Gardening with Mary: dividing your perennial plants

Over the last few weeks, we have been busy spring cleaning and cutting back all the dead growth from last year in our perennial borders. Now we can see more clearly those perennials that will benefit from division. This involves digging up and dividing the clump, removing the old sections and replanting the smaller, most vigorous pieces.

Most perennials benefit from being lifted and divided every three to five years to maintain health and vigour. It also offers an opportunity to multiply your plants, rather than buying more, for planting up a new garden bed or to create a greater mass planting for added impact.

Perennials that benefit from regular division include Astilbe, Hosta, Siberian iris (Iris sibirica), day lily (Hemerocallis), sedums (Hylotelephium), Rudbeckia and asters.

Some perennials resent disturbance and are not suitable for dividing, especially those with tap roots. These include acanthus, bleeding heart, sea holly and peony.

Signs that a perennial plant needs to be divided include smaller leaves and diminished flowering. Often, you’ll see a doughnut, a bare or dead spot in the middle of the crown, and active growth around the outside. We will also divide large clumps that are outgrowing their allotted spot and beginning to encroach on other plants.

This year, we have several groups of bergenia and lamb’s ear (Stachys lanata), that have pushed up out of the ground, exposing straggly, woody stems.

These will be dug up and replanted deeper so that the rosettes of leaves sit in neat clumps on the ground surface — the woody stems are shortened and these will regrow roots when buried in the soil again.

Divide border sedums

Every two years, we also dig up and divide border sedums, such as Hylotelephium ‘Herbstfreude’, as this keeps their growth more compact and the stems are less prone to flopping when in flower.

We divide most perennials in spring when there is less foliage for the plant to support and the soil is moist. This gives the transplants the entire growing season to become established.

Division of spring-flowering perennials, like brunnera, epimedium and pulmonaria, can be deferred until autumn, if it is done early enough for the roots to establish before winter, but wet or cold weather later in the year can result in divided sections rotting.

For beginner gardeners, the first time you divide perennials you may feel unsure of what you’re doing. Have no fear! Most perennials are very resilient and recover quickly.

Break apart

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.

Begin by digging up the entire clump, once your plant shows signs of growth in spring (3-5 cm of new shoots is fine). Carefully divide the crown and rootball into two or more sections, depending on the size of the clump.

Sometimes the roots are easy to break apart with your hands. Congested rootballs of plants like day lilies (Hemerocallis), Siberian iris or hostas need a bit more force and can be sliced up with a garden spade or we often use a kitchen steak knife to carve them into smaller pieces.

Occasionally, a handsaw may be required for plants with woody crowns, such as lupins or astilbes. Be prepared for a bit of huffing and puffing.

For plants with fibrous roots, like sedums, asters and heleniums, insert two garden forks into the crown, back to back, as pictured. Push the forks in opposite directions to break the root mass into two sections. Further sub-divisions can then take place. Each division needs to have leaves and roots in order to grow. If growth is advanced, and is likely to wilt, cut back excess leaves and flower stalks.

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.

Replant your divisions as soon as possible, making sure that the roots don’t dry out. Water each division immediately after planting and keep the plants well watered while they settle in, especially during dry spells. If you have loads of extra divisions, consider potting up a few of them — they will make welcome and inexpensive gifts when visiting other gardeners.

Is our beautiful Japanese maple acer being damaged by lichen on the branches? - Mary O’Malley, Co Mayo

Lichen is not a disease and it does not harm its host plant. It favours damp, damaged or dead wood and commonly appears on trees or shrubs that are weak, stressed, or lacking in vigour due to maturity or unfavourable conditions.

Check for and address any signs of environmental stress or plant health issue that could predispose your maple to lichen colonisation.

Lightly prune, if necessary, to remove dead wood and improve air circulation, ensure adequate watering and mulching, and give your plant a feed of liquid seaweed to boost its vigour.

Prune roses as soon as possible if not already done. It’s also a good time to mulch and feed roses.

Pot up tubers of begonias into pots or trays of potting compost and grow them on in a warm place, protected from frost, until planting out.

Recut lawn edges with a spade or half-moon edging tool to smarten them up for the new season.

Tie in climbers to their support framework as new growth develops, covering the space evenly. Most climbers flower more profusely if tied in horizontally.