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Growing Tips—Cutting-Edge Plants, Tools & Techniques

Plants & Gardens News Volume 17, Number 4 | Winter 2002

by Greg and Pat Williams

Vinegar Kills Weeds

Experiments conducted by U.S. Department of Agriculture scientists have shown that vinegar with 10 to 20 percent acetic acid concentration can kill many common weeds, including giant foxtail, smooth pigweed, and velvetleaf. Spraying vinegar along corn rows resulted in almost complete control of weeds with no damage to the corn plants. Complete coverage of large areas with vinegar would be prohibitively expensive, but spot applications would be financially feasible. Vinegar produced from natural sources by natural fermentation is suitable for organic production. Note: Vinegar with more than 5 percent acetic acid concentration may be hazardous on contact with skin or eyes or if vapors are inhaled. Handle with appropriate precautions.

For additional information, visit the USDA web site or contact Dr. Jay Radhakrishnan at 301-504-6105.

Fungal Threat From Birdseed

Ohio State University plant pathologists warn that sunflower seeds used as birdseed sometimes harbor sclerotia (overwintering forms) of the fungus Sclerotina sclerotiorum, which can attack many cultivated and native plants. Five out of ten samples from bags of sunflower seeds marketed commercially as birdseed were found to contain live sclerotia. The sclerotia are not digested by birds and can remain viable for a long time. Because there are currently no sunflower cultivars with significant resistance to the fungus, minimizing the presence of sclerotia in birdfeeders will require strict monitoring of commercial seed after harvest.

Source: A.E. Lighthiser, S.G.P. Nameth, and L.H. Rhodes, "Birdseed: A Potential Disease Problem for the Plantscape, Yard, and Garden?" HortScience 37(3), June 2002, 599 (American Society for Horticultural Science, 113 S. West St., Suite 200, Alexandria, VA 22314-2851).

Daylily Rust Update

Since Asian daylily rust (Puccinia hemerocallidis) was discovered in the Southeast two years ago, it has been reported in more than 20 states and has developed into a serious threat to daylilies (Hemerocallis) in much of the eastern United States. Rust symptoms progress from water-soaked spots on both sides of leaves to orange or brown pustules (or yellow flecks on resistant cultivars). When wiped with facial tissue, the pustules produce an orange-yellow stain on the tissue. Below are lists of "very resistant" and "very susceptible" cultivars, based on field research conducted at the University of Arkansas.

Source: R. Clark, "Daylily Rust," Hort Notes 13(10), July 8, 2002, 3 (UMass Extension, French Hall, 230 Stockbridge Rd., Amherst, MA 01003-9316).

Rose Recommendations

Based on extensive field trials, Texas A&M University, in cooperation with the Texas Nursery and Landscape Association, has identified a number of rose cultivars with exceptionally high disease and insect tolerance or resistance. Designated EarthKind™ Roses, these cultivars appear to be adaptable to a wide range of soil types and rarely require pesticide sprays.

They include 'Marie Daly', a dwarf shrub that grows about three feet high and wide and produces fragrant pink semi-double blooms; 'Dadler', a small shrub that grows about four feet high and wide, producing cherry-red semidouble blooms; and 'Belinda's Dream', a medium shrub that grows about five feet high and wide and produces fragrant pink double blooms.

If you'd like to grow these plants, you can further minimize disease risks by planting them where they'll receive direct sun for at least eight hours daily. Provide free air movement around them and don't irrigate with sprinklers late in the day or at night. Also, mulch around them with organic materials to a depth of three or four inches. Fertilization in March and June (with at least half of the nitrogen in slow-release form) and in late August (with readily available nitrogen) is recommended.

Source: S. George, "The Importance of EarthKind™ Roses," The Yellow Rose 19(2), July 2002, 20–25 (Dallas Area Historical Rose Society, P.O. Box 831448, Richardson, TX 75083-1448).

Sweet Relief for Transplants

Researchers at Bartlett Tree Laboratories in Charlotte, North Carolina, have been testing the effects of adding small amounts of various kinds of sugar to the root zones of newly transplanted trees. Their aim is to accelerate root growth and reduce the effects of transplanting stress, with the underlying notion that supplying sugar directly to the roots makes up for diminished photosynthesis following transplanting. In trials with birch trees, adding sugar has indeed boosted root growth, but for some tree species (such as beech and oak), sugar has been detrimental. And for all species, too much sugar causes problems. For birches, sucrose gives the best results; other sugars are less effective. More research is needed to determine the range of species that benefit most from adding sugar when transplanting and to determine optimal amounts of added sugar.

Source: G. Percival and E.T. Smiley, "One Lump or Two?" Grounds Maintenance 37(4), April 2002, Contractor 18, 20 (Primedia Business, 9800 Metcalf Ave., Overland Park, KS 66212).

Mulch Matters

Field trials in South Carolina have shown that concentrations of important nutrients beta-carotene (vitamin A precursor) and ascorbic acid (vitamin C) in carrot roots are higher when white or yellow mulch is used instead of black, red, blue, or green mulch. Carrots dressed with white mulch also had about 50 percent heavier roots than those dressed with red, blue, or green mulch. The carrot cultivar 'Scarlet Nantes' was chosen for the trials and grown in raised beds supplied with drip irrigation. Black plastic mulch was painted with exterior enamels to produce the white, blue, green, and yellow colors.

Source: G.F. Antonious and M.J. Kasperbauer, "Color of Light Reflected to Leaves Modifies Nutrient Content of Carrot Roots," Crop Science 42(4), July-August 2002, 1211–16 (Crop Science Society of America, 677 S. Segoe Rd., Madison, WI 53711).

Avoiding Ash Anthracnose

Ash anthracnose, due to the fungus Gnomoniella fraxin, is a common problem in landscape plantings. Symptoms include leaf blotches and sometimes premature leaf drop. The dissemination of anthracnose fungal spores is enhanced by wet weather in the spring.

Eight ash species (Fraxinus americana, F. angustifolia, F. chinensis, F. excelsior, F. mandshurica, F. pennsylvanica, F. quadrangulata, and F. tomentosa) growing at the Morton Arboretum, in Lisle, Illinois, were rated for anthracnose-symptom severity in 1997, 1998, and 1999, in late June and early July (when symptoms were most pronounced). In addition, relative timing of budbreak and leaf emergence for the eight species was observed in the spring of 1998.

Contrary to some previous reports, white ash (F. americana) was found to be less susceptible to anthracnose than green ash (F. pennsylvanica). Blue ash (F. quadrangulata) was found to be the least susceptible of all species in the trial, while Chinese ash (F. chinensis) appeared most susceptible. Some correlation was seen between anthacnose susceptibility and leaf emergence. The researchers who conducted the study suggest that trees leafing out early might show more pronounced anthracnose symptoms simply because their leaves are exposed to inoculum for a longer period of time. Interestingly, a search for anthracnose fruiting bodies in fallen leaves proved unsuccessful for the resistant species F. quadrangulata.

Source: K. Jacobs and D. Danielson, "Variation in Field Susceptibility of Native and Exotic Ash Species to Anthracnose," Journal of Arboriculture 28(1), January 2002, 35–40 (International Society of Arboriculture, P.O. Box 3129, Champaign, IL 61826-3129).

Catnip Repels Bugs

Catnip (Nepeta cataria) is recognized for its intoxicating effects on cats. Less widely known are its insect-repelling effects. One researcher has reported that the essential oil of catnip, nepetalactone, is avoided by many insects and has even been found in the "defensive secretions" of some other insects. Experiments conducted recently at Iowa State University showed that catnip essential oil is comparable in repellency to the widely used insect repellent DEET when tested on German cockroaches.

Source: C.J. Peterson, L.T. Nemetz, L.M. Jones, and J.R. Coats, "Behavioral Activity of Catnip (Lamiaceae) Essential Oil Components to the German Cockroach (Blattodea: Blatellidae)," Journal of Economic Entomology 95(2), April 2002, 377–80 (Entomological Society of America, 9301 Annapolis Rd., Lanham, MD, 20706).

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