Grow and Care for Rock Cress

Grow rock cress in full sun, in gritty soil that drains. Rock cress is Aubrieta. Shops also say false rockcress or purple rock cress. White wall rock cress on a tag is usually Arabis.

Give it full sun and gritty soil that drains

rock cress is an evergreen mustard with small hairy gray-green leaves and four-petaled flowers in purple, violet, pink, red, or white. Give it full sun and soil that drains fast. Gritty, rocky, or sandy ground is the right home. Lime or chalk suits it, and lean ground is better than a rich, wet bed. [1] Once the roots take, water only when the soil is dry. Soggy soil around the crown is the primary cause of root rot. In a hot, wet summer the mat often thins, even when the winter was fine. [2]

Plant it in a wall or on a slope

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 mat sits on the face of the stone. Water until new roots hold. [3]

1. Plant in spring, in full sun, after the soil drains.
2. Set the crown at soil level. In a wall, press the root ball into a mix of soil and grit so the mat sits on the face of the stone.
3. Space plants 10 to 15 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.

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.

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 early summer

Non-flowering stem cuttings taken in early to mid-summer root easily in sand. Cuttings are more reliable than a packet of seed, and they work better than dividing a clump that is still small. If the mat is crowded, divide it every one to three years after flowering and replant the pieces in grit.

1. Take non-flowering stem cuttings in early to mid-summer, after you shear the mat.
2. Remove the lower leaves, stick the cuttings in sand, and keep the sand just barely moist until roots form.
3. Divide a crowded clump every one to three years after flowering, and replant the pieces at once.

Not wall rock cress — and not a lilac bush

White wall rock cress on a U.S. tag is usually Arabis. That plant makes a slightly taller mat with toothed gray-green leaves and white or pink flowers. This plant is Aubrieta. It makes a lower, denser cushion and flowers in purple, blue, red, or pink. See How to Grow Mountain Rock Cress if the tag says Arabis. [4]

A lilac bush is Syringa, a woody shrub with scented panicles — see How to Grow Lilacs if that is the plant you bought. Lilacbush printed on this tag is still rock cress. Yellow basket-of-gold is Aurinia — see How to Grow Basket-of-Gold if the flowers are yellow. Sweet alyssum is Lobularia — see How to Grow Sweet Alyssum if the tag is the annual edging. Candytuft is Iberis. It has smooth, dark, leathery leaves and flat flower heads, not a hairy gray-green cushion. Snow-in-summer on a shop list is usually Cerastium, a gray felted mat, not Aubrieta.

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. Creeping thyme is Thymus — see How to Grow Creeping Thyme if the tag is thyme, not rock cress.

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.