Tips for Dividing Plants
Many hardy plants mature and fatten into bushy clumps that can be divided to create new rooted plants. Unlike growing from seed, division uses an existing plant to produce more plants that are exactly like the original. It is also a reliable way to rejuvenate perennials that are old and no longer flowering well. Choosing plants to divide The plant you select to divide must be healthy in every respect. Whereas few ailments are passed on through seeds, a diseased parent plant used either for division or for taking cuttings will usually result in diseased offspring.
When buying new herbaceous perennials, look for large plants: these are an economical option if you can divide them into two or three portions before planting. In choosing a suitable clump of plants for division, check that the roots are healthy. They should be flexible and undamaged; most healthy roots also have numerous short, pale, hairlike side roots growing along them. Discard plant segments that look dry or withered, with dark roots that crumble or break easily, since these may be symptoms of disease. Discard the tired, old parts of the plant, which are usually in the center of the clump, and select the young, vigorous sections toward the edges for replanting.
When to Divide
Most perennials can be divided at any time while they are dormant and unlikely to suffer any slowdown in growth, but different times of year are better for different divisions.
- Early spring is the best time to divide most perennials, especially the fleshy-rooted types. The worst weather is usually over and growth will resume quickly. Some plants will already have buds, which can help you to distinguish the strongest parts for replanting.
- Once flowers start to die down in early-flowering plants— such as doronicum, pulmonarias, and Primula denticulata—you can start to divide them. This is also the best time to split clumps of bearded irises.
- Between late spring and early summer, divide marginal and aquatic pond plants—this is when their growth revives.
- During late summer, some perennials, such as hostas, often make new roots, and it is a good time to divide them. Most of the growth is over and divisions will quickly settle in. To be on the safe side, keep late divisions in a cold frame or nursery bed to protect them from harsh winter weather, and move them to their final positions in spring.
- Avoid winter, as the weather can be wet or very cold, with a risk that divisions will rot.
Herbaceous perennials may develop solid, woody crowns, clusters of smaller crowns, or simply a mass of fibrous roots. Chop through solid crowns with a spade, or cut them with a knife. Ensure that each segment has plenty of healthy roots and several strong growth buds. Divide looser clumps with garden forks, or hand forks if they are small. You may be able to simply tease them into smaller portions with your fingers. Some plants, such as alpines, produce “offsets,” complete young plants alongside the parent. Loosen these with a hand fork, separate them from the parent plant with a sharp knife or pruners, and replant.
Divisions that fit comfortably in one hand are the best size for replanting, but you can use larger ones for rapid establishment. Grow on smaller fragments with one or two buds in a nursery bed without any competition for a season before planting them out.