balm, and lamb’s ears are several examples.
Clumping root systems originate from a central cluster with multiple growing points. Hostas, daylilies and many ornamental grasses are included in this group. These perennials can be cut with a knife or spade or pried apart with two spading forks.
Rhizomes grow at or below ground level. (Bearded irises have rhizomes, for instance.) These should be divided in mid- to late summer, about one month after the irises finish flowering. The older, shriveled rhizomes can be removed with a knife. Keep young, firm rhizomes that have a fan of leaves and several healthy roots. The foliage should be trimmed back by ⅓ so that the division is not top heavy.