Aster Identification: Telling Asters From Michaelmas Daisies and Chrysanthemums

The names, briefly

The flower is the fastest clue

Look at the ray florets, the "petals". Asters and Michaelmas daisies have many narrow ray florets, usually in more than one row, around a small yellow disc. The overall look is fine and crowded, like a small shaggy daisy.

Fleabanes take this further. They have so many extremely fine ray florets that the flower looks feathery or almost hairy, and they are usually smaller.

Chrysanthemums are more substantial. Fewer, broader ray florets, a bigger and more prominent disc, and often a distinctly different petal shape — spooned, quilled, or curled at the tips in the decorative forms.

Once you have looked at a few, the difference between an aster and a chrysanthemum becomes obvious. It is the fineness of the petals.

Then the flowering month

• Early to midsummer flowering, in a garden setting, points to a true Aster.
• Flowering from late August into October is the Michaelmas daisies. The name is literal — they flower around the feast of St Michael at the end of September.
• Flowering from late spring points to a fleabane, which is usually also smaller and looser in habit.
• Chrysanthemums flower from September onwards and, unlike most of these, will keep going until the first hard frost.
• If it is flowering in high summer and it looks like an aster, check whether it is actually a Boltonia or a Kalimeris, both of which are sold as asters.

The leaves, which confirm it

Aster and Symphyotrichum leaves are usually lance-shaped, alternate up the stem, and often slightly rough to the touch. The Michaelmas daisies have narrow, dark green leaves that are almost stiff.

Chrysanthemum leaves are the ones that give it away instantly, because they are deeply lobed and strongly aromatic when crushed. Crush one and the smell is unmistakable — that is a chrysanthemum, not an aster.

Fleabane leaves are smaller, softer and often held in a basal rosette as well as up the stem.

One more cue worth knowing: many Michaelmas daisies have a distinctly hairy stem, and the hairs catch the light.

The mildew problem

Michaelmas daisies are notorious for powdery mildew, and it is the main thing that ruins them. It shows as a grey-white powdery coating on the leaves, usually starting at the base of the plant where the air is still.

The practical fixes are unglamorous. Lift and divide the clumps every two or three years, because a dense old clump always mildews. Water at the roots rather than over the leaves. Thin the stems in early summer so air moves through. And at the first sign, cut the worst stems right back, because they will flower again.

Some varieties resist it far better than others. If you are planting fresh, that matters more than flower colour.

My grandmother called every purple daisy in her garden an aster, and she was not wrong in the way gardeners used to use the word. She was just using a name that has since been taken apart by botanists.

The big change is that most of what was called an aster is now in a genus called Symphyotrichum. The Michaelmas daisies, the New York asters and the New England asters all moved. The true asters, like Aster amellus, are a much smaller group now.

For anyone actually standing in front of the plant, the useful question is not the Latin name. It is: which group is this, and when does it flower? That tells you who it is and how to look after it.