Garden Q&A

Question: Some of my coneflowers (very ordinary pink ones) are misshapen - the spikes grow too large, close together and irregular, and often they don't get petals. Is this a disease or a sign of distress? What can I do about it?

Answer: Bad news. The affected plants should be removed and destroyed.

They have a condition called aster yellows, for which there is no treatment. In addition to the distortions you describe, coneflower or echinacea flower heads may be almost entirely green, quite congested with only tiny petals, or send up secondary flowers from the center of the cone.

Since aster yellows is spread by leafhoppers from one plant to another, leaving the diseased specimens in place would imperil other plants. Like mosquitoes that carry say, West Nile virus, the leafhoppers are transmitting a tiny organism called phytoplasma when they feed on plants. (I say this not to absolve the leafhoppers - just being precise.)

Victor Piatt, a horticulturist at the Mount Cuba Center in northern Delaware, is in the last months of a three-year trial of various echinacea species and hybrids. He reports a fair amount of aster yellows this year in the trial gardens, exacerbated, he suspects, by the monoculture of row after row of coneflowers, unlike a home garden setting.

Piatt reports that E. purpurea (the common name is purple coneflower though the flowers are usually pink) is by far the most susceptible, including selected white cultivars. Most other species and hybrids discourage leafhopper with hairy (sometimes almost bristly) stems and leaves. He also points out that the symptoms tend to show up mid growing season, even though the infection happened earlier - meaning that it's entirely possible to buy plants with "latent" aster yellows.

My white coneflowers, second- and third-generation seedlings of the cultivar "White Swan," exhibited a bad case of aster yellows this year. Other susceptible flowers are asters, chrysanthemums, coreopsis, cosmos, marigolds and petunias, along with carrots, potatoes, onions, tomatoes, dandelions and weed plantain. I grow or have grown all those ornamentals and have never seen any symptoms akin to the distorted flowers on the echinacea. For what it's worth, a non-exhaustive Web crawl found far more references to aster yellows on coneflowers than anything else.

About those echinacea trials at Mount Cuba: Seven species and 49 cultivars have been under evaluation, including many of the new catalog beauties in yellows and oranges, to determine which perform well in this climate. The final report is due by the end of this year, but a preliminary report, posted last year, has much valuable information. (Go to www.mtcubacenter.org, click on Research, then click on Plant Evaluations and scroll down to the echinacea link.)

These sorts of trials are invaluable for gardeners, especially since too many cultivars of various plants are being introduced too quickly these days, before their true garden-worthiness has been established. In this regard, the study will be immediately out of date - the wholesaler Terra Nova has several new coneflowers (great pictures, I must admit) in the pipeline for garden centers to sell next spring.

Q: Will Roundup kill stilt grass? What else can I do to get rid of it?

A: Roundup, the nonselective herbicide, certainly will kill stilt grass, but there are other tactics to employ to deal with this increasingly prevalent invasive from Japan. It is taking over untended meadows and parks, since its comparatively dense foliar cover inhibits native grasses and meadow plants.

Stilt grass, especially the leaves, looks vaguely like a delicate miniature bamboo. It grows to 2 feet if uninhibited, and readily re-roots when stems touch the ground.

The good news is that it is an annual; just like crabgrass, it totally dies over the winter. Each year's crop is from seed scattered the previous late summer or fall. So, if you have stilt grass, the most important thing is to keep it from going to seed. If it's in your lawn, mow it and - unlike crabgrass - it will not be able to bloom and produce seed. One mowing isn't enough - keep the stuff short until the really cold weather hits. But elsewhere, pull it by hand. If you use Roundup, you end up with lots of dead stilt grass (and dead everything else), which you then have to pull up, so why go through two steps? Just weed it out; the roots are pretty shallow.

The bad news is that the seeds remain viable for three or more years, so even if you remove every bit of stilt grass from your garden, there are seeds in the ground waiting for next spring to germinate. Next April, use a preemergent herbicide, which will prevent both crabgrass and stilt grass seed from sprouting successfully. Depending on your circumstances, it may be smart of get two formulations of preemergent, one for lawns, the other for flowerbeds.

Pay attention to the margins - the narrow areas where lawn and flower beds meet, likewise lawn and fences, lawn and untamed "natural" areas, etc. Preemergent spreaders may not get all the way to the edge of a lawn, for which the stilt grass seeds will be grateful. If your neighbor has a bumper crop of stilt grass, gently educate him or her, for the seed travels.

All in all, if you are vigilant about preemergent application, close mowing and weeding for three years, you can get rid of the wretched stuff. Most of it. Unless it turns out that the seeds stay viable for a lot longer than three years.

- Michael Martin Mills