Soil pH is measured on a scale from 0 to 14, with 7 sitting at neutral. Anything below 7 is acid, anything above is alkaline. Most asters — the Michaelmas daisies, New England asters and their many garden cultivars — grow best in soil that sits between 6.5 and 7.5. That is neutral to slightly alkaline, which is a narrower window than many perennials demand.
Get the pH wrong and the plant will not simply refuse to grow; it will grow badly in ways that are easy to misread. In acid soil below about 6.0, phosphorus becomes locked up and unavailable, so asters produce pale, weak growth and flower sparsely. Very acid conditions can also release aluminium into the soil solution, which damages fine roots. The plants become thin, leggy and far more prone to powdery mildew. If you garden on sandy heathland, over granite or in the wetter west, your soil is likely to be on the acid side and worth testing before you buy a single plant.
A pH test is only as good as the sample you give it, so take a little care. Do not test straight after liming, feeding or spreading mushroom compost, as the reading will be skewed.
In expensive probe meters are tempting but unreliable, especially in chalky or very wet soil. Colour-match kits are cheap and reasonably accurate to about half a pH point. If you want a definitive figure, send a dried sample to a laboratory for a full analysis, which also tells you about nutrient levels and texture.
If your border reads between 6.5 and 7.5, plant with confidence and simply keep the organic matter topped up. Between 6.0 and 6.5, asters will manage, but a light dressing of lime will pay dividends. Below 6.0, lime is genuinely necessary rather than optional. Above 7.5, do not panic — asters will tolerate mildly alkaline soil, though at pH 8.0 and above they can struggle to take up iron, showing yellowing between the veins of new leaves. In that case, work in plenty of garden compost or leafmould rather than trying to acidify the whole border.
Ground limestone, sold as garden lime, is the safest choice. It is calcium carbonate, slow-acting and gentle. Hydrated lime shifts pH quickly but is caustic, unpleasant to handle and easy to overdo, so leave it on the shelf. Dolomitic lime is useful where magnesium is also low.
Retest six to eight weeks later, and again the following year. Lime moves slowly through the soil, so expect to repeat the treatment periodically rather than once and forever.
Once the pH is right, plant in spring or early autumn, spacing taller cultivars 45cm apart and compact ones 30cm. Water them in well and mulch with well-rotted compost, keeping it clear of the stems.
Asters earn their place partly through the way they combine with copper and bronze foliage. Think of heucheras in shades of caramel and amber, the deep copper leaves of Physocarpus, dusky sedums and bronze fennel. Asters themselves take on a bronze cast in their lower foliage as summer fades, and that warm tint carries beautifully into the pinks, purples and lilacs of their autumn flowers.
Spring: mulch with 5cm of compost, stake tall varieties early and watch for slugs on new shoots. Late May: cut stems back by a third — the Chelsea chop — for bushier plants and later, longer flowering. Summer: water in dry spells, because drought stress is what brings on powdery mildew. Autumn: deadhead or leave seedheads for the birds, then cut stems down to about 15cm once flowering finishes or in early spring. Lift and divide clumps every two to three years in spring to keep vigour. Retest pH every few years, particularly on chalk or after heavy compost additions, and top up lime as needed. Get that foundation right and your asters will reward you for years.
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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