Grow and Care for Mountain Rock Cress

Grow mountain rock cress in full sun, in gritty soil that drains. Mountain rock cress is Arabis. Shops also say wall rock cress. Purple rock cress on a tag is usually Aubrieta.

Give it full sun and gritty soil that drains

mountain rock cress is an evergreen mustard with toothed gray-green leaves and tiny four-petaled white flowers in spring. Give it full sun and soil that drains fast. Gritty, sandy, or rocky ground is the right home. Alkaline soil suits it, and lean ground is better than a rich, wet bed. [1] Once the roots take, water only when the soil is dry. Wet clay around the crown is the usual cause of rot. In a hot, wet summer south of Zone 6 the mat often thins, even when the winter was fine. [2]

Plant it in a wall or on a slope, then leave the roots alone

Set it where water cannot sit at the crown: a stone wall, a bank, or a rock-garden pocket. Put the crown at soil level. In a wall, press the root ball into a gritty pocket so the rosettes sit on the face of the stone. Water until new roots hold. Then leave it. This plant does not like a later move.

1. Plant in spring or early fall, in full sun.
2. Set the crown at soil level. In a wall, press the root ball into a mix of soil and grit so the rosettes sit on the face of the stone.
3. Space plants 12 to 18 inches apart so the mats can meet without smothering a tiny alpine next to them.
4. Water until the planting is established, then let the soil dry between waterings.
5. Leave the plant in that spot. Do not lift and move the roots later.

Grow it in a trough that drains

A shallow trough or a terracotta pot with a hole in the bottom can hold one mat on a patio. Use a gritty mix, not a peaty mix that stays wet. Water when the mix is dry, and dump any water that sits in a saucer. This is an outdoor wall plant, not a houseplant.

1. Choose a trough or terracotta pot with a hole in the bottom.
2. Fill it with a gritty mix, and plant one mat.
3. Water when the mix is dry. Empty standing water from the saucer.

Cut the mat back after the flowers fade

When the spring flowers finish, cut the mat back. Spent stems left in place turn into long bare branches with leaves only at the ends, and the center of the clump opens up. Cut it after bloom, not in fall. A fall cut leaves the evergreen rosettes open to winter wet. [3]

1. Wait until the flowers fade, usually late spring.
2. Cut the mat back by about half, or at least shear off the spent stems and the long shoots.
3. Do not prune in fall. Leave the evergreen rosettes through winter.
4. If the center has already gone bare, divide the clump after bloom, or take cuttings and replant the fresh pieces.

Root a stem cutting in spring

Non-flowering stem cuttings taken in spring root easily in a sandy mix. Cuttings are more reliable than a packet of seed. If the mat is crowded, divide it every two or three years after flowering, or in fall, and replant the pieces in grit.

1. Take non-flowering stem cuttings in spring, after you shear the mat.
2. Stick them in a sandy mix that drains, and keep the mix just barely moist until roots form.
3. Divide a crowded clump every two or three years after flowering, and replant the pieces at once.

Not purple rock cress — and not basket-of-gold

Purple rock cress on a U.S. tag is usually Aubrieta. That plant makes a lower, denser cushion and flowers in purple, blue, or red. Mountain rock cress is Arabis. The flowers are white, or sometimes pink, on a slightly taller mat with gray-green toothed leaves. [4]

Yellow basket-of-gold is Aurinia. It is not this white mat, and it is not sweet alyssum. Sweet alyssum is Lobularia — see How to Grow Sweet Alyssum if that is the annual edging on the tag. Candytuft is Iberis. It has smooth, dark, leathery leaves and flat flower heads, not a gray-green toothed rosette. Snow-in-summer on a shop list is usually Cerastium, a gray felted mat, not Arabis.

Saxifrage is a different rock-garden genus. See How to Grow Saxifrage if the tag says Saxifraga. Moss phlox is Phlox — see How to Grow Phlox if you bought a creeping phlox, not this mustard mat.

Set the next plant in a sunny wall pocket or on a slope, in grit, water until it roots, then cut the mat back after the flowers fade.