Gadenin日本語Get the appKnow which of the two you are growing
Everything sold as sedum or stonecrop falls into two habits, and they want different places. The creeping kinds run flat along the ground, root as they go and rarely reach more than four inches (10 cm) tall. The tall kinds make an upright clump of stems eighteen to twenty-four inches (45 to 60 cm) high that carries a broad flat flower head in late summer. If you know which you have, most of the advice below sorts itself out.
The quickest test is in early spring. Creeping types come through winter as visible evergreen or semi-evergreen mats with leaves already on them. Tall types die back to a knuckle of fat buds at ground level and push up new shoots from nothing.
| Habit | Height | Best position | Typical failure |
|---|---|---|---|
| Creeping mat types | One to four inches (2 to 10 cm) | Gravel, wall tops, roofs, path edges, containers | Thins and goes bare where shaded or wet |
| Tall upright border types | Eighteen to twenty-four inches (45 to 60 cm) | Open sunny border, back of a gravel bed | Splits open in the middle when the flowers form |
Full sun, and more of it than you think
Six hours of direct sun is the working minimum for both habits, and eight is better. Sedum stores water in its leaves, so it does not compete for light by growing broad leaves the way most border plants do. It simply stretches. Stems get longer between the leaves, the tissue is softer, and a soft stem cannot hold up a flower head.
Morning sun is worth more than the same hours in the afternoon in a hot climate, because the plant dries off overnight dew early and the leaf surface stays healthy. In cool, damp summers, put it in the brightest and most open spot you have, where air moves across it. A sedum in the lee of a fence in still air is a sedum that gets mildew.
Leaves that look gray, blue or purple in a catalogue and green in your garden are telling you about your light. The waxy bloom and the red pigments both develop under strong sun and heat, and they fade in shade.
Why the tall ones fall open in the middle
This is the single most common complaint about border sedum, and it is not a disease. The clump grows well, the buds form, and then as the flower heads gain weight the stems lean outwards from the center until the plant is a ring with a hole in it. Two things cause it, and they usually happen together.
- Too much shade Anything under about six hours of sun stretches the stems. Look at the spacing between leaf pairs on last year's dead stems: wide gaps mean the plant was reaching. Move it in autumn or early spring to somewhere more open.
- Soil that is too rich Manure, a general fertilizer, or a bed that gets fed for the roses either side will all push soft growth. Stop feeding it entirely. On a fed border, the plant flops every year until you stop.
- Too much water at the wrong time Irrigation in early summer, when the stems are extending, produces long weak growth. Water an established clump only in a real drought, and not at all once buds show color.
- Cut it back in early summer Where you cannot change the position, shorten every stem by a third to a half in early summer, before the flower buds form. It branches below the cut, comes back shorter and sturdier, and flowers a week or two later on smaller but more numerous heads. This works reliably and is the standard fix.
Drainage decides everything else
Sedum survives drought easily and dies in winter wet. The roots are shallow and fine, and they rot in cold saturated ground faster than in warm. If your soil sits in puddles after rain in winter, no amount of sun will save the plant.
Fix it by raising the planting position rather than by digging in organic matter, which holds water. Mound the bed four to six inches (10 to 15 cm), work in grit or coarse sand until the mix crunches, and top the surface with half an inch of gravel so the crown and the lower leaves never sit on wet soil. Creeping types in particular rot at the point where a stem touches damp ground under a fallen leaf.
| Site | Verdict | What to do first |
|---|---|---|
| South-facing gravel bed | Ideal for both habits | Plant and leave alone |
| Top of a dry wall or a raised edge | Ideal for creeping types | Push rooted pieces into a gritty crevice |
| Open border, average garden soil | Good for tall types | Add grit, stop feeding the area |
| Under a deciduous tree | Poor | Autumn leaf fall smothers mats. Sweep or move it |
| Low ground, wet in winter | Unsuitable | Raise the bed or grow it in a pot instead |
| Container with a saucer under it | Unsuitable as it stands | Take the saucer away and raise the pot on feet |
Containers, walls and roofs
Creeping sedum is one of the few plants that grows in less than an inch of substrate, which is why it covers green roofs. In a container, it does not need depth so much as it needs to dry out. Use a shallow, wide pot in terracotta rather than a deep plastic one, a mix of two parts loam-based compost to one part grit, and no saucer.
Tall types in pots need a heavier container than they look like they need, because a full clump in flower is top heavy and one gust of wind on a light plastic pot will lay it flat. A twelve inch (30 cm) pot suits a single clump for two or three years before it needs splitting.
Write down where each clump sits and how much sun that spot actually gets through the day, checked once at midsummer. Flopping is a position problem, and next year you will not remember whether the bed got four hours or seven.