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How to Divide Perennials

© Jojo Sigurgeirson

Most gardeners are clear on the notion of dividing perennials to get more, but in actual fact, it is necessary to the health of many types. Perennials such as chrysanthemums can become overcrowded if left to grow without division, and eventually, the centre core of the root mass may die and rot. This crowding can also effect the way the plant takes up nutrients from the soil and reacts to insect infestation. If hostas are left without dividing, the central core of roots and crowns can become so woody and dense that leaves no longer grow there, forming a characteristic circle of growth.

Tulips and Daffodils also need to be dug up and divided periodically or they will become overcrowded just like perennials roots, and can cease to flower because of this.

This is true for so many perennials that division is necessary on almost all types. There are some perennials that don't live well through the dividing and replanting process, such as hollyhocks, columbines, delphiniums and lupines, and these are best started from seeds saved from the previous year. Woody perennials such as Lavender, Sage, Thyme and Rue are best taken from cuttings, and rarely send up off-shoots. So striclty speaking, not all perennials are divisible.

How to tell if your perennial can be divided
The trick is to get down on your knees and look at the point where the stem goes into the ground. If it's just one major stem, then skip it. It will kill the plant to attempt to divide it. There's nothing to divide. In this case, either wait until it has formed two stems or figure out some other way to increase this plant.

Whichever types of perennials you have, a good rule of thumb is to divide every 3 to 4 years. Vigorous plants such as yellow loosestrife (Lysimachia punctata), certain ornamental spurges (Euphorbia spp.) and most Polygonums can be divided every year just to keep them under control.

Another common rule of thumb revolves around the question of when to divide. Generally speaking, perennials that bloom in the spring and early summer should be divided in the fall. Perennials that bloom in late-summer and fall (Such as asters and many grasses and heleniums) should be divided in the spring. This gives each perennial a resting period before blooming season.

At the same time, gardeners in cold regions should note that on most perennials spring is the best time to divide. This is because colder winters are harder on perennials, and sometimes a plant may not have enough time to get settled before cold winter sets in. If you divide in the spring, do it just when new growth is visible.