Gadenin日本語Get the appA dying rosette is often not a problem at all
Before treating anything, check whether the rosette in trouble had stretched upward into a tall cone. Each sempervivum rosette flowers once and then dies. In its final season it elongates into a thick column, opens a head of starry pink or greenish flowers, and collapses over the following weeks. This is how the plant is built and nothing you did caused it.
The clue that separates this from a genuine problem is the direction of failure. A flowering rosette dies from the top down, slowly, while its ring of offsets around the base stays firm and green. A rot or a pest kills from the base upward, quickly, and usually takes the offsets nearest it too. Cut the spent flower stem out at the base, pull away the dead rosette, and let the offsets fill the gap.
Vine weevil, the one that actually kills colonies
Adult vine weevils are dull black beetles that feed at night and notch the edges of leaves. The notches are cosmetic. The damage is done by the larvae, fat white C-shaped grubs with a tan head, up to a third of an inch (8 mm) long, which live in the mix and eat the roots and then the base of each rosette from below. Container-grown houseleeks in a peat-based mix are a favorite target.
The symptom is unmistakable once you have seen it. A rosette that looks perfectly healthy lifts straight out of the pan with no resistance and no roots, and the underside is hollowed into a shallow cup. Often several rosettes in one pan go the same way within a fortnight. If you can pick up a rosette like a coin off a table, tip the whole pan out and search the mix.
The response is thorough rather than clever. Empty the container, pick out and destroy every grub you find, discard all the old mix rather than reusing it, wash the pan, and replant the sound rosettes into fresh gritty mix. A high proportion of mineral grit in the mix makes the container far less attractive to egg-laying adults, which is one more reason not to grow these in ordinary compost. Biological control with parasitic nematodes, watered into the mix in mild soil above 41F (5C), works well as a follow-up in late summer or early autumn.
Rot, and how to read it early
Rot is the other reliable killer and it is almost always a winter problem in a mild wet climate rather than a cold one. Water sits in the tightly packed center of a rosette, temperatures hover just above freezing, air does not move, and bacterial and fungal rot takes hold in the crown. From the outside the first sign is a rosette that has lost its glossy firmness and looks slightly translucent at the base.
There is no treatment once the crown has gone soft. What you can do is act on the first rosette rather than the fifth. Pull out the affected rosette entirely, take away the mix immediately around it, and improve the conditions for the rest: tilt the pan so water runs out, move it somewhere windier and sunnier, put an open-sided cover over it, and stop watering completely until spring.
The full symptom list
Most of what appears on a houseleek falls into a short list, and the right response is usually environmental rather than chemical.
| What you see | Likely cause | What to do |
|---|---|---|
| Rosette elongates into a tall cone, then flowers | End of that rosette's life, entirely normal | Cut the stem out afterwards and let the offsets fill in |
| Rosette lifts out with no roots, hollow underneath | Vine weevil grubs | Tip out the pan, destroy grubs, discard all old mix, replant in fresh grit |
| Base turns translucent then brown, leaves fall away | Crown or root rot from standing wet | Remove it, cover the pan, stop watering until spring |
| White fluff in the leaf joints and around the roots | Mealybug | Dab with alcohol on a cotton bud, repeat weekly, repot if roots are infested |
| Rusty orange spots and pustules on older leaves | Rust, encouraged by still damp air | Remove affected leaves, increase spacing and air movement |
| Rosettes pale, loose and stretched sideways | Not enough light | Move to full sun over a fortnight to avoid scorch |
| Whole rosettes torn out and scattered on the surface | Birds or squirrels working the loose grit | Push them back in, weight the surface with larger stones |
Prevention, and the notes that make it work
Nearly every entry in that table is prevented by the same three habits: a mix that is mostly grit, a position in full sun with moving air, and no water from mid-autumn to early spring. There is no spray program worth running on this plant, and a healthy starved houseleek in the right place goes years without incident.
Keep a short log of losses rather than of treatments. For each rosette you lose, note the date, the pot, and which of the three patterns it matched: flowered and died, lifted with no roots, or went soft at the base. After one winter that list is unambiguous. If most losses are hollow rosettes, you have a weevil problem in a particular container. If most went soft in the same fortnight of drizzle, the fix is a cover and a windier spot, and you will know to put it in place a month earlier next year.