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Publication #HS717

Weed Management Principles in Commercial Vegetable Production1

Peter Dittmar and Nathan Boyd2

Weeds compete with vegetable crops for light, water, and nutrients. This competition decreases plant vigor, yield, and crop quality (Figure 1). They interfere with hand harvest and can complicate or prevent machine harvest. Weeds also serve as alternative hosts to diseases, viruses, and nematodes. Lists of weed hosts for viruses and nematodes are available in the EDIS documents “Common Weed Hosts of Insect-Transmitted Viruses of Florida Vegetable Crops” (ENY-863) and “Weed Hosts of Root-Knot Nematodes Common to Florida” (ENY-060). There are also a range of books and EDIS documents that can help identify weeds. Two books that are helpful for the identification of weeds in Florida are Weeds of the South and Weeds of the Northeast.

Figure 1. 

Wild radish interference in sweet corn


Credit:

P. J. Dittmar


[Click thumbnail to enlarge.]

The first step in weed management is frequent and proper scouting. Fields should be frequently scouted early in the production year when the crop is more susceptible to competition. Weed populations are greater along the edges of the field and will vary due to microecosystems in the field (e.g., wet spots or different soil types). Scouting should be completed in a zig-zag pattern across the field for the entire length of the field.

Proper identification and understanding of life cycles is important for selecting the correct method and timing of weed control. Weeds may be annuals, biennials, or perennials. Annual weeds emerge from seed, grow, and flower within a single year. Summer annuals emerge in the spring and grow through the heat of the summer months. Summer annuals include pigweed, morningglory, crabgrass, pusley, and goosegrass. Winter annuals emerge during the fall and grow during the winter months. Biennial weeds emerge from seed and grow during the first year but do not flower and produce seeds until after a dormancy period. Biennial weeds include wild carrot, cutleaf evening primrose, and common mullein. Perennial weeds can grow and produce flowers for multiple years. Perennial weeds produce vegetative structures (stolons, rhizomes, tubers, or large roots) that generate new plants. Perennial weeds include nutsedge, Bermuda grass, Brazil pusley, and creeping beggerweed.

Decision tools like economic thresholds and weed-free periods can guide the timing of weed control. The economic threshold is the weed density that causes a significant decrease in crop yield. The threshold is different for each weed/crop complex (Table 1). Weeds may be present in the field, but the loss in the crop yield from competition may be less than the expense of control.

The second guide for timing of weed control is based on the critical weed-free period. To determine the weed-free period, researchers plant the crop and remove the weeds that germinate at specific intervals during the crop year (Figure 2, blue line). Typically, removing the weeds later in the season has a negative impact on plant growth or yield. In other plots, the crop is kept weed-free for a specific interval then weeds are allowed to germinate. The weed-free period is typically early in the crop cycle when the crop is less competitive (Table 2). Weeds that emerge later in the season have less of an impact on the crop. A threshold is determined, usually 5% or 10%, and a weed-free period is determined based on the period when plant growth is above the threshold. The weed-free period is the time required for the field to be weed-free for optimum plant growth or yield. Transplanted crops can become established and competitive more quickly than directly seeded crops.

Figure 2. 

Example of a critical weed-free period using a 10% reduction threshold. The critical weed-free period is the time between the black lines when crop growth or yield is below 100% of the weed-free. Weeds removed before or weeds that emerge after the critical period have >90% growth of the weed-free.


[Click thumbnail to enlarge.]

Weed management practices can be separated into five categories: preventative, cultural, mechanical, biological, and chemical. The most successful weed management programs will incorporate more than one type of weed control.

Preventative

The first step is site selection. Select a field with low weed populations and treat areas with problems such as poor drainage prior to crop establishment. Control or mow the weeds at the edges of fields or irrigation furrows to prevent seed formation. Seeds can be moved by equipment, wind, animals, and water, and may spread throughout the field. Weed seed can also move between fields on tractors, blades of cultivators, heads of harvest equipment, and other methods. All equipment should be cleaned after completing a task in a field with a high weed population. When possible, limit travel in the field to periods when weed seeds are not mature, and work your cleanest fields first and move toward the ones with the greatest weed populations. Purchase crop and cover crop seed from reputable sources to limit the amount of weed seed contamination.

Cultural

A healthy crop is important for competing against weeds. Use healthy transplants or seed with excellent germination to insure quick canopy closure. Plants stressed by improper watering (too wet or too dry), diseases, or nematodes are less competitive (Figure 3). Proper nutrition is important: minimize fertilizer in the row middles where crops won’t benefit but weeds will. Select the proper row spacing to allow for quicker canopy closure.

Figure 3. 

A wet low area of the field decreasing potato health and grass weeds emerge in the row middles


Credit:

P. J. Dittmar


[Click thumbnail to enlarge.]

Crop and variety selection has an impact on weed growth. Crops that are tall or have large leaves shade the soil surface and prevent weed seed germination. Crops such as cabbage, bean, and corn are very competitive crops. Onions and carrots allow more light to the soil surface and are less competitive. The same principal of light penetration to the soil surface can be applied to crop varieties: a compact variety will be less competitive compared to other varieties with larger growth habits.

Multiple vegetable crops are grown with polyethylene mulch as part of the cultural practices. The horticultural benefits of plasticulture are reduced water loss, better nutrient management, and a barrier for fumigation. The weed management advantage is the control of grass and broadleaf weeds. The plastic mulch prevents light penetration to the soil surface and inhibits weed germination. White plastic allows light, so select mulch that has a black underside to prevent light penetration. Grass and broadleaf weeds still grow in the crop holes, and yellow and purple nutsedge can pierce through the plastic mulch.

Repeating the same crop for multiple years with the same weed management control will select for certain weed species. Crop rotation allows for different weed control options to be used in the field. Choose a rotation based on crop competitiveness, use of mulch or cultivation, and different herbicide modes of action. Observe plant back restrictions of herbicide or injury that may occur in the crop that follows.

Cover crops should be included in the crop rotation. These shade the soil surface and prevent weed germination. Some cover crops, such as rye, have allelopathic compounds, which are plant chemicals that prevent seed germination. Additional information can be found in EDIS publication HS387 “Annual Cover Crops in Florida Vegetable Systems Part 1. Objectives: Why grow cover crops?”

Mechanical

Mechanical weed control includes plows, cultivators, mowers, hoes, and hand-weeding. Chisel and moldboard plows are used at the beginning of the season and cultivate deep into the soil profile. This process buries weed seeds below the germination zone. Light cultivation with a field cultivator controls small weeds by cutting the weeds and is shallow to prevent weed seeds from being brought to the soil surface. A single cultivation provides excellent control of annual weeds; however, cultivation may break apart pieces of perennial weeds and cause the weed to spread. Repeat cultivation is important to encourage continuous growth, which reduces the carbohydrates in the storage structure of the weed.

Basket, tine, or finger cultivators lightly disturb the soil surface and control small weeds by breaking roots or foliage. Basket cultivars will provide control in the row middles, however, weed control in the crop row will be minimal (Figure 4). Tine or finger cultivators may provide better weed control in the crop row.

Figure 4. 

Weed control in the row middles with a basket cultivator, nutsedge is still in the crop row


Credit:

P. J. Dittmar


[Click thumbnail to enlarge.]

Use mechanical weed control only when it will be most effective, because mechanical weed control degrades soil structure, dries the soil surface, and prunes crop roots.

Biological

Biological control relies on biological agent to damage a weed species. This method uses insects, plant pathogens, or animals. Several control agents are host specific, controlling certain weed species (e.g., the TSA beetle (Gratiana boliviana) for tropical soda apple, and the leaf mining fly (Hydrellia pakistanae) or the tuber weevil (Bagous hydrillae) for hydrilla). Because of these narrow feeding habits, this method is typically used in natural and aquatic areas for a single invasive species. Other biological agents consume several species (e.g., goats and grass carp). This can be difficult to manage if the crop species is also consumed. Biological control is not used in vegetable production due to the multiple weed species in the field; however, research is being conducted and new techniques may emerge in the future.

Chemical

Proper herbicide selection can be an effective weed control tool. Herbicides are classified by their mode of action, which is how they affect plant growth. Herbicides are separated by application placement, selectivity, and translocation.

Application placement includes foliar- or soil-applied herbicides. Foliar-applied herbicides control the weeds after emergence above the soil surface (postemergence). Proper coverage of the foliage is important for foliar applied herbicides, and a surfactant is often required for proper absorption of the herbicide. Soil-applied herbicides control the weeds before emergence above the soil surface (preemergence). Soil-applied herbicides are applied to the soil surface or require incorporation into the soil surface. Incorporation reduces vaporization of certain herbicides or places the herbicide closer to the weed seed. Incorporation methods include irrigation, rainfall, or light cultivation. Poor incorporation will result in reduced efficacy.

Herbicide selectivity results in control of a specific type of weed such as broadleaf or grass weeds only. Auxin herbicides (2,4-D, clopyralid) control broadleaf weeds only and are common in grass crops or turfgrass. Carfentrazone and certain sulfonylureas provide excellent control of broadleaf weeds with low to no injury to grass crops. Grass only/Gramineae herbicides/ACCase herbicides (clethodim, sethoxydim, fluaziflop) control only grass weeds and can be applied over the top of broadleaf weeds.

Herbicides can be grouped as translocating or contact herbicides. Translocating herbicides (glyphosate, halosulfuron) move from the contact point to another part of the plant. This is important when controlling perennial weeds, which require root death for complete control. Contact herbicides (carfentrazone, paraquat) kill the area around the contact point; complete coverage is important for these herbicides.

Herbicide-resistant weed species have become more problematic. Paraquat-resistant American black nightshade, paraquat-resistant goosegrass, and glyphosate-resistant Palmer amaranth have been documented or observed in Florida vegetable crops. To prevent resistance, growers should incorporate nonchemical control methods, rotate modes of action, choose products with multiple modes of action, use correct rates, and monitor constantly.

Other EDIS publications that provide information about herbicides for vegetables and fruits include “Estimated Effectiveness of Recommended Herbicides on Selected Common Weeds in Florida Vegetables” (HS706), “Calibration of Chemical Applicators Used in Vegetables” (HS1220), and “Factors Affecting Herbicide Use in Fruits and Vegetables” (HS1219).

Conclusion

For decades, the vegetable industry relied on methyl bromide for weed control. The phase-out of methyl bromide has proven that reliance on a single method of weed control can be detrimental to an individual field and the vegetable industry as a whole. The vegetable industry faces additional challenges to weed control. Erosion and fuel prices restrict the use of mechanical weed control. Immigration policies affect hand weeding. Herbicide resistance and public policy affect chemical control. A successful weed management program will incorporate multiple control practices with preventative, cultural, biological, mechanical, and chemical methods.

References

Amador-Ramirez, M.D. 2002. Critical period of weed control in transplanted chili pepper. Weed Res. 42:203–209.

Baziramakenga, R. and G. D. Leroux. 1994. Critical period of quackgrass (Elytrigia repens) removal in potatoes (Solanum tuberosum). Weed Sci. 42:528–533.

Bhowmik, P. C., and K. N. Reddy. 1988. Interference of common lambsquarters (Chenopodium album) in transplanted tomato (Lycopersicon esculentum). Weed Technol. 2:505–508.

Blackshaw, R. E. 1991. Hairy nightshade (Solanum sarrachoides) interference in dry beans (Phaseolus vulgaris). Weed Science 39:48–53.

Buker, R. S. III, W. M. Stall, S. M. Olson, and D. G. Schilling. 2003. Season-long interference of yellow nutsedge (Cyperus esculentus) with direct-seeded and transplanted watermelon (Citrullus lanatus). Weed Technol. 17:751–754.

Ciuberkis, S., S. Bernotas, S.Raudonius, and J. Felix. 2007. Effect of weed emergence time and intervals of weed and crop competition on potato yield. Weed Technol. 21:213–218

Gilbert, C. A., W. M. Stall, C. A. Chase, and R. Charudattan. 2008. Season-long interference of American black nightshade with watermelon. Weed Technol. 22:186–189.

Friesen, G. H. 1978. Weed interference in pickling cucumbers (Cucumis sativus). Weed Sci. 26:626–628.

Mallet, S. J. and R. A. Ashley. 1988. Determination of summer squash’s tolerance: A critical period study. Proc. Northeast Weed Sci. Soc. 42:204–208.

Miller, A. B., and H. J. Hopen. 1991. Critical weed-control period in seeded cabbage (Brassica oleracea var. capitata). Weed Technol. 5: 852–847.

Monks, D. W. and J. R. Schultheis. 1998. Critical weed-free period for large crabgrass (Digitaria sanguinalis) in transplanted watermelon (Citrullus lanatus). Weed Sci. 46:530–532.

Morales-Payan, J. P., B. M. Santos, W. M. Stall, and T. A. Bewick. 1997. Effects of purple nutsedge (Cyperus rotundus) on tomato (Cycopersicon esculentum) and bell pepper (Capsicum annuum) vegetative growth and fruit yield. Weed Technol. 11:672–676.

Morales-Payan, J. P. 1999. Interference of purple and yellow nutsedges (Cyperus rotundus L. and Cyperus esculentus L.) with tomato (Lycopersicon esculentus L.). PhD dissertation. Univ. FL., Gainesville.

Motis, T. N., S. J. Locascio, and J. P. Gilreath. 2004. Critical yellow nutsedge-free period for polyethylene-mulched bell pepper. HortScience 39:1045–1049.

Santos, B. M., J. A. Dusky, W. M. Stall, D. G. Shilling, and T. A. Bewick. 1997. Influence of smooth pigweed and common purslane densities on lettuce yields as affected by phosphorus fertility. Proc. Fla. State Hort. Soc. 110:315–317.

Seem, J. E., N. G. Creamer, and D. W. Monks. 2003. Critical weed-free period for ‘Beauregard’ sweetpotato (Ipomoea batatas). Weed Technol. 17:686–695.

Terry, E. R. Jr., W. M. Stall, D. G. Shilling, T. A. Bewick, and S. R. Kostewicz. 1997. Smooth amaranth interference with watermelon and muskmelon production. HortScience 32:630–632.

VanGessel, M.J. and K. A. Renner. 1990. Redroot pigweed (Amaranthus retroflexus) and barnyardgrass (Echinochloa crus-galli) interference in potato (Solanum tuberosum). Weed Sci. 38:338-343.

Weaver, S. E., and C. S. Tan. 1983. Critical period of weed interference in transplanted tomatoes (Lycopersicon esculentum): Growth analysis. Weed Sci. 31:476-481.

Weaver, S. E. 1984. Critical period of weed competition in three vegetable crops in realtion to management practices. Weed Res. 24:317–325.

Woolley, B. L., T. E. Michaels, M. R. Hall, and C. J. Swanton. 1993. The critical period of weed control in white bean (Phaseolus vulgaris). Weed Sci. 41:180–184.

Tables

Table 1. 

Effect of weed density on yield of various vegetable crops

Crop

Weed, population

Yield reduction

Reference

Bean, white

Hairy nightshade, 2 plants/m

105

Blackshaw 1991

Cabbage

Velvetleaf, 1.2 plants/m2

52 to 76%

Miller and Hopen 1991

Lettuce

Common purslane, 16 plants/6 m of row

44 to 48%

Santos et. al 1997

Lettuce

Smooth pigweed, 2 to 4 plants/6 m of row

20%

Santos et. al 1997

Pepper, bell

Purple nutsedge, 0.63 plants/m2

10%

Morales-Payan et al. 1997

Potato

Barnyardgrass, 1 plant/m

19%

VanGessel and Renner 1990

Potato

Redroot pigweed, 1 plant/m

22 to 30%

VanGessel and Renner 1990

Tomato

Purple nutsedge, 0.46 plants/m2

10

Morales-Payan et al. 1997

Tomato

Common lambsquarters, 16 plants/m

17%

Bhowmik and Reddy 1988

Watermelon

American black nightshade, 2 plants/m2

80%

Gilbert et al. 2008

Watermelon

Yellow nutsedge, 2 plants/m2

10%

Buker et al. 2003

Table 2. 

Weed-free periods

Crop

Weed

Period1

Reference

Bean, white

Various

Second trifoliate to first-flower stages

Woolley et al. 1993

Cabbage, seeded

Velvetleaf

0 to 4 WAE

Miller and Hopen 1991

Cabbage, transplanted

Various

3 to 5 WAE

Weaver 1984

Cucumber, pickling

Various

0 to 36 DAE

Friesen 1978

Cucumber, pickling

Various

0 to 4 wk. after seeding or 3 to 4 consecutive wk.

Weaver 1984

Pepper, fall planting

Yellow nutsedge

2 and 7 WAT

Motis et al. 2004

Pepper, spring planting

Yellow nutsedge

3 and 5 WAT

Motis et al. 2004

Pepper, transplanted chili

Various

1.3 to 11.9 WAT

Amado-Ramirez 2002

Muskmelon, seeded

Smooth amaranth

1.0 to 3.9 WAE

Terry et al. 1997

Squash, summer

Various

4 to 6 WAE

Mallet and Ashley 1988

Sweetpotato

Various

2 to 6 WAT

Seem et al. 2003

Potato

Various

Planting to 25 d after flowering

Ciuberkis et al. 2007

Potato

Quackgrass

0 to 68 DAE

Baziramakenga and Leroux 1994

Tomato

Various

28 to 35 d. after transplanting

Weaver and Tan 1983

Tomato

Yellow nutsedge

3 to 6 WAT

Morales-Payan 1999

Watermelon, transplanted

Large crabgrass

0 to 6 WAT

Monks and Schultheis 1998

Watermelon, seeded

Smooth amaranth

0.5 to 2.97 WAE

Terry et al. 1997

1 WAT= weeks after transplanting, WAE= weeks after emergence, DAE= days after emergence

Footnotes

1.

This document is HS717, one of a series of the Horticultural Sciences Department, UF/IFAS Extension. Original publication date June 1995. Revised July 2014. Please visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Peter Dittmar, assistant professor, Horticultural Sciences Department, Gainesville, FL 32611; and Nathan Boyd, assistant Professor, Horticultural Sciences Department, Gulf Coast Research and Education Center, Wimauma, FL.

The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition. Use pesticides safely. Read and follow directions on the manufacturer's label.


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U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.