Aster Year Round: A Season by Season Schedule from Spring Division to Autumn Cleanup

The shape of an aster year

An aster is a herbaceous perennial that dies back to nothing over winter, rebuilds a full-sized plant between spring and late summer, then flowers through autumn for five or six weeks until a hard frost stops it. It spends three quarters of the year producing nothing you would look at, which is exactly why the jobs that matter are all done long before there is a flower in sight.

Most of the outcome is fixed by midsummer. Whether the clump was split this spring decides the flower size and half the mildew risk, whether it was cut back in early summer decides the height and whether you need a stake, and whether the soil stayed damp through midsummer decides how much of the lower foliage is still alive when the flowers open. Nothing you do in autumn can correct any of them.

Use the table as a checklist and the sections below it for the reasoning. Seasons are given rather than dates, so the sequence reads correctly in either hemisphere and in any climate; the only fixed points you need from your own garden are when your last spring frost falls and when your first autumn frost usually arrives.

Spring: division is the job of the season

Start before the shoots break through by raking away everything left around the crown, because mildew spores overwinter in old leaf litter and reinfect the new growth from below. Top-dress with an inch (2.5 cm) of compost at the same time and mark where each crown sits while you can still see the old stubs.

As the shoots appear, work out which clumps need splitting. Any plant in its third year or older is due, and the confirmation is visible from above: many more stems than last year, all thinner, with a woody or hollow center and the vigorous growth pushed out into a ring. Lift the whole thing with a fork driven in a spade's width outside the shoots, split the outer ring into pieces of three to five shoots each with a good share of root, throw the exhausted middle away, and replant eighteen to twenty-four inches (45-60 cm) apart.

Feed once, in mid spring, with a balanced general fertilizer as the shoots reach six inches (15 cm), then lay two inches (5 cm) of mulch over the root area with the material pulled back an inch or two from the stems. That is the entire feeding program for the year. Anything richer, and particularly anything high in nitrogen, produces soft leaves that mildew easily and long stems that flop.

Protect the emerging shoots from slugs on new plants and fresh divisions. An established clump can lose a few shoots without anyone noticing; a division with four cannot. Finish spring by putting ring supports or twiggy sticks over any clump you have decided not to cut back, while the stems are still short enough to grow up through them.

1. Clear the crown before growth starts Rake away every scrap of last year's stems and leaves and bin it. This is the cheapest mildew control available, because it removes the spore reservoir sitting directly under the new shoots.
2. Divide every clump in its third year Lift, split the vigorous outer ring into pieces of three to five shoots with plenty of root, discard the woody center, and replant 18-24 inches (45-60 cm) apart at the same depth the clump was growing.
3. Feed once and mulch One dressing of balanced fertilizer as the shoots reach six inches (15 cm), then two inches (5 cm) of compost or bark mulch over the root area, kept off the stems themselves.
4. Support what you are not going to cut Put a ring support or three or four twiggy sticks over any tall clump you intend to leave uncut, while the stems are short. A collapsed clump cannot be tidied up convincingly after the event.

Summer: the cut back, and the watering that prevents the gray

Early summer is the one moment you can change the shape of the plant. When the stems are knee high, roughly twelve to eighteen inches (30-45 cm), and before any bud clusters are visible at the tips, shear the whole clump back by about a third. It branches from every cut stem and finishes shorter, stiffer and bushier, standing up without a stake and flowering one to two weeks later. Cut some clumps and leave others and you stretch the autumn display considerably.

From then until the flowers open, watering is the job. The plant is building all its stem and every flower bud through the hottest, driest weeks of the year, and a clump whose roots go dry in that period both flowers thinly and mildews badly. One long soak a week that wets the top eight inches (20 cm), at the base of the plant, in the morning, beats any amount of surface sprinkling and does not leave the foliage wet overnight.

Once a week, part the stems and look at the lowest leaves in the middle of the clump. That is where powdery mildew starts and you will see it there a fortnight before anything shows on the outside. Pick off and bag any leaf with a floury patch; early stripping often stops an infection, and a spray is only worth applying while the plant is still largely clean.

Do not feed in summer. A late nitrogen dressing produces exactly the soft growth the fungus wants at exactly the wrong moment, and it undoes the stiffness the early summer cut gave you.

Autumn, winter and the notes that make next year easier

Autumn is when the plant pays for the year. Deadhead spent flowers if you want the clump tidy and to stop it seeding around, but leave the stems themselves standing. The foliage is still moving reserves down into the crown, and cutting a clump to the ground the week the flowers brown gives you fewer and weaker stems next spring.

Once the leaves have yellowed properly, cut the stems to two or three inches (5-8 cm) and clear every leaf and stem fragment away from the crown. Bin or burn it. Do not compost it and do not leave it as a mulch, because this is the second half of the mildew cycle and skipping it undoes everything the spring division bought you.

Through winter the plant needs nothing except not to sit in standing water. Cold is rarely what kills an aster; a waterlogged crown on heavy soil is. If you have lost one that way before, lift and replant on a slight rise in spring, working coarse grit and compost through the area first. Mark every crown position before the top growth disappears, because the plant is invisible from now until well into spring.

Then use the quiet weeks to go through your notes clump by clump: which clumps you divided and when, which you cut back in early summer and by how much, the finished height of each, the dates the first and last flowers opened, when mildew first appeared and on which plants, and the date of the first hard frost. Those records answer next spring's questions before you have to guess. They tell you which clump is due for division, which position in the bed is genuinely too still, which plant is susceptible enough to be worth replacing, and whether your season leaves room for the delay the early summer cut costs you.

The single most valuable line in the notebook is the year each clump was last divided. Asters go woody in the middle on a three-year cycle, and that date tells you what to do next spring more reliably than looking at the plant does.