Wall rock cress

Wall rock cress: how do you recognise it?

Habit

Low, spreading mats about 20 cm tall and up to 50 cm wide, with sprawling stems.

Leaves

Small, obovate leaves with a toothed edge, grey-green and softly hairy; evergreen.

Flower

Loose clusters of small, four-petalled flowers 1–1.5 cm across, white or pink depending on the cultivar, from March to May.

Scent

The flowers have a light, sweet scent, especially on a sunny, still day.

Fruit

Narrow, flat seed pods (siliques) with small seeds, typical of the cabbage family.

Roots

Shallow and finely branched; they like to creep into the gaps between stones.

Flowering: March–May · Plants: March–April and September–October · Pruning: May–June

Why grow it?

• One of the first plants to flower each year: a white carpet in the rock garden from March to May.
• Early nectar and pollen for bees, bumblebees and the first butterflies.
• Evergreen: the grey-green cushion stays visible all winter.
• Drought tolerant and ideal for walls, steps, edges and the cracks between stones.
• A fast ground cover that fills bare patches and leaves little room for weeds.

Wall rock cress: where does it grow best?

• Full sun gives the best flowering; in shade the cushion becomes loose and flowers poorly.
• Poor, free-draining soil: sandy or loamy, acid, neutral or alkaline. Chalky soil suits it perfectly.
• Dry to moderately moist; it dislikes wet feet in winter.
• Ideal on and against walls, in rock gardens, on banks and between steps or paving.
• Plant 30–40 cm apart, about 6–8 plants per m².

Light

Sun

Soil type

Sand or loam

Acidity

Not fussy

Moisture

Dry or normal

Size

0.2 m tall, 0.5 m wide

Hardiness

Hardy (to -30 °C)

Goes well with

• Aubrieta: Purple and blue cushions next to the white, with the same flowering time and site.
• Basket of gold: Golden yellow next to white, spilling over a wall together.
• Moss phlox: Pink and lilac shades in the same sunny rock garden.
• Species tulips: Small, colourful tulips that push up through the white carpet.
• Grape hyacinths: Blue flower spikes in April beside the white cushions.

Wall rock cress: when and how to plant it?

Plant in March–April and September–October.

1. Plant in March–April or in September–October, when the soil is neither frozen nor waterlogged.
2. Loosen the soil and, on heavier ground, mix in coarse sand or fine gravel for good drainage.
3. Dig a hole slightly wider than the pot. Do not add fertiliser or fresh compost.
4. Set the plant at the same depth as it was in the pot. In a wall, slide the root ball into a crevice with some sandy soil.
5. Firm the soil and water in. In the first year, only water during long dry spells.
6. A mulch of gravel or grit keeps the base dry and the leaves clean.

Care

Watering

Rarely needed once established. Only during long droughts in the first summer or when grown in a pot.

Feeding

Not needed: on rich soil it becomes floppy and flowers less. At most a thin layer of compost in March on very poor soil.

Trimming

Shearing the whole cushion after flowering keeps the plant compact and dense (see Pruning).

Rejuvenating

After 3–4 years the centre often goes bare. Divide the plant or replace it with young cuttings.

In a pot

Use potting compost mixed with one third coarse sand and make sure there is a drainage hole.

Tip: Straight after flowering, shear the cushion back by half with a small pair of hedge shears: it stays dense and fills out again with fresh leaves before summer.

Wall rock cress: when to prune it?

Pruning months: May–June.

When: Straight after flowering, in May–June.

How:

• Cut or shear off all the faded flower stems and reduce the cushion by about half.
• A small pair of hedge shears makes quick work of it; do not cut into the bare, woody parts.
• Shorten long, sprawling stems that grow over neighbouring plants.
• In March, remove any dead or rotten patches from the cushion.

Caution: If you skip the trim, the cushion opens up after a few years and the centre goes bare.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Cuttings

In June–July, take 5 cm cuttings from non-flowering shoots, insert them in sandy cutting compost and keep it slightly moist. They root within a few weeks.

Division

In September or March, lift a cushion, split it into pieces with their own roots and replant straight away.

Wall rock cress: how does it get through the winter?

Hardy to −30 °C.

• Fully hardy, to about −30 °C: no protection is needed in Belgium.
• The biggest danger is a wet winter: give it free-draining soil, or plant it on a slope or a wall.
• Remove fallen leaves from the cushion; they hold moisture and make the foliage rot.
• In a pot: stand it under a roof overhang or against a wall, so the compost does not get waterlogged.

Diseases and pests

• Bare centre: The middle of the cushion dies and turns brown. Trimming every year after flowering and replacing old plants in time helps.
• Rot: Brown, soft patches in a wet winter or on heavy soil. Gravel in the soil and a dry mulch prevent it.
• White blister: Glossy white pustules on leaves and stems. Cut off affected parts and put them in the household waste, not on the compost heap.
• Aphids: On young shoots in spring. Natural predators usually deal with them; otherwise spray them off with water.

Through the year

• January: Rest. The evergreen cushion stays visible.
• February: Remove fallen leaves from the cushion.
• March: First flowers. Planting. Cut out dead patches.
• April: Full flower, busy with bees. Planting.
• May: End of flowering. Then trim back hard.
• June: Trim back if not yet done. Take cuttings.
• July: Take cuttings. Water young plants in dry weather.
• September: Planting and dividing.
• October: Planting. Top up the gravel mulch.
• December: Rest. Make sure no water stands on the plant.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Butterflies

Nectar for butterflies.

Did you know?

• Wall rock cress belongs to the cabbage family, like cabbage and radish: its four petals form a cross.
• Botanists sometimes treat it as a subspecies of alpine rock cress (Arabis alpina).
• Its Dutch name, ‘rijstebrij’ (rice pudding), probably refers to the white carpet of flowers that covers the cushions in spring.