Autumn asters: colour and a last harvest when others fade

By the end of August the border falls quiet. The sunflowers tip over, the marigolds set seed, and an exhausted late-summer green settles over many gardens. And then, almost overnight, the asters open: violet, pink, white, a whole field of little stars humming like it did in June. Autumn asters are the plants that switch the garden back on when others switch off. For you, and for the insects that now need every open flower.

Why the aster blooms so late

The name says it all: autumn aster is an umbrella term for several species from the large aster genus that reach their flowering peak only when the days grow shorter. Botanically it is mainly two Americans behind this, the New England aster (Symphyotrichum novae-angliae) and the New York aster (Symphyotrichum novi-belgii), plus the native European Michaelmas daisy (Aster amellus) and the delicate cushion aster.

These plants respond to day length. Only when the nights grow noticeably longer, in August, do they set their buds. That is no accident but a strategy: a plant that flowers only in September faces little competition for pollinators and no heat-stressed bloom in high summer. For you that means a flowering season from early September into October · CW 36 to 42, often right up to the first harder frost.

The last harvest of the year

There is a reason beekeepers and wildlife gardeners love the aster. In September and October food for insects grows scarce. The big summer nectar flows are over, lime and bramble long since finished. Right into that gap the aster falls with a lavish offer of nectar and pollen. For honeybees it is the last chance to top up the winter stores. For bumblebee queens and many wild bees it is the filling station before hibernation.

For maximum insect value, choose the single-flowered varieties with an open flower centre. Double cultivars look lavish, but their stamens have been bred into petals, and then there is neither pollen nor reachable nectar. A simple New England aster with a yellow centre is a laid table for a bumblebee, a densely doubled variety often just decoration.

The right spot

Asters are no divas, but two things they will hold against you: too little sun and wet feet in winter. In a fully sunny spot with at least six hours of sun they stand firm, flower densely and stay largely healthy. In part shade they grow long, lean and topple more easily.

The classic weak point of the New York aster is powdery mildew, that mealy-white coating on the leaves in late summer. It is rarely fatal but unsightly. Two levers help you: an airy, sunny position and the choice of variety. New England asters and the newer Symphyotrichum hybrids are far more robust than the old New York aster cultivars.

Planting in early autumn

The best time to plant is early autumn · CW 32 to 38. The soil is still warm, some rain usually falls, and the plant roots in calmly before winter. Pot-grown stock from the nursery can be planted almost all year round, and bought in flower in autumn it even gives an instant effect in the border.

1. Dig a generous hole

   About twice as wide as the root ball. Loosen the base so the roots find their way down easily.

2. Improve the soil

   In heavy clay mix in a handful of mature compost and some sand. On lean soils compost alone is enough.

3. Water the root ball

   Dip the root ball briefly before planting until no more air bubbles rise. A dry ball soaks up water poorly later.

4. Plant and firm

   Plant as deep as the ball stood in the pot. Firm the soil all around and shape a shallow watering basin.

5. Water in thoroughly

   Right after planting, water deeply and keep watering during dry spells in the first weeks.

Set taller varieties like the New England aster at the back, low cushion asters at the front edge. A spacing of 40 to 50 cm for the tall ones and 25 to 30 cm for the small ones looks airy at first but fills in completely in the second year.

Partners for a long late-summer border

An aster on its own is lovely, but it shows its full effect in company. Combine it with plants that either flower at the same time or hold their own with structure and autumn colour. That creates a border that carries from September into November.

The tall stonecrop (Hylotelephium spectabile) is the ideal neighbour. Its flat flower plates stand like little landing pads between the aster stars, it also draws in insects and stays attractive as a seed head all winter. Ornamental grasses such as fountain grass suit it too, their spikes glowing in the backlight, along with the coneflower, whose dark seed heads set a calm contrast to all the little asters.

Care through the year

Asters are grateful and long-lived if you do them three small favours. The first is optional but effective: if you cut the tall varieties back by about a third in late May · CW 22 (the so-called Chelsea Chop), they branch more densely, stay lower and do not fall apart in autumn. That saves you the staking later.

The second favour is dividing. After three to four years many asters go bare in the centre and flower only around the edge. Then dig up the clump in spring, split it with a spade into fist-sized pieces and replant the strong outer sections. That rejuvenates the plant and gives you free supplies for the whole border.

Watering matters above all in the first season and during long dry spells. An established aster gets by on normal summer rain. A handful of compost in spring covers its nutrient needs, and you need no mineral fertiliser.

Which aster for which spot

The genus is large, and not every aster suits every place. Roughly, you can orient yourself by four types, then you make the right choice in the border.