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If a sedum dies, the odds are overwhelming that it rotted. The plant stores water, so it has no defense against a root system that is already saturated, and cold wet soil in winter is the worst case because the roots are inactive and cannot draw anything down. The tissue simply collapses.
You will notice it as a stem that goes soft and dark at ground level, or as a section of a creeping mat that turns brown and lifts away with no roots attached. There is no treatment for rotted tissue. The job is to cut it out, take healthy cuttings from anything still firm above the damage, and fix the drainage before you replant.
| Symptom | Cause | Response |
|---|---|---|
| Stem black and soft at soil level | Crown rot from wet soil | Cut the stem out entirely. Take cuttings from the firm top |
| Whole clump collapses after winter | Waterlogging while dormant | Replant elsewhere on raised, gritty ground |
| Brown patch in a creeping mat | Debris or fallen leaves held moisture | Clear the litter, cut out the dead, replug with rooted pieces |
| White powdery film on leaves and stems | Mildew in still, humid air | Thin crowded stems for airflow, water at soil level only |
| Small brown spots with a darker margin | Leaf spot after a wet spell | Remove affected leaves. It usually stops when it dries |
Vine weevil in containers
This is the pest that catches people out, because there is no warning above ground. Creamy white grubs with brown heads, curved into a C and about a third of an inch (8 mm) long, eat the roots through autumn and winter. The plant looks normal until spring, then wilts despite moist compost and lifts out of the pot with no roots left at all.
The adults are dull black beetles about a third of an inch long that feed at night and leave neat semicircular notches around the edges of leaves. Notched leaves are your only advance notice, so act on them. Any potted sedum standing outside is at risk, and a plant in the open ground far less so.
- Check the notches Look at leaf edges in late summer for U-shaped bites taken out of the margin. That means adults are present and eggs are going into the compost.
- Tip out and inspect In autumn, knock a suspect pot out and look through the root ball. Grubs are obvious against dark compost. Pick out every one you find.
- Repot into fresh mix Wash the roots clean, discard all the old compost rather than reusing it, and repot into a fresh gritty mix in a clean pot.
- Use a gravel mulch A half inch layer of gravel on the surface makes egg laying harder and is worth doing on every container anyway.
Aphids on the buds
Tall border sedum attracts aphids to the developing flower heads in early summer, exactly when the flat corymbs are forming. They cluster on the stem just below the head and on the tightly packed buds, where they are easy to miss until the surface below turns sticky with honeydew and then black with sooty mold.
A firm jet of water knocks most of them off, repeated every few days until they stop returning. Squash small colonies between finger and thumb. Ladybird larvae and hoverfly larvae clear them entirely if you leave them alone, so avoid spraying, particularly since sedum flowers are one of the best late-season nectar sources you can offer bees and butterflies.
Slugs, snails and mammals
The vulnerable moment is early spring, when the fat new shoots of a tall type push through the soil. They are soft, sweet and at ground level, and slugs can take a whole crown down to stumps in two damp nights. The plant regrows, but the stems will be uneven and late. Once the stems are a hand high and firmer, interest drops away sharply.
A ring of sharp grit or crushed grit around each emerging crown works well and is consistent with the drainage the plant wants anyway. Go out with a torch on the first mild damp evenings of spring and pick them off, which is the single most effective hour you can spend. Creeping mats are also grazed by rabbits and deer where those are present, usually in dry weather when the fleshy leaves are the wettest thing on offer.
| Pest | When it strikes | Sign | Control |
|---|---|---|---|
| Slugs and snails | Early spring shoots | Ragged stumps, slime trails | Grit ring, torchlight collecting on damp evenings |
| Vine weevil grubs | Autumn to spring, in pots | Wilting in moist compost, no roots | Repot into fresh mix, remove grubs by hand |
| Aphids | Early summer on buds | Sticky honeydew, sooty mold | Water jet, leave predators to work |
| Mealybugs | Any time, sheltered crowns | White woolly specks in leaf joints | Dab with a cotton bud dipped in alcohol |
| Rabbits and deer | Dry spells | Mat grazed flat, clean bites | Wire guard until established |
Knowing when to give up on a plant
Sedum is cheap to replace and trivial to propagate, so do not nurse a badly rotted clump. If the crown is soft when you squeeze it, the plant will not come back, and leaving it in place spreads the problem to neighbors through the wet ground it is sitting in.
Take cuttings from any firm stem tips first, since those root in two or three weeks and give you the same plant back. Then lift and dispose of the remains, improve the drainage of that spot with grit and a raised profile, and replant next spring.
Record what you found and the date, especially vine weevil grubs and the first slug damage each spring. Both are seasonal and repeat in the same containers and the same corners of the garden year after year, so last year's notes tell you exactly when and where to look.