How to Grow Aubretia in Australia

Aubretia (Aubrieta × cultorum) is a mat-forming evergreen perennial in the cabbage family, Brassicaceae, grown for sheets of purple, blue and pink flowers in spring. It suits dry-stone walls, rockeries and pots in cool, temperate, Mediterranean and dry inland gardens, where it shrugs off frost and dry spells. Soil that stays wet is what most often kills it, because the crown rots.

Three mistakes that matter

1. Planting where water lingers. The crown (where stems meet roots) rots in soil that stays wet after rain, above all in humid summers and wet winters. Plant on a slope, a wall top or a raised rockery, with coarse sand or fine gravel mixed into the hole.
2. Expecting flowers in the first summer. A full flush needs a well-grown plant followed by several weeks of winter cold, so seedlings rarely bloom before their first winter. Sow in autumn for flowers the following spring, and treat a spring sowing as a plant for the next year.
3. Leaving the mat untrimmed. An unshorn mat turns straggly and often goes bald in the centre, with flowers only around the rim. Shear it by about half as the last flowers fade (see Trimming and Shaping).

When to plant

Seed germinates best when the mix sits at 18 to 21°C, and it will start from 15°C. After an autumn sowing, the young plant needs a winter chill to flower well. Growers hold it at 4 to 10°C for 6 to 10 weeks. One sowing is enough: the plant is perennial and flowers every spring once established.

Sow early in your zone's autumn window, so plants have 10 to 13 weeks to fill out before the cold. In most districts that means before Anzac Day, though Perth, where autumn heat lingers, sows into May.

Site, soil and setup

Light shade is tolerated but thins the flowering, except in hot inland and Mediterranean districts, where afternoon shade through summer protects the mat from scorch.

Check soil pH before adding lime, since alkaline ground needs none. Ground where puddles linger after rain is waterlogged soil, so plant high instead (see Three mistakes that matter).

Container growing

Aubretia grows about 10 to 15 cm tall and spreads 30 to 45 cm, trailing over the rim of a pot. Give one plant a pot or trough at least 40 cm across, holding about 20 L, with plenty of drainage holes. Fill it with potting mix blended half and half with coarse sand or fine gravel, then top it with gravel. Any of the aubretia seeds suit pots: 'Hybrida Blue', 'Hybrida Pink', 'Hybrida Purple' or the mixed colours.

• Water: when the mix is dry a third of the way down, fill to the rim, let it soak in and fill again. In hot spells that can mean every day or two.
• Feed: worm castings at planting, then fish and seaweed blend at 25 mL per 9 L fortnightly from early spring until flowering ends.
• Position: full sun from autumn to spring, moved into afternoon shade for summer in hot districts. Empty saucers within a few hours of watering.

Sowing Seeds

Start aubretia in a tray or punnet under cover, where you can hold warmth and moisture steady around the fine seed.

Starting under cover

• Fill a tray with fresh seed raising mix in a clean container and water it before sowing.
• Drop 2 or 3 seeds into each cell, or scatter them thinly, and press them onto the surface.
• Leave the seed bare or barely cover it with a sprinkle of fine mix, because light helps it germinate.
• Mist gently and keep the mix moist but not soggy; seedlings appear in 8 to 21 days.

Sowing direct

In the autumn window, seed can go straight into a fine, weed-free pocket of gritty soil where it is to grow. Mix it with dry sand so it spreads evenly (see sowing tiny seeds), press it onto the surface and mist whenever the surface dries.

Transplanting

• Prick out (lift and separate) seedlings into 8 cm pots once the seed leaves are firm enough to hold; handle them by the leaves.
• Plant out about 6 to 8 weeks after sowing, once roots hold the mix together.
• Harden off (accustom plants raised under cover to sun, wind and cooler air) over about 2 weeks.
• Space plants 25 to 30 cm apart, with the top of the root ball level with or just above the soil.
• Water in with seaweed solution at 30 mL per 9 L.

Care by growth stage

Drainage and a cold winter decide this plant, far more than feeding. Weeks count from an autumn sowing and shift between zones.

If growth stalls in very poor soil, scratch in a light dose of all-purpose controlled release granular (NPK around 14-5-10) in early spring. Grow seedlings in bright light at about 14 to 18°C, which keeps them short and sturdy.

Trimming and Shaping

Trim straight after flowering, while the plant has the whole growing season to make new shoots. Cutting later in the year can cost flowers the next spring, so leave the mat alone through winter.

• Young, compact mats: a light trim of up to 5 cm with shears, keeping the cushion tight.
• Established mats: cut back by about half, which stops seeding and closes up the centre.
• Straggly or bald mats: a hard cut to near the base, so fresh growth fills the gap.
• Stems kept for seed: leave a few near the edge uncut until the pods ripen, then trim them too.

Some varieties reward the trim with a light second flush of flowers.

Companion planting

The best partners share full sun, gritty soil and a dislike of wet feet.

Grows well with

• Arabis, another low wall and rockery plant that tolerates hot, dry conditions and poor soil.
• Houseleeks (Sempervivum), succulents that store water and want the same lean, fast-draining pocket.
• Dwarf bulbs, whose short upright stems rise through the low mat without competing for light.

Keep apart

• Cabbage-family vegetables, which host cabbage aphid (Brevicoryne brassicae) and white blister.
• Thirsty bedding plants, whose daily watering keeps aubretia crowns wet.
• Vigorous groundcovers, which run over the mat and smother it.

Flowering and Seed Saving

Flowering starts in spring after the first winter, when short stems rise above the leaves and the mat can almost disappear under colour.

• Watch the pods form along the stems left uncut (see Trimming and Shaping).
• Pick the pods on a dry day once they look ripe; unripe pods do not ripen once picked.
• Drop them straight into a paper bag and label it.
• Dry the bag somewhere warm and airy until the pods split and the seed falls free.
• Sieve out the chaff and blow away the dust.

Store the seed in a labelled paper envelope inside an airtight container, somewhere cool and dry. Bees move pollen between colours, so seed from a blue grown beside a pink may not come true. A cutting keeps a colour exactly (see Tips).

Common problems

Few or no flowers. A mild winter or a spring sowing gives the plant too little cold, and part shade thins the flush further; see When to plant.

Mat rotting after rain. Wet soil around the crown; see Three mistakes that matter.

Leggy seedlings. Weak light or too much warmth after germination stretches the stems. Read more

Brown, crisped patches in summer. Afternoon sun on a hot wall or dry pot scorches the mat; move pots and shade walls from December to February.

Pests and diseases

Insects rarely trouble aubretia; wet weather brings most of its problems.

Aphids (Aphididae). Soft insects up to 3 mm long that cluster on shoot tips and flower stems from late winter into spring. Squash small colonies or hose them off, and leave ladybirds, lacewings and hoverflies to finish the job. Read more

Grey mould (Botrytis cinerea). Fuzzy grey-brown mould spreading from dead flowers and leaves in cool, damp or dewy weather. Pick off spent flowers and fallen leaves promptly, and keep water off the mat.

White blister (Albugo candida). White, powdery pustules on leaf undersides and stems in wet weather, with distorted shoots. Cut off affected parts and bin them, since resting spores stay in the soil for several months.

Root and crown rot (Pythium and Rhizoctonia species). Shoots wilt and die back while the roots blacken in wet soil. Lift and bin dead plants, then replant somewhere with sharper drainage.

Damping off (Pythium, Rhizoctonia and other soil fungi). Seedlings collapse at soil level in wet, crowded, stale trays. Read more

Prevention first

• Plant into gritty, fast-draining soil with a gravel collar around each neck.
• Water the soil early in the day, never the mat, so leaves dry before evening.
• Clear fallen leaves and spent flowers off the mat after wet weather.
• Bin diseased material, because home compost heaps rarely get hot enough to kill spores.

Tips

1. To keep a favourite colour, take softwood cuttings 5 to 10 cm long after the spring trim. Under a clear cover they root in 2 to 4 weeks.
2. Mature mats often throw small plantlets around their edges; pot them up and plant them out once they have a good root system.
3. The flowers draw bees, so a mat on a wall beside the vegie patch brings them past the crops in spring.