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Early spring, as the new shoots appear and before they extend, is the best time for both habits. The plant is about to grow, so damaged roots replace themselves within weeks and a divided clump closes the gap the same season. Early autumn is a workable second choice in mild climates, leaving six weeks of root growth before the cold, but in a wet or cold winter area an autumn division is a risk not worth taking.
Do not repot a tall type in mid summer while it is building flower heads. It will sulk, the flowers will be poor, and the stems will flop from the disturbance. If a pot is clearly too small in summer, water it and wait.
| Signal | Habit affected | Action |
|---|---|---|
| Clump flops open every year despite full sun | Tall upright types | Divide in early spring, replant the outer pieces |
| Flower heads smaller than in previous years | Tall upright types | Congested clump. Divide |
| Bare woody center with growth only at the edges | Creeping mat types | Lift, discard the old middle, replant rooted edges |
| Roots circling out of the drainage holes | Both, in pots | Repot one size up in spring |
| Pot dries out within two days in summer | Both, in pots | Pot bound. Repot or divide |
| Growing well and flowering well | Both | Leave it. Sedum resents pointless disturbance |
Pot size and shape
Sedum roots run sideways in the top four to six inches (10 to 15 cm) of soil and rarely go deeper. A deep pot gives you a column of compost below the roots that stays wet after every watering, and that is where rot starts. Choose width over depth: a shallow bowl or a half pot, six to eight inches (15 to 20 cm) deep, suits almost everything.
Go up one size only, an inch or two (3 to 5 cm) more across the top. Overpotting is the classic mistake because the plant looks small in its new pot and the surplus compost holds water it cannot use. Terracotta is better than plastic here because it breathes and dries out, and for a tall type it also adds the weight that stops the pot blowing over in flower.
The mix
Aim for something that crumbles apart in your hand rather than holding together in a ball. A reliable starting point is two parts loam-based potting mix to one part coarse grit or perlite, plus a handful of extra grit for creeping types, which want even sharper conditions than the tall ones.
Do not add manure, worm castings or a controlled-release fertilizer. Sedum grown lean is compact, well colored and self-supporting. The same plant fed is floppy, pale green and prone to mildew, and there is no way to undo the feeding once the growth is soft.
| Component | Proportion | Purpose |
|---|---|---|
| Loam-based potting mix | Two parts | Structure and a small reserve of nutrients |
| Coarse grit, three to six mm | One part, more for creeping types | Free drainage and air at the roots |
| Gravel top dressing | Half an inch (1 cm) over the surface | Keeps the crown and lower leaves off wet compost |
| Fertilizer of any kind | None | Feeding causes flopping and soft growth |
Doing the job
The work is quick, and the plant is far tougher than it looks. Broken pieces are not waste, so keep a tray beside you as you go.
- Let it dry out first Do not water for several days beforehand. Dry compost falls away from the roots cleanly and dry tissue bruises less, so there is less to rot afterwards.
- Lift and shake Tip the plant out or lift it with a fork from well outside the clump. Shake and tease away most of the old compost so you can see what you are working with.
- Split it For a tall type, pull or cut the crown into pieces with three to five buds and a fist of root each. Throw away the woody, budless middle. For a creeping mat, simply tear off rooted sections the size of your palm.
- Trim damaged roots and pause Cut off any black or mushy roots to clean white tissue. Leave the divisions in a shady, airy place for a few hours so the cuts dry over before they meet compost.
- Replant at the same depth Set the crown level with the surface, never buried. Firm gently, top with gravel, and stand the pot somewhere bright and sheltered.
- Wait a week before watering Water lightly once after about a week, then return to the normal regime. Watering a freshly cut root system immediately is the usual reason a division rots.
Afterwards
Expect a divided tall type to be shorter and to flower slightly later in its first year, which is a benefit rather than a problem since it will stand upright. A creeping mat will look sparse for a month and then knit together. Neither needs feeding to recover.
Note the date you divided each clump and how many pieces you took. Tall sedum needs it again roughly every three to four years, and the interval is much easier to keep track of on paper than by looking at a plant that has declined so slowly you did not notice.
Every scrap left over will grow. Push spare pieces of creeping sedum into gravel, wall crevices or the gaps between paving before you throw anything away.