Dividing Herbaceous Perennials
Over time, many herbaceous perennials will form large clumps that can engulf other plants, outgrow their allotted spot, develop a bare spot at the centre of their crown or become reluctant to flower. It’s easy to give them a new lease of life and restore their vigour and flowering or control their spread by a propagation technique known as division.
This involves digging up and dividing the clump, removing the old sections and replanting the smaller, most vigorous pieces. This is also an inexpensive and effective way to increase your plant stock. You can use excess divisions for filling in bare spots in your garden or for sharing with friends and neighbours
Most perennials benefit from being divided every three to five years. Perennials that particularly benefit from regular division include Astilbe, Hosta, Siberian iris (Iris sibirica), day lily (Hemerocallis), sedums (Hylotelephium), Rudbeckia, lamb’s ear (Stachys lanata), asters, shasta daisy (Leucanthemum), Achillea, Monarda and Phlox.
When to divide?
Spring is the best time to divide a perennial because there is less growth on the plant than later in the growing season and the weather is mild. To ease the stress on the plant, try to time your division to coincide with a stretch of cloudy, overcast days as dividing on a hot sunny day can cause it to dry out. Ideally, divide plants when there are a couple of days of rain showers in the forecast to provide enough moisture for the new transplants.
Generally, perennials that flower after Midsummer’s Day (24 June) are suitable for dividing in spring, while those that bloom before this date are best left until autumn. However, many gardeners prefer to divide everything in spring because wet or cold weather later in the year can result in divided sections rotting.
You will need:
- a spade
- 2 garden forks
- sturdy old bread
or steak knife
1. Dig up the whole plant by cutting around the clump with a spade, digging the whole depth of the spade into the ground to sever and loosen the roots.
2. Use your spade or garden fork to lever and lift the plant out of the ground, shaking off loose soil.
3. Look for natural division points and plan where you will make dividing cuts, taking into consideration not to make the pieces too small. Perennials like sedums, asters or heleniums, with fibrous roots, are easy to divide by plunging two forks into the centre of the clump, back to back. Split in two by pushing the forks forward. Plants with a loose network of roots, such as heucheras and hardy geraniums, are usually easy enough to tease apart by hand. Perennials with thick, sturdy roots need a bit more force. Congested rootballs of clumps of day lilies (Hemerocallis) or red-hot pokers (Kniphofia) can be sliced up with a spade or carved into smaller pieces with a knife.
4. Subdivide large divisions into smaller divisions, using a knife if necessary, making sure each new piece has several shoots and roots. Each division needs to have both leaves and roots in order to grow. If growth is advanced, and is likely to wilt, you can cut back excess leaves and flower stalks.
5. Select pieces to replant that have at least two to three strong growing points and a healthy and plentiful root system. Use pieces from the outside of the clump as these will be the youngest and most vigorous. Discard the old woody central portion and broken pieces.
6. Replant divisions promptly so the roots don't dry out, giving each new plant plenty of room and appropriate spacing from neighbouring plants. Firm in well and finish by watering generously.
7. The new plants will grow away quickly at this time of year. Water regularly for a few weeks after planting until roots re-establish.
A word of caution
Not all perennials can be divided. Some simply resent root disturbance and can be difficult to re-establish after transplant. Those that produce multiple stems emanating from a single crown or with long taproots are not suitable candidates. These include:
- Lamprocapnos or bleeding heart
- Eryngium or sea holly
- Ornamental thistles like Cirsium
- Papaver orientale or Oriental poppy