Keyword: Symphyotrichum

Our Asters are green and getting ready to bloom but the
lower 8″ are “rusty” or burnt looking. Did our bark mulch hurt them? Too much
fertilizer? This is their third year in the same spot.

 

There are quite a few potential causes of the rusty leaves you are
seeing. It might be entirely normal, as mature Asters (renamed Symphyotrichum) can start looking a bit ragged in late summer. It could be due to excessive heat,
overwatering (symptoms include yellowing and dropping of lower leaves),
or fertilizer burn (Asters are sensitive to soluble salts in chemical
fertilizers). It could be a fungal disease, rust, or Aster yellows, a
common disease caused by a microscopic organism (phytoplasma) and spread
from plant to plant by leafhoppers. With Aster yellows, you would notice
a loss of green in the leaf veins, and yellowing of new leaves.
Sometimes, infected outer leaves turn a rusty or reddish purple color. A
good general practice to keep your Asters looking full and less leggy is
to cut them back by one-half to two-thirds when they have reached 12 to
16 inches in late spring/early summer. (Source: The Well-Tended Perennial
Garden by Tracy DiSabato-Aust, Timber Press, 1998).) Here are
links to resources which describe other possible causes of the leaf
problem.

Connecticut Agricultural Experiment Station

Missouri Botanical Garden

To determine the exact cause, it might be worth bringing a sample to a
Master Gardener Clinic.

Can tall asters be sheared or lopped off early in the growing season to control height and make them bushier? I have Aster novae-angliae ‘Wild Romance’. I love the color and bloom-time, but would like them shorter.

 

Yes, asters (now renamed Symphyotrichum) can be pruned. This is sometimes referred to in England as “the Chelsea chop,” and it is a technique that may be used for a number of different perennials, as this article by Bunny Guinness in the Telegraph describes. An excerpt appears here:

“Plants now commonly manicured by their snip happy owners are Campanula lactiflora, sedums, rudbeckias, echinaceas, asters and heleniums. These have their shoots chopped back by around a third in late May/June. The basic rule is that perennials which only flower once should not be chopped or you will lose the flowers; varieties such as peonies, irises and aquilegias.”

Here is similar information previously available from the Colorado State University Cooperative Extension website:

“…control the height and shape of an aster by pruning. Gardeners can pinch asters like mums, regularly removing little bits of new growth until the first of July. However, an easier approach is to cut the aster back by one half in mid-June. At this time, the aster can be shaped. Outer stems can be cut lower than inner ones to produce a nice mounded plant. This shaping tends to encourage bloom near the base of the aster and discourage ugly brown stems. Although this pruning may sound extreme, it tends to delay flowering by only a few days and produces a much prettier plant.”