By late August, the border is doing two things at once: it is pushing out the last of its summer growth, and it is setting buds for an autumn finale. Asters in particular — Michaelmas daisies, now lumped under Symphyotrichum in the catalogues — are guilty of the second. Their stems are soft, their sap is sweet, and their tightly packed buds offer aphids shelter that a leaf simply cannot match.
Aphids breed fastest in warm, still, slightly humid weather. A single female can produce a colony in a fortnight without ever needing a mate, which is why you can inspect a plant on Sunday and find the growing tips thick with greenfly by the following weekend. Ants make it worse: they farm aphids for honeydew and will actively carry them onto fresh shoots, defending them from predators as they go.
If you grow copper-toned foliage — a Physocarpus 'Diabolo', a dark Cotinus, a bronze Heuchera, or one of the burgundy sedums — you have an unexpected advantage. Honeydew and the sooty mould that follows it show up far more clearly against dark, coppery leaves than against green ones. Treat those plants as your early-warning system and check them first.
Make a habit of walking the border once a week with a mug of tea in one hand and nothing else. Turn leaves over. Aphids cluster on the underside, along the midrib and in the soft creases where the leaf meets the stem, because that is where the tissue is thinnest and easiest to pierce.
Look for four things:
Pay particular attention to aster tips, the new growth of dahlias, and the soft flower stems of penstemon and Japanese anemone. Check the plants nearest the house or a warm wall first; they run a week or two ahead of the rest of the border.
For a light to moderate infestation, water is the most underrated tool in the shed. A strong jet knocks aphids off, and once on the ground most will not climb back. The trick is to be firm but not brutal — you want to dislodge insects, not shred flower buds.
Use a hose with a rose attachment, or simply part-cover the nozzle with your thumb to soften and spread the stream. Work from underneath, angling upwards into the leaf undersides, and move along the row steadily. Do it in the morning on a dry, sunny day so the foliage dries quickly and you are not inviting botrytis into the bargain. Avoid soaking open blooms, especially on asters, where petals bruise and mark easily.
Repeat every two to three days for a fortnight. You are not aiming for eradication in one pass; you are interrupting the breeding cycle often enough that numbers never recover. Keep going until you see the first ladybird larvae patrolling the stems.
Nature will catch up, and your job is mainly to stay out of its way. Ladybird larvae — those small, armoured, slightly alarming creatures that look nothing like their parents — eat dozens of aphids a day. Lacewing larvae are even more voracious, and hoverfly larvae, legless and green, suck aphids dry while disguised as harmless grubs.
To encourage them:
Once you see larvae on the hunt, ease off the hose. The balance usually tips within a week or two, and the colony collapses on its own.
Aphids prefer soft, sappy growth, and soft growth comes from too much nitrogen. Feed flowering plants with a potassium-rich tomato feed instead, and keep nitrogen-heavy general fertiliser for leafy crops. Water at the roots rather than over the foliage, and mulch to keep moisture even — drought-stressed asters are far more attractive to pests than well-grown ones.
Position copper-toned foliage where it earns its keep: as a dark backdrop that makes the lilac, pink and blue asters sing, and as a diagnostic plant you will actually notice. Deadhead spent flowers to keep energy in the plant, but leave a few seedheads standing for birds and overwintering insects. Hold off cutting stems down until spring, then tidy the border properly, clearing debris where eggs and overwintering females hide.
Do that, and your late-season border will carry colour well into October without you ever reaching for a bottle.
Copper Aster shares practical, down-to-earth guidance on practical uk gardening with aster flowers, copper-toned foliage, and seasonal border care. for readers across the UK.
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