How to Propagate Spiraea: Division of Suckers, Cuttings and Layering

Dividing the clump: the fastest route

Because spiraea thickens and spreads from the crown, an established plant is really a colony of stems with their own roots. That makes division the quickest way to get a new plant, and it does double duty as the job that keeps the shrub inside its allotted space. You are removing growth you would otherwise cut off anyway.

Do this in the dormant season, from late autumn through late winter, when the plant has no leaves to support. Choose a section at the edge of the clump with several stems and visible roots. Push a spade straight down between it and the parent, all the way through, then work the spade under the piece and lever it out. You do not need to dig up the whole shrub, and the parent barely notices as long as you take from the outside rather than the middle.

Cut the removed section's stems back by about half before replanting, so the reduced root system is not trying to support a full top. Plant it immediately at the same depth it was growing, water it in thoroughly, and keep it watered through its first summer. A division taken this way is a fully grown plant from the first season and will usually flower the same year.

If the piece comes away with very few roots, pot it up rather than planting it out, keep it in a sheltered spot out of harsh sun, and let it build a root system for a season before it has to compete in open ground.

Take divisions from the outer edge of the clump, never from the center. Cutting into the middle of the crown leaves an open wound in the oldest, slowest-healing wood and creates the hollow center you are otherwise trying to avoid.

Softwood and semi-ripe cuttings

Cuttings are worth the effort when you want a dozen plants for a hedge rather than one or two. Spiraea roots readily at two stages: softwood in late spring and early summer, when the new growth is still flexible but snaps rather than bends, and semi-ripe in late summer, when the base of the shoot has begun to firm and turn woody while the tip is still soft.

Softwood cuttings root fastest, often in three to four weeks, but they wilt within minutes of being cut, so collect them early in the morning into a bag with a damp cloth and get them into compost the same hour. Semi-ripe cuttings take longer, six to ten weeks, but they are far more forgiving of imperfect handling and are the better choice for a first attempt.

Take shoots 4-6 inches (10-15 cm) long from non-flowering growth. Flowering shoots root poorly, because the plant is putting its energy into the flowers rather than into producing roots. Trim each cutting just below a leaf joint, strip the leaves from the bottom half, and pinch out the very tip if it is soft and drooping. Rooting hormone is not required but speeds things up and improves the strike rate.

Insert them around the edge of a pot of free-draining cutting mix, roughly half peat-free compost and half perlite or coarse sand, spaced so no leaf touches another. Water once, then cover with a clear bag or propagator lid and keep the pot out of direct sun in a bright, warm place. The compost should stay just moist. Cuttings sitting in wet compost rot at the base before they root.

Layering: the plant does the work for you

Layering asks least of you and works almost every time, because the stem stays attached to the parent and keeps being fed while it roots. Bend a low, flexible stem down to the ground, scrape a shallow wound an inch (2-3 cm) long on the underside where it touches, pin it down with a wire staple or a stone, and cover that section with two inches (5 cm) of soil. Leave the tip sticking out and pointing upwards.

Spring and early autumn are both good times to start a layer, and one shrub can carry several at once without any effect on its flowering. The stems on the outside of the clump that already arch down towards the ground are the obvious candidates, and on an untended plant you will often find one that has layered itself where a stem has been resting on bare soil.

Check it after a full growing season by scraping the soil away gently. If there are roots, sever the stem on the parent side, leave the new plant in place for another few weeks to adjust, then lift it and move it. If there are no roots yet, cover it again and give it until the following season. Nothing is lost by waiting.

Growing the young plants on

Rooted cuttings need weaning off their humid cover gradually. Once you see new leaves at the tip and feel resistance to a gentle tug, open the bag or vent the propagator for an hour a day and extend it over a week or two until the cover comes off. Pot each cutting into its own small pot of ordinary compost at that point, and keep them somewhere sheltered and bright.

Young plants from cuttings are best kept in pots through their first winter, protected from hard frost and from sitting wet, then planted out in spring once growth restarts. Plant them out too early, at the end of their first season, and the small root system is very exposed to a hard freeze. Give them a year in a pot and they go in as sturdy plants.

1. Label everything at the moment you cut it Write the date and, most importantly, the parent shrub's flowering season on the label. That last detail decides how the new plant will be pruned for the rest of its life and is impossible to recover later from an unlabelled pot.
2. Take more than you need Aim for twice the number of plants you actually want. Losing a third of a batch of cuttings is normal and no reflection on your technique. Extra rooted plants are easy to give away.
3. Pot up before the roots circle Move each rooted cutting into a slightly larger pot as soon as roots show at the drainage holes. A young plant left too long in a small pot sets up circling roots that follow it into the ground.
4. Record the strike rate Note how many cuttings you took, the method, the week, and how many rooted. Two seasons of those numbers tell you which timing works in your own conditions better than any general guidance can.