Midwest Vegetable Guide - 2022 - 57

Weed Management Strategies
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 'offbalance'
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 warmseason
crops.
Critical Weed Free Period1
First third of cultivation period
First half of cultivation period
2 weeks after first half of
cultivation period
4 weeks after first half of
cultivation period
None
Crop
Potato, Pumpkin, Sweet
Corn
Asparagus, Beans, Cole
Crops, Lettuce, Pepper,
Tomato
Carrots, Celery, Beet, Peas
Garlic, Leek, Onion
Chives, Parsley, Fennel,
Swiss Chard, Spinach
1Modified 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.
A good introduction to mechanical cultivation be found in the
publication Steel in the Field available online at sare.org/wpcontent/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.
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.
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https://www.sare.org/wp-content/uploads/Steel-in-the-Field.pdf https://www.sare.org/wp-content/uploads/Steel-in-the-Field.pdf https://northcentral.sare.org/resources/in-row-mechanical-weed-control-options-for-farmers-large-and-small/ https://northcentral.sare.org/resources/in-row-mechanical-weed-control-options-for-farmers-large-and-small/ https://eorganic.org/node/3411 https://edepot.wur.nl/24553

Midwest Vegetable Guide - 2022

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