The great perennial divide
Herbaceous perennials are plants that die back to the ground in the winter. Warm spring days coax them to break their dormancy and start growing.
This is the ideal time to divide summerand fall-blooming perennials.
Not sure if your plants need to be divided or where to start?
We divide perennials for three reasons: to control size, to increase stock or to rejuvenate an older planting.
Dividing and replanting keeps rapidly spreading plants under control. It is an economical way to increase your supply of your favourite perennials: plant the divisions in other areas of the garden, share them with friends or donate potted divisions to a local plant sale. Established clumps of perennials will put out fresh growth and bloom more freely after being divided.
Most perennials should be divided every three to five years. Vigorous growers such as chrysanthemums, asters and obedience plant may need to be divided every one or two years or they will crowd themselves into non-flowering clumps of leaves and roots. Bleeding hearts and peonies rarely need to be divided unless you want to increase your stock.
Signs that perennials need dividing are flowers that are smaller than normal and centres of the clumps that are hollow and dead. Established clumps of hosta, daylily and ornamental grasses will spread out and circle a hollow where mother plant once stood — overgrown patches are easy to spot in the spring, before the leaves unfold and mask them. Plants that are growing and blooming well should be left alone unless you need extra plants.
Divide perennials as soon as the growing tips of the plant have emerged, but before they leaf out completely. Fresh divisions may bloom a little later than usual for the first year. Choose a cloudy day, ideally with some light rain in the forecast over the next few days. Dividing plants on a hot sunny day will put them under extra stress.
Use a sharp pointed spade or spading fork to dig deeply around the plant about eight to15 centimetres away from the shoots. If possible, lift the whole clump to be divided. If the plant is very heavy, cut it into several pieces in place with your spade before lifting it.
Shake or hose off loose soil and remove dead leaves and stems. This will help loosen tangled root balls and make it easier to see what you are doing. Perennials have different types of root systems, each one needs to be treated differently.
Plants with spreading root systems include asters, bee balm, lamb’s ear, coneflowers and many other common perennials. As they spread, the centres will become crowded and less floriforous; some can be invasive unless divided frequently.
Large, vigorous plants with thickly intertwined roots can be separated with digging forks. Put two forks back to back in the centre of the plant and use them to pry the pieces apart. I find this process too difficult, so I slice the root mass into four or six chunks. Discard small or weak and woody divisions.
Hostas, astilbes, daylilies and many ornamental grasses have thick, fleshy roots that can be a challenge to divide. Use a heavy, sharp knife to slice the fleshy roots apart. Keep several developing eyes or buds with each division.
If the root mass is very large, or tight and tangled, lift the clump 30 to 60 cm off the ground and drop it to loosen the root mass, then pull the individual plants apart. Plants that have very tough, vigorous root systems (agapanthus, red-hot pokers and ornamental grasses) may have to be divided with a shovel, saw or ax. You can also vigorously hose off soil to make the root system easier to work with.
Perennials that develop deep taproots, such as butterfly weed (Asclepias), euphorbias, oriental poppies, false indigo (Baptisia) and columbines (Aquilegia) should not
be divided.
Lenten and Christmas roses (Helleborus) are very difficult to move when more than a few years old, but their seedlings are easy to move. I have been monitoring an established clump of hellebores for seedlings, over the past year or two. Last summer, I discovered a single seedling and gently moved it to another part of the garden. It did not bloom in sequence with the mother plant this spring, so I thought I would have to wait until next year to see any flowers. I discovered a pair of tiny buds on the seedling over the weekend, so I will have flowers after all.
Why is this significant? Hellebores can be expensive, seedlings provide a free crop of extra plants, if you are willing to wait for the mother plant to settle in and provide seedlings. This year, there are about a dozen seedlings near the mother plant. Baptisia and columbines will also self-seed. Look for tiny baptisia seedlings beneath the mother plant, columbines seem to be more free spirited, and pop up in unexpected places around the garden.
After dividing a plant, never allow the divisions to dry out. Keep a pail of water nearby to moisten divisions until they are planted. Trim all broken roots with a sharp knife or pruners, and remove any weeds that have permeated the root mass.
Plant the divided sections immediately in the garden or in containers. Replant divisions at the same depth they were originally growing. Firm soil around the roots to eliminate air pockets. Water well after planting and top dress the area with compost.