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Organic Herbicide, Clematis Wilt, Overwintering Pelargoniums

Corn gluten meal, a yellow powder that is a by-product of the wet milling process, has long been used as an additive to animal feed...
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Can you tell me about an organic herbicide made from corn gluten? S.T., Sharon, CT

Answer: Corn gluten meal, a yellow powder that is a by-product of the wet milling process, has long been used as an additive to animal feed. More than 15 years ago, however, Dr. Nick Christians, a turfgrass professor at Iowa State University, discovered that it also inhibits root development in certain germinating weed seeds. Patented by the university in 1991, corn gluten meal is now available as an herbicide under a variety of names including Safe ’n Simple (Blue Seal Feeds) and Concern Weed Prevention Plus (Necessary Organics, Inc.). It is labeled as a preemergent herbicide for use on turfgrass, field crops, and home gardens. Among the weeds controlled by corn gluten meal are crabgrass, dandelions, smart weed, redroot, pigweed, purslane, lamb’s quarters, foxtail, and barnyard grass. Both powdered and pelleted forms are available.

Timing is very important to achieve the best control. Corn gluten meal must be applied just prior to weed seed germination. If applied too early, its effect will be limited by soil microbes. Similarly, if the herbicide is applied too late, there will be little reduction of root growth. Soil moisture also plays a part. Christians has found that corn gluten meal is most effective at controlling weed emergence during periods of drought. When there is abundant soil moisture, many of the weeds survive despite their stunted root systems. Nevertheless, research has yielded as high as 80-percent control of weeds over a three-year period.

Those seeking 100-percent control may opt for synthetic herbicides. Corn gluten meal herbicide (which contains 10% nitrogen by weight) must be applied at a higher rate than conventional “weed-and-feed” herbicides. This translates to as much as a threefold increase in cost per square foot. Nevertheless, gardeners in search of an organic herbicide may want to experiment with corn gluten meal, trying it on small areas to see if they are pleased with the results.


This spring the new growth on my Clematis ‘Henryi’ suddenly wilted and turned black. The clematis next to it was fine. Could this be the dreaded clematis wilt? J.G., Osceola, MO

Answer: Yes, indeed. Large-flowered clematis are highly susceptible to this notorious fungal disease. The disease strikes in early summer, often when plants are on the verge of flowering. Although wilted leaves and stems that droop and turn black are the first signs, the fungus initially attacks the stem close to the soil line, invading cracked, damaged, or weak tissue. Lesions or discolored areas associated with the fungus girdling the stem can usually be seen below the first pair of wilted leaves.

Despite its devastating appearance, clematis wilt is rarely fatal. Even when all top growth is killed, new healthy shoots can emerge from basal buds below the soil surface. (This is the reason for the recommendation that clematis be planted with the crown at a depth of 2 1/2 inches below soil level.)

Prune back infected stems to healthy tissue and discard the trimmings. Fertilize and water to encourage regrowth. To encourage the production of woody tissue on the lower stem, pinch out the tips of new shoots when they reach two to three inches in length. Carefully tie the shoots that emerge from below the pinch to avoid damage from wind and rain.

At present, there is no registered fungicide for controlling clematis wilt, and in any case, fungicide application would have to be made in the spring before symptoms appear. Alternatively, you may want to plant some of the smaller-flowered clematis such as C. alpina, C. macropetala, C. viticella, and their hybrids, which are less susceptible to clematis wilt.


Is it true that pelargoniums can be overwintered bareroot and upside down in a cool place? R.P., Southold, NY

Answer: The thick, fleshy stems of pelargoniums lend themselves to survival during periods of drought, hence this old-fashioned method of overwintering the plants in cold climates. The location must remain above freezing, yet be cool and moist to avoid excessive dehydration of the bare roots. Periodic moistening of the roots may be necessary. Light is neither necessary nor desirable provided the temperature is cool enough. Despite this drastic treatment, the pelargoniums can be repotted in the spring, their tops cut back by half or two-thirds, and returned to bright light and regular watering. New roots will emerge from the green stems below the soil surface, and new shoots from above. Obviously, the ready availability each spring of new pelargonium plants at nurseries and garden centers has made this a much less common practice than it once was.