Gadenin日本語Get the appWhen it is actually due
Sempervivum is not repotted because it has outgrown a pot in the usual sense. Its roots are shallow and modest, and a colony will happily fill a pan and stay there for years. What eventually forces the job is the mix: after two or three seasons the fine particles wash down, the grit compacts, and the surface stops draining fast enough. That is the trigger, and it comes round about every two to three years.
The other reason is crowding, but crowding is not automatically a problem. A pan packed edge to edge with rosettes is precisely how the plant looks best and colors best, because competition keeps every rosette small and stressed. Only break it up when the offsets are climbing over one another in layers, or when the center of the colony has died out and left a bare patch. Do the work in spring or early autumn, in the mild growing weather, never in the middle of summer heat or in a freeze.
The container
Pick width over depth every time. A pan two to three inches (5 to 8 cm) deep and as wide as you like is a better home than a conventional pot twice as deep, because it holds less water and dries out faster after rain. Terracotta and unglazed stoneware are better than plastic for the same reason. Alpine pans, seed trays with extra holes drilled, shallow bowls and stone troughs all work.
Drainage holes matter more than the material. If a container has one small hole, add more. Stand the finished pot on pot feet, tiles or a bed of gravel so the holes are never sealed against a hard surface, and never use a cache pot or a saucer for anything that stays outdoors.
The mix
Aim for something that looks and feels far too poor to grow anything in. Roughly half to two thirds coarse mineral material and the rest a low-nutrient compost is the target. Ordinary all-purpose potting mix used neat holds too much water, slumps within a season, and feeds the plant into producing soft green growth.
| Component | Proportion | Why it is in there |
|---|---|---|
| Coarse grit or crushed stone, 2 to 6 mm | About half the volume | Drainage, weight, and stability for a top-heavy colony |
| Perlite or pumice | Up to a quarter | Air pockets that survive compaction |
| Loam-based or all-purpose potting mix | A third at most | Enough body to hold the fine roots |
| Grit top dressing, quarter inch (5 mm) deep | Surface layer | Keeps the base of each rosette dry and clean |
| Fertilizer of any kind | None | Feeding costs color and produces rot-prone growth |
That last row is not an oversight. Houseleeks are one of the very few garden plants that are better for never being fed. If you have been feeding yours and the rosettes are large, soft and uniformly green, stop, and the color will return over the next season.
Doing the job
The whole operation takes twenty minutes and the plants are unusually forgiving of rough handling. Work dry, with dry mix and a dry plant, and do not water for several days afterwards.
- Lift the colony out dry Turn the pan over and let the whole mass drop into your hand. If the mix is dry it will break apart easily. Never soak a plant to loosen it first, because damaged roots plus wet mix is how rot begins.
- Break the colony into single rosettes Pull the mat apart with your fingers and separate the rosettes, snapping the connecting stolons. Discard any rosette whose base is brown, soft or hollow, and any that is stretching upward into a cone, because that one is about to flower and die.
- Trim and inspect the roots Shake all the old mix off and look closely at what falls out. Cut back long, dead, papery roots to an inch (2 to 3 cm) of live root. Any fat white grubs in the debris are vine weevil and every scrap of old mix must go in the bin rather than the compost pile.
- Set the rosettes at the right height Fill the pan with fresh mix, then plant each rosette so the lowest ring of leaves sits just clear of the surface. Burying the base of a rosette in damp mix is a direct route to rot. Space them so they will touch within a season rather than leaving room to spread.
- Top dress and leave dry Work a quarter inch (5 mm) of grit right up under the leaves of every rosette so no compost touches them. Then put the pan in a bright, sheltered spot and give no water at all for 5 to 7 days while the cut roots seal.
Afterwards, and what to write down
For the first fortnight keep the pan out of the hardest afternoon sun and out of heavy rain, then return it to full sun and treat it normally. Expect the rosettes to look slightly loose for a few weeks while new roots take hold. If a rosette lifts when nudged after a month, push it back and firm the grit around it rather than digging it up again.
Write the repotting date on a label pushed into the pan, along with the rough mix ratio you used. Two or three years from now, when you are wondering whether this pot is due, that label answers the question in a second. If you make up two pans with different amounts of grit, note which is which, and compare how each looks after its first winter. That single comparison will settle your own house recipe better than any general advice.