Look closely at Sedum forsterianum 'Oracle' and each shoot resembles a tiny blue spruce. The needle-thin leaves are arranged in tight whorls, and together they make a soft, cool grey-blue carpet that holds its colour all year and warms to pink and plum when the frosts arrive.
The parent species, rock stonecrop, is one of the few stonecrops native to Britain, growing wild on cliffs and rocky ground in Wales and the west of England. Here you will learn how to recreate those conditions, how to keep the mat dense and tidy, how to divide it and which plants make the best partners.
Sedum forsterianum 'Oracle' at a glance
- Botanical name Petrosedum forsterianum 'Oracle' (synonym Sedum forsterianum 'Oracle')
- Common name Rock stonecrop, Oracle stonecrop
- Plant type Evergreen, mat-forming succulent perennial
- Hardiness RHS H4 for the species (about -10C), though it tolerates much harder frost in well-drained soil
- Height and spread 10 to 15cm in leaf, flower stems to about 20cm, spreading 30cm or more
- Flowering Flat heads of starry yellow flowers in early to midsummer, roughly June to July
- Foliage Needle-like blue-grey leaves in spruce-like whorls, tinged pink to purple in winter
- Position Full sun
- Soil Free-draining, poor to moderately fertile, tolerates acid or alkaline
- Good for Gravel gardens, wall tops, rockeries, pots and green roofs
Colour through the seasons
Spring growth is a fresh glaucous blue, and by June the mat is studded with upright stems carrying flat clusters of bright yellow stars. These are a welcome source of nectar for bees and hoverflies during early summer.
After flowering the foliage returns to a steady blue-grey. As nights cool in autumn, the shoot tips flush rose, then deepen to purple through winter, giving welcome colour when most of the garden has gone brown.
The right spot for rock stonecrop
Give it as much sun as you can. In full sun the whorls stay compact and the winter colouring is richer, while in partial shade the stems lengthen and the mat can look thin by midsummer.
Natural rock stonecrop grows on exposed, well-drained ledges, so think tops of walls, sunny banks and gravel. It is unfussy about pH, which makes it useful on both chalky Lincolnshire soils and more acidic sandy ground.
Planting in soil, gravel or pots
Plant between April and September when the soil is workable. On heavy ground, raise the planting area or mix in coarse grit, aiming for soil that drains within minutes after a downpour.
In containers, a mix of two parts peat-free compost to one part grit suits it well. A wide, shallow pot gives the creeping stems room to spread and spill over the rim.
- Space plants 25 to 30cm apart for groundcover.
- Set the crown level with the soil surface, never buried.
- Water in once, then top-dress with gravel to keep the stems off damp soil.
Go easy on water and fertiliser
After its first summer, Oracle in open ground rarely needs watering at all. The succulent needles hold enough moisture for long dry spells, and over-watering causes more losses than drought ever does.
Pots need a soak whenever the compost feels dry to the touch in summer, and almost nothing in winter. Skip fertiliser in the ground. A half-strength general feed once in late spring is enough for plants that have been in the same container for a while.
Trimming and keeping it tidy
Once the yellow flowers fade, the stems turn straw coloured. Cut them back to the foliage with shears in late summer and the mat looks fresh again within a couple of weeks.
Older clumps can go woody and bare in the middle after three or four years. When that happens, lift the plant in spring, discard the tired centre and replant the vigorous outer pieces.
Dividing and rooting cuttings
The trailing stems root wherever they touch the ground, so new plants are easy to come by. Spring and early summer are the best times, giving young pieces months to establish before winter.
- Cut 5cm tips from healthy shoots and strip the lower whorls of needles.
- Leave them to callus for a day, then insert into a tray of gritty compost.
- Alternatively, lift a rooted section from the edge of the mat with a trowel and replant it straight away.
- Keep cuttings in bright light and water only when the compost dries out.
Winter wet and other troubles
Cold alone rarely harms it, but heavy soil that stays waterlogged from November to March can rot the roots. If patches turn brown and mushy, improve the drainage and replant healthy pieces in spring.
In pots, watch for vine weevil, which eat the roots and leave the plant loose in the compost. Aphids may gather on flower stems in warm weather and are easily hosed away. Sedums are generally considered low risk to pets and children, though the plant is not intended for eating.
Planting partners for blue foliage
The cool blue tones look superb against dark slate chippings and sit well beside purple and burgundy plants. Try it next to Sedum spathulifolium 'Purpureum', bronze sempervivums, Heuchera 'Palace Purple' or deep red Sedum 'Dragon's Blood'.
In a hot, dry border it makes a soft edge in front of lavender, Nepeta and Erigeron karvinskianus. For a wider choice of plants that suit these conditions, see our guide to the best drought-tolerant plants for UK gardens.
Questions about Sedum forsterianum 'Oracle'
Is Sedum Oracle the same as Petrosedum forsterianum?
Yes. Botanists moved rock stonecrop and its relatives into the genus Petrosedum, so you will see both names on labels. 'Oracle' is a selection of the species chosen for its particularly blue foliage.
Does Sedum forsterianum Oracle stay blue all year?
It keeps its grey-blue base colour all year, but the tips change with the seasons. Expect pink and purple tints from autumn through winter, especially in a sunny, exposed position, followed by fresh blue growth in spring.
How do I stop Sedum Oracle going leggy?
Legginess usually means too much shade or soil that is too rich. Move it to full sun, avoid feeding, and trim back stretched stems after flowering. Replanting the outer rosettes every few years also keeps the mat dense.
Is rock stonecrop native to the UK?
The species Petrosedum forsterianum is native to parts of Britain, mostly on rocky ground in Wales and western England. 'Oracle' is a garden selection, but it shares the same tough, drought-tolerant nature.
If you would like a carpet of blue that changes colour with the seasons, Oracle is an easy and rewarding choice. You can find it, along with plenty of other creeping and upright stonecrops grown on our Lincolnshire farm, in our Sedum collection.