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2022 Midwest Vegetable Guide

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Weed Management Strategies Midwest Veg Guide 2022 57 Rotating between crops will improve crop growth and competitiveness. Related vegetables should not be grown in the same location in successive years (see Botanically Related Vegetables). This is important both to ensure healthy crop growth for weed competitiveness, but also to avoid buildup of problematic weed species that are well adapted to specific vegetable crops. Crop rotation helps keep weeds 'off- balance' by applying different selective pressures including variations in the timing of tillage and type of herbicides applied. Wild proso millet is an example of a problem weed where rotation is important for management. Rotation from sweet corn to early-planted peas, or alfalfa almost completely eliminates wild proso millet because these crops are established before the soil is warm enough for wild proso millet seed germination. A rotation from sweet corn to broadleaf crops also allows the use of postemergence grass herbicides to manage wild proso millet. Competitive Cultivars and Planting Methods. Each crop has a Critical Weed Free Period (table below) for which weed competition should be eliminated to ensure a good crop. Use adaptive, vigorous varieties resistant to diseases and insects. Unhealthy plants cannot effectively compete with weeds. Varieties suited for cultivation in regions covered by this publication are listed in some sections of this guide. Narrower row spacings and proper plant densities assure crop canopy closure. Closed canopies shade out later emerging weeds and prevent germination of weed seeds that require light. Weeds seldom are a problem after canopy closure. Proper row spacing and plant density also allow row cultivation. Correct planting time is another cultural method that can improve crop competitiveness. Crops can be divided into warm- or cool-season plants, depending on the optimum temperature for their growth. Planting date affects the time until emergence and the crop's early seedling vigor, both of which are important in determining crop competitiveness. Cool-season crops germinate at cooler soil temperatures, so compete better against early emerging weeds than warm- season crops. Critical Weed Free Period 1 Crop First third of cultivation period Potato, Pumpkin, Sweet Corn First half of cultivation period Asparagus, Beans, Cole Crops, Lettuce, Pepper, Tomato 2 weeks after first half of cultivation period Carrots, Celery, Beet, Peas 4 weeks after first half of cultivation period Garlic, Leek, Onion None Chives, Parsley, Fennel, Swiss Chard, Spinach 1 Modified from Practical weed control in arable farming and vegetable cultivation without chemicals PPO 352 (2006), from Wageningen University & Research Applied Plant Research unit, at edepot.wur.nl/24553 . Mulches. Mulching can be useful in managing weeds. Mulches can be classified as either natural (e.g., straw, leaves, paper, and compost) or synthetic (plastics). Because natural mulches are difficult to apply over large areas, they are best for small, specialized areas. Natural mulches should be spread evenly at least 1 to 1.5 inches thick over the soil to prevent light penetration. Natural mulch materials must be free of weed seeds and other pest organisms and be heavy enough so they will not be easily displaced by wind or water. A major advantage of natural mulches is that they add organic matter to the soil and do not need to be disposed of at the end of the season. Synthetic mulches are easy to apply, control weeds within the row, conserve moisture, and increase soil temperature. Black or clear plastic mulches are the most common and are effective in improving early- season growth of warm-season crops such as tomato, cantaloupe, watermelon, or pepper. Fast early-season growth of these crops improves their competitive ability against weeds. Plastic mulches used in combination with trickle irrigation can also improve water use efficiency. A disadvantage of plastic mulch is disposal at the end of the season. Many landfills do not accept plastic mulches. Photodegradable plastic mulches have been developed, but their season-long persistence has been a problem, and they degrade into small pieces of plastic that contaminate the environment. Biodegradable plastic mulches are available. Mechanical Practices Mechanical weed management relies on a wide range of primary and secondary tillage implements including moldboard plows, disks, rotary hoes, row cultivators and various 'in-row' tools. It typically begins with primary tillage with a moldboard or chisel plow, followed by disking, field cultivating and bed formation. These operations are important for successful production of most vegetable crops for several reasons including formation of a good seed bed, incorporation of soil amendments, and breaking the life cycle of pests. For weed management, these tillage operations help eliminate established winter annual and perennial weeds, as well as summer annuals that emerge prior to planting. A good introduction to mechanical cultivation be found in the publication Steel in the Field available online at sare.org/wp- content/uploads/Steel-in-the-Field.pdf, and video footage demonstrating the nuances of many of these tools and techniques can be seen in the North Central SARE-funded videos In-Row Mechanical Weed Control Options for Farmers Large and Small, as well as in the Weed 'Em and Reap Part 1: Tools for Organic Weed Management in Vegetable Cropping Systems video series found at eorganic.org/node/3411.

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