How to Propagate Sempervivum from the Offsets Around Each Rosette

Why this plant is the easiest thing in the garden to increase

The common name of the genus means always living, and it earns it in an unusual way. The individual rosette is not long lived at all, but it surrounds itself with children before it goes. Through spring and summer a mature rosette pushes out short horizontal stems, called stolons, each ending in a miniature rosette that sits on the surface and starts putting roots down into the grit beneath it. A vigorous plant makes anywhere from three to a dozen in a season.

This means propagation is not really a technique so much as a harvest. There is no rooting hormone, no propagator, no bottom heat and no waiting for callusing. You take a small plant that is already rooting, put it somewhere else, and it carries on.

Taking and planting the offsets

The best time is spring or early autumn, in mild growing weather. Offsets can be moved at almost any point in the growing season, but ones taken in the middle of high summer heat sit and sulk, and ones taken in late autumn have no time to root before winter wet arrives.

1. Pick offsets that are already anchored Look for ones about the size of a large coin that have made contact with the surface and put out white roots. An offset still dangling in the air on a stolon will root too, but takes several weeks longer than one that has already started.
2. Detach with a clean snap or a cut Follow the stolon back to the parent and cut or snap it, leaving a short stub attached to the offset rather than a long trailing stem. Keep any roots that come with it and do not wash them.
3. Let the cut dry for a day Leave the offsets on a dry surface out of direct sun for 24 hours so the cut stub seals. Skip this and the wound sits wet in the mix, which is the one way this easy job goes wrong.
4. Set them into gritty mix Push each one into a shallow pan of the usual half-grit mix so the base of the leaves sits just above the surface, not buried. Space them an inch or two (3 to 5 cm) apart, which is close enough that they will knit into a mat within a season.
5. Withhold water for a week, then treat as adults Give no water for 5 to 7 days, then a single soak, then return to the normal dry-out-completely routine. Keep the pan in bright light but out of the fiercest afternoon sun for the first fortnight.

When the parent is about to flower

There is one moment when taking offsets is urgent rather than optional. If a rosette starts to stretch upward into a tall cone, that rosette is going to flower, and after flowering it will die. It cannot be stopped and it is not a disease. What it can do is take the offsets around it with it if the whole clump is disturbed at the wrong moment.

The right response is to leave the flowering rosette alone until the display is over, then cut the dead stem out at the base and pull the collapsed rosette away. Underneath it you will find a ring of offsets that have been growing all along, and those are your replacement plants. If the gap left in the middle of a pan looks bare, lift two or three offsets from the edge and plant them into the hole.

A stretching, cone-shaped rosette is not a sign that you did something wrong. Flowering is the natural end of a mature rosette, and every colony loses a few each year while gaining many more.

Seed, and why most people skip it

Sempervivum does set viable seed, and sowing it in spring on the surface of a gritty mix at around 60F (15C) gives germination in two to four weeks. The catch is time and predictability. A seedling takes two to three years to reach a size worth looking at, and because the genus crosses freely, seedlings from a garden plant look nothing like their parent.

That unpredictability is exactly why some growers do it, since raising seed is how new forms appear. If you want more of the plant you already have, take offsets. If you want surprises and have three years to spare, sow seed and pot on the strongest seedlings once they are the size of a fingernail.

Keeping track of what came from where

Houseleeks look confusingly similar once they have been spread around a garden, and a colony you assembled from several sources becomes an unlabelled mystery within two years. Note the date each batch of offsets was taken, which parent pan or patch they came from, and where they went. A pencil label pushed into the pan is enough.

It also pays to note when a rosette starts to stretch for flowering, because after a few years those notes show a pattern. Most rosettes flower in their third or fourth season, and knowing roughly where each of your clumps is in that cycle tells you which pans will need refilling with offsets next year and which are safe to leave alone.