A Review of Cultural Weed Control Practices

In this month's Penn State Extension - The Vegetable & Small Fruit Gazette,

Mike Orzolek reviews methods & practices for weed control.

Effective weed control in vegetable production involves more than the use of herbicides. An effective weed management program includes the use of both cultural and chemical practices. There are several cultural practices that help to suppress both annual and perennial weeds including: liming, band application of fertilizer, moldboard plowing, cultivation(s), crop rotations, and use of black or colored polyethylene mulch.

Also see: Weed Control Strategy Differences in Alternative/Organic vs Conventional Farming by Brad Majek and Jack Rabin.
Weed prevention is the most basic and easiest method for successful weed management in your vegetable crop. Avoid weed establishment in all your fields by not allowing weeds to go to seed (reproduce) and eliminate all individual weed survivors. In addition, identifying and mapping weed infestations in all your fields, especially perennial weeds, and maintaining accurate records over many years will help you make both crop production as well as effective weed management decisions in the future. It is also very important to recognize and eliminate new weeds before they multiply and establish.

Use of sanitary procedures to prevent the spread of weeds and their seeds will long term reduce the number of weed seeds in your field’s soil. Clean all field equipment between sites or infestations. Cleaning in this case refers to the washing or steaming of all tillage equipment after use between different field locations; especially land that is rented. It is recommended that you examine all transplants, seed, and imported soil or media for potential weeds as plants or seed contamination. If found, remove all weeds and seed from either transplants or media before establishing the crop in the field. Screen irrigation water where weed seed contaminates surface water transported in canals and rivers or stored in lakes or ponds. This can be a major source of weed seed contamination over time. Control weeds and seed sources around all production fields and/or plant production sites.

Liming fields to maintain a soil pH of 6.0 to 6.8 will help reduce the carryover of triazine herbicides and increase the activity of other types of herbicides. In addition, soil pH will affect the activity of soil microorganisms that help breakdown herbicides. In general, bacteria and actinomyces are favored by soils having a medium to high pH, and their activity is seriously reduced below a pH of 5.5. Fungi tolerate all normal soil pH values. In normal soils, therefore, fungi predominate at pH 5.5 and below. Above pH 5.5, the fungi are reduced through competition with bacteria and actinomyces. Thus a warm, moist, well-aerated, fertile soil with optimum pH is most favorable to microorganisms that can rapidly decompose organic herbicides.

Banding of fertilizer will help to maintain an actively growing and healthy vegetable crop and reduce nutrient availability to weeds growing between the crop rows. In addition, normal recommendations are that you would band half the recommended broadcast fertilizer application. Example, if your soil test recommended the application of 100 – 200 – 200 lbs/A of N-P-K broadcast, then the recommended banding rate would be 50 – 100 – 100 lbs/A of N-P-K. Not only are you reducing the nutrients that weeds would have available for growth, but you would also cut your fertilizer bill by 50%.

Moldboard plowing will physically uproot weeds, especially perennial weeds with large taproots, resulting in damaged roots, low carbohydrate reserves, and desiccation of the weeds. Additional cultivation after plowing during the winter if possible will increase the level of desiccation that weeds will be exposed to during the winter. After crop establishment and during the growing season, cultivation either once or several times in the field will uproot weed seedlings (especially if they are under 2 inches tall), increase soil aeration and reduce soil surface crusting. The only disadvantage of relying solely on cultivation to control weeds is that wet soil conditions could prevent the use of a tractor and cultivating equipment due to the potential of soil compaction and the ability of plants to recover in moist soil after physically being uprooted.

Select manageable fields (identify weeds and choose crop according to feasibility of weed management strategies; e.g., avoid planting onions into perennial weeds). Rotate crops and disrupt weed life cycles or suppress weeds in competitive crop followed by planting a noncompetitive crop.

Plant winter cover or competitive fallow crops in rotation to improve soil structure, organic matter content and crop management strategies(specific cultivars are being evaluated). Consider legumes to supplement nitrogen requirements. Consider specific varieties of cereals with natural plant toxins (allelopathy); vegetation must remain uniform on soil surface; either perennial or large-seeded crops can be planted through undisturbed mulch. Consider crops or cultivars that winter-kill after vigorous growth during fall to avoid springtime controls.

Transplant slow-growing vegetable crops, especially a crop like cabbage. If a slow-growing crop like cabbage is direct seeded in the field, slow germination and emergence of the cabbage will allow significant growth of volunteer weeds that will compete successfully with the cabbage resulting in reduced yield and or quality of the cabbage.

Place and time fertilizer, especially nitrogen. Band or spot fertilizer beside plants or seed (reduces availability to surface-germinating weeds). Time additional side-dressings for maximum crop growth or to minimize weeds. Encourage the development of crop canopies that shades weeds and suppresses weed germination.

Select crops or varieties that form a canopy quickly. Space plants in equidistant (triangular) arrangements and vary density depending on crop management constraints or harvest requirements (e.g., product quality). Combine broadleaf and taller, narrowleaf crops (corn or beans with pumpkins). Relay plantings or harvest short-duration crops within longer maturing crops (bush beans with corn; cucumbers with peppers; tree crops with vegetables). Manage appropriate living mulch (grass or legume) between perennial crop rows. Improve pasture management by reseeding and/or fertilizing with or without control measures to reduce weed infestation (weeds often are a symptom of poor management).