Rhodanthemum Care: Watering, Feeding, Shearing and Keeping the Mound Tight

Water only when it is genuinely dry

The single most useful habit with this plant is to leave it alone. An established Rhodanthemum in the ground needs no watering at all in ordinary weather; its roots run out sideways into the surrounding soil and find what they need. Kill it with kindness and it goes exactly the way it always goes, with a soft brown collar and a mound that lifts away in your hand.

When it does need water, in a long dry spell, in a container, or in the first season after planting, water properly and then stop. Push a finger through the grit into the soil at the edge of the mound. If it is dry two inches (5 cm) down, soak the root area thoroughly and then do not water again until it is dry to that depth once more. In a hot summer that might be once a week; in a cool damp one it might be never.

Water at the base, in the morning, and keep it off the foliage. Those silver hairs hold droplets for hours, and wet leaves lying on wet soil through a warm still night is how the crown rot starts. An overhead sprinkler running over a Rhodanthemum every evening in summer will finish it in one season.

In a container, ordinary rules apply until winter and then they reverse. Through the growing season a pot dries fast and needs regular attention; through the cold, wet months it should be kept nearly dry and, if you can, out of the rain altogether. A pot standing in a saucer of winter rainwater is a guaranteed loss.

Feed almost nothing

This is a plant adapted to poor stony ground, and it performs best when it is slightly hungry. Lean conditions give you the things you actually want: dense silver foliage, a tight compact mound and flowers held just clear of the leaves on short stems. It is one of the few garden plants where doing less produces a visibly better result.

Rich soil or a generous nitrogen feed does the opposite, and the damage is easy to recognize. The leaves turn greener and lose their silver cast, the growth gets soft and lax, the mound opens out and flops apart in the middle, flowering drops off, and the soft tissue is far more vulnerable to rot at the first spell of damp weather. Every one of those symptoms is reversible only by cutting back and waiting.

If you feed at all, make it one light dressing of a low-nitrogen general fertilizer in early spring as growth restarts, and nothing else for the rest of the year. A plant in a container that has been in the same mix for two or three seasons is the main case for feeding; one in open ground rarely needs anything.

Refresh the grit mulch each spring instead. An inch (2.5 cm) of fresh grit spread up to the stems does more for the plant than fertilizer: it keeps the collar dry, holds the silver foliage clear of the soil, suppresses weed seedlings that would otherwise root into the middle of the mound, and reflects heat back up into the plant.

If your Rhodanthemum has gone green, soft and floppy, it is being fed and watered too well. Stop both, shear it back by a third, and it will usually come back silver and tight within a couple of months in the growing season.

Deadhead through the season, then shear once

During flowering, snip spent blooms off with their stems, cutting back into the leafy part of the mound rather than leaving bare stalks sticking up. Done regularly this keeps the plant producing more buds and stops it looking scruffy, and it takes a couple of minutes a week on a mature plant.

The bigger job is a single shear after the main flush fades. Take shears or scissors and remove roughly a third of the mound's height all over, cutting into leafy growth so that every cut stem has green shoots or visible buds below it. The plant responds by branching from those points, thickening up, and very often producing a second wave of flowers later in the season.

Skip that shear and you get the classic failure of an old Rhodanthemum: the outer ring keeps growing while the center goes woody, bare and gray, so within two or three years you have a hollow doughnut of foliage with a dead patch of stems in the middle. Once that has happened it cannot be pruned back into shape, and that is the difference between a plant that looks good for four years and one that looks tired after two.

The rule that keeps you out of trouble is never cut into bare old wood. Unlike a shrubby herb such as lavender, a stem cut below the last green shoot on this plant simply dies back; it does not break from the base. Always leave green growth below every cut, shear little and often rather than hard and late, and never shear at the end of the season, when the plant needs its remaining foliage to get through winter.

1. Deadhead as the flowers fade Cut spent flower stems back into the leafy mound rather than snapping the heads off. Regular deadheading through the season keeps buds coming and stops the plant putting energy into seed.
2. Shear by a third when the main flush ends Once most of the flowers are over, take about a third off the whole mound with shears. Work over the plant evenly rather than cutting one side hard; the aim is a shallower dome, not a haircut with steps in it.
3. Check every cut has green below it Part the foliage and look. If your line would fall below the lowest green shoot on a stem, raise it. Bare stems on this plant do not resprout, and a hole cut into the mound stays a hole.
4. Water once afterwards, then leave it Give a single soak after shearing if the soil is dry and skip the feed entirely. The plant will look bare for a week or two before breaking into new silver growth from the cut points.
5. Do not shear late in the season A cut made as the weather turns leaves soft new growth exposed to cold and damp and strips the plant of the foliage it needs to overwinter. If the main flush ended late, skip the shear that year and do it early next season.
6. Take cuttings at the same time Late summer shoots are ideal cutting material and you are removing plenty of it anyway. Root a few every year so that the loss of the parent plant is an inconvenience rather than a disaster.

Short-lived by nature: take cuttings and keep notes

Treat this as a plant you will replace rather than one you will keep. Three or four years of good performance is normal, and by the fourth or fifth the center is usually woody and the flowering thinner however well you have shorn it. Nothing you do prevents that, and the answer is not to fight it but to have young plants coming along.

Cuttings are easy and take about a month. In late summer, pull or cut non-flowering shoots two to three inches (5-8 cm) long, strip the lower leaves, and push them into a gritty free-draining mix of about half sharp sand or perlite and half compost. Keep them somewhere bright and out of direct midday sun, water sparingly, and do not cover them with a plastic bag; the humidity that helps most cuttings rots this one. Roots form in three to five weeks. Grow the young plants on in pots and set them out the following spring.

Division is possible on a plant that has spread and rooted where the stems touch the ground, but it is far less reliable than cuttings and the woody center of an old plant is not worth splitting. If you want more plants, take cuttings; if you want to rescue a dying one, take cuttings from whatever is still healthy immediately rather than waiting to see whether it recovers.

Keep a short written record for each plant, because with a short-lived species the pattern only becomes visible across several of them. Note the date it was planted and where; when the first flowers opened and when the main flush finished; the date you sheared it and how it responded; when and how badly the center started to open up; whether it came through the winter and, if it did not, what the weather had been doing in the weeks before it collapsed. After two or three plants those notes tell you exactly how long a Rhodanthemum lasts in your conditions, and therefore what year to strike replacement cuttings so you are never without one.

Take cuttings while the parent is healthy, not when it starts to look ill. Material from a plant that is already rotting at the collar frequently carries the problem with it and fails in the pot.