Why compact asters suit a patio

Asters have a reputation for flopping, mildewing and generally misbehaving in the open border. In a pot, on a warm paved patio, the rules change. The plants stay within reach, you control the moisture far more precisely, and the late flowers — soft lilac, deep violet, magenta, pink, even white — arrive exactly when the rest of the garden is winding down. From mid-September into October, a few well-grown pots will carry more colour than most of the beds put together.

The trick is to treat them as container plants first and perennials second. That means choosing dwarf varieties, using a compost that holds moisture without turning to sludge, and watering with real consistency through August and September, when the plants are flowering and their roots are filling the pot.

Choosing dwarf varieties that stay upright

Go for the Aster dumosus types and the shorter Symphyotrichum novi-belgii cultivars. These naturally form tight, bushy mounds of 30–45cm, which sit well in a pot and do not need staking in the way that taller asters do. Taller selections such as ‘Little Carlow’ are lovely plants, but they belong in a border with neighbouring perennials to lean against.

• Growth habit: look for “dwarf”, “compact” or “mounding” in the description, and a height under 50cm.
• Flower density: dwarf asters that flower freely — dozens of small daisies rather than a few large blooms — give the best container display.
• Disease resistance: choose varieties noted for resistance to powdery mildew, which is the main scourge of asters in a damp UK autumn.
• Colour planning: lilac, violet and magenta sit beautifully against copper and bronze foliage; white forms light up a shady corner.

Compost, pots and planting

Use a loam-based compost, such as a John Innes No. 2 for planting and No. 3 for a mature specimen that will stay in the same pot for a couple of years. Loam-based composts hold water and nutrients far better than peat-free multipurpose alone, and they have the weight to keep a top-heavy aster upright on a windy patio.

Choose a pot at least 30cm across and 30cm deep for a single plant; a wide, shallow bowl or a large trough suits three plants spaced 25cm apart. Terracotta is ideal because its weight helps stability, and the porous walls allow a little air to the roots. Add crocks or a few large stones to the base, then mix in a handful of horticultural grit to improve drainage.

• Plant so the crown sits level with the compost surface, never buried.
• Water the pot thoroughly after planting and top-dress with a 2cm layer of gravel or bark to reduce evaporation.
• Raise pots onto feet so surplus water drains freely and slugs have less easy access.

Watering, feeding and pinching

Consistency matters more than quantity. Aim to keep the compost evenly moist from spring onwards, watering at the base rather than over the foliage, which helps to keep mildew at bay. In a hot spell, a 30cm pot may need water every day, sometimes twice. If the compost dries out completely, the plant will drop lower leaves and the flowering stems will become woody and short.

Feed with a high-potash tomato feed every fortnight from midsummer until the flowers open, then stop. Too much nitrogen produces soft, leafy growth that flops and invites mildew. In late May or early June, pinch out the growing tips of each stem to encourage branching. This single job produces a bushier, more flower-laden plant and is the best insurance against top-heaviness. If a plant still leans, push three or four short twiggy stems around the edge of the pot as discreet support.

Copper-toned foliage partners

Asters and copper foliage are made for each other. The warm bronze and caramel tones pick up the pink and purple in the flowers and stop the whole arrangement looking thin at the base, where asters can become bare. In a large pot, underplant or surround the aster with:

• Heucheras in caramel, amber or bronze shades, which hold their colour well into winter.
• Bronze sedges and leathery-leaved carex, whose arching foliage softens the pot rim.
• Purple-brown sedums, whose flat flower heads follow on from the asters.
• Bronze fennel or dark-leaved grasses, for height and movement behind the mound.

In the border, the same copper palette works as a bridge between summer colour and autumn asters. Position dark foliage where low sun will strike it in the afternoon, and the asters will appear to glow against it.

Year-round border and pot care

Leave the stems standing through winter rather than cutting them down in October. The seedheads hold structure, feed the birds and protect the crown from hard frost. Cut back to about 15cm in early spring, once new shoots appear at the base, and divide congested plants every two or three years at the same time.

In the border, mulch with well-rotted organic matter in spring, once the soil has warmed, and keep plants 40–50cm apart to allow air to move between them. Thin out crowded stems in midsummer to reduce mildew. Slugs and snails will graze new shoots, so set a night patrol or use a barrier around pots. Aphids on soft tips can be blasted off with a jet of water.

Overwintering pots is mostly about drainage. Asters are hardy, but sitting in cold, wet compost kills more of them than frost ever does. Move pots against a sheltered wall, keep them on feet, and water sparingly until growth restarts. Repot every second spring into fresh loam-based compost, and your compact asters will reward you with that generous late-season flourish for years.