Aubrieta on retaining walls: turn grey stone into a flowering feature

With the right perennial, a problem area can become a brilliantly colourful feature.

Many gardens have one: a plain retaining-wall barrier made from concrete blocks or natural stone, grey, angular and functional. It may be practical from a structural point of view, but it is rarely attractive. Especially in spring, as borders and lawns come into bloom, such a bare wall can look completely out of place. Yet one remarkably tough, purple-flowering cushion plant is enough to turn the wall into a living focal point in very little time.

The heroine of stone crevices: why Aubrieta loves walls

The plant in question is generally sold as Aubrieta, also known as rock cress. Its botanical name is Aubrieta deltoidea. This is a perennial, low-growing plant that forms dense cushions. It reaches around 10 to 15 centimetres in height, while spreading sideways across stones and joints.

From roughly mid-April into early summer, Aubrieta produces countless small flowers. Depending on the variety, these shine in violet, purple or pinkish violet. At the height of flowering, the foliage is barely visible: the entire plant becomes a single carpet of colour.

“A grey stone wall becomes a cascading veil of flowers that visually softens the hard edge completely.”

Aubrieta naturally grows on rocky slopes and in stony ground. That is precisely why it thrives on the top of a retaining wall. It prefers:

• poor, sandy and gravelly soil
• excellent drainage with no waterlogging
• plenty of sun and warmth
• dry spells once it is established

Its flexible shoots make their way into the smallest gaps, root there and then trail down in loose garlands. In this way, it gradually covers the wall without loosening or damaging the stones.

The best position: where Aubrieta thrives on a retaining wall

Anyone looking to green up a harsh, sparse part of the garden needs the right location. The following places are particularly suitable for a retaining-wall project with Aubrieta:

• the upper edge of a dry-stone wall
• the top of a concrete-block wall
• spaces between larger natural stones
• crevices in sunny stone stairways

A south- or west-facing position is ideal, giving the plant several hours of direct sunshine each day. East-facing sites can still work well, whereas north-facing walls tend to produce pale plants with few flowers.

It is essential that water can drain away freely. The roots can rot easily if rainwater or irrigation water collects in pockets behind the wall. A retaining-wall top that allows excess water to run off quickly is ideal.

Step by step: planting Aubrieta on the top of a retaining wall

Creating a flowering strip along a wall sounds more demanding than it actually is. A few simple steps will provide the foundation for years to come.

1. Choose the planting time carefully

Two periods are especially suitable:

• Spring: after the last hard frosts, once the soil has dried out
• Autumn: during a mild spell, allowing the plants to root before winter

In either case, ordinary weather conditions are sufficient for the young perennials to establish. Once settled, they need very little maintenance.

2. Prepare the soil

Create small planting pockets on top of, or directly behind, the wall so that the plants can take root. Proceed as follows:

• Briefly lift loose stones or lightly scrape out the joints.
• Loosen any old, compacted material.
• Fill the spaces with a mixture of garden soil, a little well-rotted compost and plenty of grit or gravel.

The mix can be relatively low in nutrients. Excessive fertility produces soft growth with fewer flowers.

3. Use small perennial plants rather than seed

For wall planting, young plants in pots-usually in 9 cm pots-have proved most reliable. Seeds often dry out in the crevices and germinate inconsistently.

When planting, follow these steps:

• Loosen the root ball slightly.
• Position the perennial so that it slopes gently towards the wall edge.
• Firm the soil well, ensuring that no air pockets remain.
• Water thoroughly once to secure good contact with the soil.

Afterwards, let the surface soil dry out between waterings. Aubrieta copes surprisingly well with short dry periods.

Low maintenance, but not entirely hands-off: keeping the flower carpet dense

Once Aubrieta is established, it requires little attention. However, it should not be left completely unattended, as the cushions will gradually become old and woody.

Pruning after flowering

Use shears immediately after the main flowering period in May or June. Cutting the shoots back firmly by around half their length encourages the plant to form fresh new cushions.

“Anyone who cuts back their Aubrieta once a year will be rewarded with a dense, rich green carpet and often a light second flush of flowers.”

The cut does not need to be precise to the millimetre. The key point is to shorten old, brown sections noticeably. It may look drastic at first, but vigorous new growth appears within just a few weeks.

Water, fertiliser and winter: what the plant can tolerate

• Watering: only needed during extended hot spells, particularly in its first year after planting.
• Fertilising: a light dressing of compost in spring is enough; complete mineral fertilisers tend to weaken the plant.
• Frost: Aubrieta survives even cold winters without protection, provided waterlogged soil does not freeze.

During snowless, very wet winters, a little leaf litter can serve as a loose cover to protect the roots from pressure caused by ice. Once the weather thaws, however, remove the leaves again to prevent rot.

Plants that combine particularly well with Aubrieta

A wall planting looks even more lively when one species does not dominate the whole display. Aubrieta pairs well with other drought-tolerant perennials.

The important thing when choosing companions is that they share similar requirements: poor, sunny and dry conditions. They should also not be too vigorous, so that they do not crowd out the Aubrieta.

Benefits for insects and the stability of the wall

The violet carpet is not merely attractive. It also supplies food and shelter for many insect species. Bees and hoverflies visit the flowers enthusiastically in spring, when nectar is not yet abundant everywhere.

At the same time, the rooting growth helps stabilise the uppermost layers of soil on the wall top. The plant is no replacement for structural calculations, but it does prevent every small amount of material from immediately washing away during heavier rain. This reduces erosion and loss of grit.

Common mistakes - and how to avoid them easily

People planting Aubrieta on a wall often encounter the same issues. Three problems appear time and again:

• Soil that is too nutrient-rich: this results in excessive foliage, fewer flowers and a greater susceptibility to disease.
• A position that is too shady: cushions become open and straggly, producing only scattered blooms.
• No pruning: plants become hollow in the centre and eventually fall apart.

Anyone who keeps these pitfalls in mind will rarely have serious trouble with Aubrieta on retaining walls. The plant even forgives the occasional missed watering or a few weeks of complete neglect.

Who will benefit from an “Aubrieta wall project”

This plant is especially interesting for homeowners whose gardens have steeper slopes supported by retaining walls. Rather than replanting containers every year, Aubrieta creates a permanent wall border that is almost self-maintaining.

Even those with only a few square metres of garden can achieve a surprisingly strong effect with a narrow strip of Aubrieta along a driveway or cellar steps. This makes use of the garden’s vertical space instead of leaving it as a grey wall.

There is another practical advantage: on slopes where conventional flower beds repeatedly dry out, these cushion plants provide lasting colour without needing constant watering. For anyone who does not want to stand on a slope with a hose every evening in summer, that is a noticeable benefit.