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Herbicide Contaminants in Irrigation Water — Causing Crop Injury at Concentrations BELOW Detection Limits. There have been several incidences of herbicide contamination in nursery irrigation ponds resulting in devastating crop losses. Little is known about how very low concentrations of herbicide contaminants might affect nursery crops. A test was established in 2022 to test this. Three broad-spectrum, industrial / rights-of-way herbicides (suspected in the crop damage mentioned above) were tested at 0.1X, 1X, 10X and 100X the detection limits in water samples (based on a commercial testing laboratory’s report). Metsulfuron-methyl detection limits are low enough to determine if harmful concentrations are present in water. However, both Aminocyclopyrachlor and imazapyr caused crop injury at one-tenth (0.1X) of the detection limits on at least one species in the bioassay. This finding indicates that the absence of detectable herbicide residues does not preclude the possibility that crop-damaging residues of some herbicide(s) may be present. This work was presented at the 2023 Weed Science Society of America annual meeting. See the poster here: Herbicide contaminate irrigation water POSTER V3.1

Plant treated with spray is stunted with no new growth. but if applied with prototype sprayer there was no injury

Hydrangea is injured by herbicide sprays but not when applied using a directed application system

An alternative to hand weeding in container nursery crops? A prototype sprayer capable of doing directed applications to container nursery crops was designed, constructed and tested. With this prototype sprayer we applied preemergence or postemergence herbicides  in container-grown hydrangea and roses to control bittercress and spurge without injuring sensitive crop species. With such a system it is possible to apply broad spectrum postemergence herbicides like diquat or glufosinate in 1-gallon roses to remove bittercress or spurge, instead of hand weeding. Research on the development and testing of this prototype was supported by grants from the NCDA Specialty Crops Block Grant Program and the Horticulture Research institute. See results testing the spray system  in this CCBB21PRE Safety and Efficacy summary.

Nostoc control in container nurseries. Nostoc is a cyanobacterium (blue-green algae) that forms thick, gelatinous masses on roadways and production beds in container nurseries. The thick, slimy mats are a slipping hazard for nursery workers. Since 2015 we have conducted several experiments to identify effective management options. Bottom line:  glufosinate (Finale XL or Cheetah Pro) controls Nostoc! Research results were presented in this poster at the 2023 Weed Sci. Society of America annual meeting. While we have identified several other options, glufosinate is highly effective and is labeled for the sites. We also confirmed that glyphosate, pelargonic acid, peroxide-based sanitizer, diquat, flumioxazin, or indaziflam did NOT control nostoc. Results of previous research can be seen in this poster of the 2020 experimental results. This research was supported by a grant from the Horticulture Research Institute and the USDA-IR-4 program.

Fe-HEDTA (Fiesta) for weed control in nursery crops.   Preliminary research suggested that FeHEDTA applied to dormant and semi-dormant woody ornamentals did not cause significant injury to several species. Evaluations are continuing to investigate the safety of Fiesta to woody nursery crops and ornamental grasses. Research has demonstrated differences in susceptibility between weeds and between woody nursery crops. A summary of the 2017 efficacy experiment on oxalis, bittercress, groundsel and eclipta is available (report).

Biological Control of Weeds. Biological and natural products for weed control. Recent research has focused on evaluating Phoma macrostoma, MBI-005 (thaxtomin) and Fe-HEDTA (Fiesta) for weed control in turfgrass and landscape plants. Results have been presented at the  WSSA annual conference (view abstract), the 2013 International Bioherbicide Group Workshop (pages 19-23 in view abstract) and Weed Technology. A more extensive report is available in the Master of Science Thesis of Mr. Joseph Wolfe.

Sarritor (Sclerotinia minor) was evaluated for efficacy on broadleaf weeds and persistence following application to turfgrass. Using a lettuce bioassay, we observed longer persistence than had previously been reported. See the project report for details. This research was funded by the USDA IR-4 Biopesticides program.

Cost-Effective Weed Control in Container Nurseries. Strategies have been developed that can reduce hand-weeding labor costs. Cost savings are realized by (1) selecting the most effective herbicide, (2) applying the product uniformly, and (3) hand weeding frequently to prevent weed seed production and spread. Average cost savings of 39% were documented in research trials and on-farm tests. Results are documented and summarized in the “Cost-Effective Weed Control” fact sheets.

Plant Invasive Risk Assessment. An assessment protocol for Invasive species currently sold in the nursery trade was developed by Ms. Clara Englert as part of her M.S. project. This project was funded by a grant from the North Carolina Nursery and Landscape Association. Dr. Thomas Ranney was co-advisor on this project.

Longevity of weed control in container nursery crops. Using bioassay methods we have shown that preemergence herbicides applied to nursery crops provide only 4 to 6 weeks control of most weeds. Typical re-application intervals of 8 to 12 weeks will allow weeds to emerge and establish. Recent research suggests that the herbicide indaziflam remains active in container substrates longer that competing products. This research has been supported by grants from the Horticultural Research Institute, BASF Corporation, Valent Corporation, Bayer Crop Science and OHP. Publications: trifluralin dissipation; Freehand residual control.

Microstegium Control. Ecology and management of Japanese Stiltgrass (Microstegium vimineum). Studies included evaluations of preemergence and postemergence herbicide efficacy, seed dormancy and germination requirements, and native plant community responses to Japanese stiltgrass control options. Project conducted as a Ph.D. project by Dr. Caren Judge. View the Japanese Stiltgrass ecology and control dissertation.

Control options for recently introduced weeds in nursery crops. A collaborative project with Dr. Jeff Derr, Virginia Tech University. The efficacy of herbicides labeled for preemergence weed control in container nurseries was compared on several species of newly introduced weeds. This project is supported by a grant from the Horticultural Research Institute. publication: doveweed control in containers

Herbicide Safety on Nursery Crops and Landscape Ornamentals. Research collaborations with the USDA-IR-4 program and product manufacturers continues to expand herbicide registrations and register new herbicides for weed control in ornamentals. Recent foci of the program have been the development of flumioxazin (Broadstar and SureGuard), dimethenamid-p (Tower and Freehand), and indaziflam (Marengo and Specticle) for weed control in nursery crops; herbicide safety in herbaceous perennials; herbicide safety in ferns; and safety of postemergence herbicides for nutsedge and broadleaf weed control in ornamentals. Recent research has included phenoxy herbicide affects on seedhead formation in ornamental grasses; Tower herbicide potential for over the top use in woody ornamentals; safety and efficacy of natural products for liverwort control, herbicide safety on herbacous ornamentals.

Taxonomy and management of common weeds of nursery crops. Several projects include a comparison of Ranunculus ficaria sub species distribution and spread in the U.S.(manuscript in review); and a taxonomic study of species of Cardamine species found in container nursery crops. Both projects conducted by Dr. Angela Post (currently Extension Specialist in the Department of Crop and Soil Sciences at NCSU).

Extension Program

Weed management systems for nursery crops, landscapes, and Christmas trees. Programs emphasize in-service training of Extension field faculty and resource development.

Books and Bulletins:

Horticulture Information Leaflets:

Publication List

View the comprehensive list of publications.

Selected Publications

  1. Zhou, T. and J. C. Neal. 1995. Annual bluegrass (Poa annua) control with Xanthomonas campestris pv. poannua in New York State. Weed Technology 9:173-177.
  2. Uva, R. H., J. C. Neal and J. M. DiTomaso. 1997. Weeds of the Northeast. Cornell University Press, New York. 396pp.
  3. Neal, J. C., J. F. Derr, A. F. Senesac and W. A. Skroch, 1999. Weed Control Suggestions for Christmas Trees, Woody Ornamentals, and Flowers. N.C. Cooperative Extension publication AG-427.
  4. Neal, J. C. 2000. Weed control in woody liner production. Proceedings of the International Plant Propagator’s Soc. 50:528-531.
  5. Krings, A. and J. C. Neal. 2001. A Scutellaria (Lamiaceae) new to North Carolina and a key to the small-flowered Carolina congeners. SIDA 19(3):735-739.
  6. Judge, C. A., J. C. Neal, and R. B. Leidy. 2003. Trifluralin (Preen) dissipation from the surface layer of a soilless plant growth substrate. Journal of Environmental Horticulture 21:216-222 pdf
  7. Judge, C. A., J. C. Neal, and J. B. Weber. 2004. Dose and concentration responses of common nursery weeds to Gallery, Surflan and Treflan. Journal of Environmental Horticulture 22:106-112. pdf
  8. Judge, C. A., J. C. Neal. and J.F. Derr. 2005. Response of Japanese Stiltgrass (Microstegium vimineum) to application timing, rate, and frequency of postemergence herbicides. Weed Technol. 19:912-917. pdf
  9. Neal, J. C. and J. F. Derr. 2005. Weeds of container nurseries in the United States. NC Nurserymen’s Assoc. 16 p.
  10. Judge,C. A. and J. C. Neal. 2006. Early postemergence control of nursery weeds with Broadstar, OH2 and Snapshot TG. J. Environ. Hort 24(2):105-108 pdf
  11. Post, A. R., A. Krings, W. Wall, and J. C. Neal. 2009. Introduced lesser celandine (Ranunculus ficaria, Ranunculaceae) and its putative subspecies in the United States: a morphometric analysis. J. Bot. Res. Inst. Texas (formerly SIDA) 3(1):193-209 pdf
  12. Post, A.R., J. C. Neal, A. Krings, B. R. Sosinski, and Q. Xiang. 2009. New Zealand bittercress (Cardamine corymbosa; Brassicaceae): new to the United States. Weed Technol. 23:604-607. pdf
  13. Walker, L.H., J.C. Neal and J.Derr. 2010. Preemergence control of doveweed (Murdannia nudiflora) in container grown nursery crops. J. Environ. Hort. 28(1):8-12 pdf

International Experience

Participant in the International Bioherbicide Group


  • Sabbatical Leave Report
  • Screening rhizobacteria for selective suppression of annual ryegrass in canola and wheat. Hosted by Dr. Gavin Ash, Charles Sturt University, Wagga Wagga, NSW, Australia.
  • Weed management practices in Australian nurseries. Hosted by Dr. Ian Gordon, Univ. of Queensland, Gatton, Queensland, Australia

Written By

Joe Neal, N.C. Cooperative ExtensionDr. Joe NealProfessor of Weed Science and Extension Specialist Call Dr. Joe Email Dr. Joe Horticultural Science
NC State Extension, NC State University
Page Last Updated: 2 months ago
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