Aster Care: Spacing, Summer Water and the Early Summer Cut That Stops the Flopping

Give the clump room and the year gets easier

A Michaelmas daisy grows into a dense column of leafy stems that stands three or four feet (90-120 cm) tall by the time it flowers, and a mature clump can carry thirty or forty of them. Packed together, and packed against whatever is planted next to them, those stems create a still, humid pocket of air in the middle of the plant that stays damp from dusk until the middle of the following morning. That pocket is where powdery mildew begins, every year, on every susceptible plant.

So the spacing decision is not cosmetic. Set crowns eighteen to twenty-four inches (45-60 cm) apart for the tall kinds, and keep that same distance clear of any dense neighbor that will grow up into the lower third of the stems. A young aster looks ridiculous at that spacing for its first summer and correct from its second onwards, which is the usual trap: people close the gaps, and three years later every clump in the bed touches its neighbors and grays out from the ankles up.

Where you can choose, run the gaps along the line of the prevailing breeze so air moves through the group rather than around the outside of it. An open bed with sun for most of the day beats a warm sheltered corner between a wall and a hedge, however inviting that corner looks at midday. Still air is worth more to the fungus than shelter is worth to the plant.

Asters want at least six hours of direct sun, and morning sun counts for more than afternoon sun because it burns the dew off the foliage early. In real shade the plant does not die; it grows tall and leaning, opens a scatter of small flowers, and mildews badly because the leaves never dry.

Water is the mildew control nobody expects

The instinct with a fungal disease is to keep the plant dry. With asters that instinct is exactly wrong. A clump whose roots dry out in midsummer is far more likely to gray over with powdery mildew than one kept evenly moist, because drought stress thins the leaf tissue and the fungus colonises stressed leaves first. Dry soil and mildewed foliage arrive together, and the soil comes first.

Aim for soil that is never dry more than an inch (2.5 cm) below the surface through summer. Push a finger in at the edge of the clump; if it is dry to the second knuckle, water, and water enough to wet the top eight inches (20 cm). In most gardens that means one long soak a week, or two in a hot dry spell, rather than a daily splash that wets the surface and evaporates by lunchtime.

Water at the base of the plant, not over the top of it, and do it in the morning. Evening overhead watering leaves the foliage wet for ten or twelve hours in exactly the warm, still conditions the fungus wants. A can with the rose taken off, pushed under the stems and aimed at the ground, does the job in less time than a sprinkler.

Two inches (5 cm) of compost or bark mulch spread over the root area in spring is what makes the weekly watering last. Keep it pulled back an inch or two from the stems so the crown itself is not sitting in damp material through winter.

If you can only remember one thing about asters and mildew, make it this: the first sign of trouble is usually not on the leaves at all, it is soil that has been dry for a fortnight in midsummer while the plant was building its flower buds.

Cut back by a third in early summer

Tall asters flop. Left alone, a well-fed clump reaches four feet (120 cm) by late summer, and the first heavy rain of the flowering season splays it open from the middle so the stems lie outward in a ring with a bare hole in the center. Staking a collapsed clump afterwards never looks right, and the usual fix people reach for is a ring support in spring.

There is a better one. In early summer, when the stems are roughly knee high and well before any flower buds are visible, take a pair of shears and cut the whole clump back by about a third of its height. The plant responds by branching from every cut stem, and what you get in place of one tall unbranched shoot is a shorter, stiffer, much bushier stem carrying more flower heads. A cut clump usually stands up on its own with no support at all.

Two things change and you should expect both. The plant finishes noticeably shorter, often by nine inches to a foot (25-30 cm), and it flowers one to two weeks later than an uncut clump. The later flowering is often an advantage, because it stretches the display further into autumn when almost nothing else in the border is still going. If you grow several clumps, cut half of them and leave the rest, and you get a longer season out of the same plants.

Timing is the only way to get this wrong. Cut too late, once buds have formed, and you remove the flowers without gaining time to replace them. If you can see bud clusters at the stem tips, leave that clump alone this year and stake it instead.

1. Wait until the stems are knee high Roughly twelve to eighteen inches (30-45 cm) of growth, in early summer, with the stems still soft and no bud clusters visible at the tips. Cutting sooner than this leaves too little stem to branch from.
2. Take a third off the whole clump with shears Cut level across the top of the clump rather than stem by stem. Removing about a third of the height is enough; taking half delays flowering more than most gardens want and can push it past the first frosts.
3. Water and leave it alone Soak the clump after cutting and do not feed it. The plant will look blunt and unpromising for two weeks, then break into side shoots from every cut stem and thicken up quickly.
4. Cut only some of your clumps Leave one or two uncut and stake them. The uncut clumps open first, the cut ones follow one to two weeks later, and the bed carries flower for longer than if you treated them all the same.
5. Write down which clumps you cut Note the date, which clumps were cut, the finished height and the date each one opened. After two seasons those notes tell you exactly how much height and delay the cut buys in your own garden.

Feeding, deadheading and the end of the season

Asters are not hungry plants. An inch (2.5 cm) of garden compost worked around the crown in early spring and a single dressing of a balanced general fertilizer as the shoots reach six inches (15 cm) covers the whole year. What actively causes trouble is high-nitrogen feed, which produces soft, thin-walled leaves that mildew colonises easily and long weak stems that flop despite the early summer cut. Keep the lawn spreader away from the border.

Through the flowering season, pick off spent heads if you want the clump to look tidy, but there is no second flush to gain from it as there is with some perennials; asters flower once, over a long stretch in autumn, and then they are done. The stronger reason to deadhead is that many asters seed themselves freely and the seedlings are washy and vigorous enough to crowd out the plant they came from.

Leave the stems standing after flowering. The plant is still moving reserves down into the crown, and cutting a clump to the ground the week the flowers brown gives you a weaker plant with fewer stems next spring. Wait until the foliage has yellowed properly.

Then cut the stems down to two or three inches (5-8 cm) and clear every leaf and stem fragment away from the crown. Mildew spores overwinter in that litter and reinfect the new shoots from below in spring, so bin or burn the material rather than composting it or leaving it as a mulch. Record which clumps grayed over worst this year and where they sit in the bed, because the pattern repeats in the same positions and the notes tell you which corner is genuinely too still long before you would work it out from memory.

Mark the position of every crown before the top growth disappears. An aster is invisible from late autumn until well into spring, and a fork driven through a dormant crown is the most common way people lose one.