Consider your weed control strategy prior to establishing a vegetable garden in the spring. Weeds compete with vegetables for water, nutrients and light. They also harbor insects and diseases, which may then spread to vegetables. Efforts made early in the growing season will reduce the time required for hand-weeding later in the season. The weed control program may require a combination of methods to manage all weed species.
The most common weeds found in the garden can be divided into summer annuals, winter annuals and perennials. There are a few biennial weeds in South Carolina, but these are generally not a problem in gardens.
SUMMER ANNUAL WEEDS
Summer annuals are weeds that germinate in spring or early summer and flower in the summer or fall. This is the most common type of weed found in the garden. This group can be further divided into summer annual grasses and summer annual broadleaf weeds.
Common summer annual grasses include large crabgrass, goosegrass and giant foxtail. Annual grasses are easy to control if appropriate measures are taken early in the growing season but can quickly become a severe problem if not controlled when small. Large crabgrass, for example, will root into the soil at the places where the nodes of the stem contact soil, allowing this plant to quickly cover open ground. The fibrous root system of grasses makes them more difficult to pull out of the ground.
Common summer annual broadleaf weeds include smooth pigweed, common lambsquarters, purslane, galinsoga, common ragweed and tall morningglory. When controlling purslane through hoeing, remove all stems from the garden because purslane can reroot if allowed to remain on the soil surface due to the thick, succulent stem that can survive a period of drought. Galinsoga is often called quickweed, perhaps because it develops quickly and flowers while still a small plant. It seems that most gardens contain purslane, galinsoga or both. Small-seeded broadleaf weeds like pigweed are easier to control than large-seeded broadleaf weeds like morningglory. Larger-seeded weeds can germinate from a greater soil depth and can push through a shallow layer of mulch.
WINTER ANNUAL WEEDS
Winter annuals are weeds that germinate in fall and flower in the spring. These weeds are often present at the time the garden soil is prepared in the spring prior to planting. Tilling the soil will kill existing stands of common chickweed and other winter annuals. Annual bluegrass is a common winter annual grass, while henbit, common chickweed and wild mustard are common winter annual broadleaf weeds. Weeds in this category are generally not as troublesome in the garden as summer annuals.
PERENNIAL WEEDS
Herbaceous perennial weeds can be especially difficult to manage in a garden. These plants are killed back to the ground by a hard frost and overwinter through underground plant parts, such as tubers or rhizomes. Tilling the garden may spread these weeds by fragmenting and moving root pieces. Each of the root pieces may develop into a complete plant. The most common perennial weeds are common bermudagrass and yellow nutsedge. Bermudagrass, often called wiregrass, spreads by creeping stolons and rhizomes. Thoroughly remove all plant parts from the garden when hand-weeding because bermudagrass can reroot into the soil. Yellow nutsedge is often called nutgrass, but this plant is a member of the sedge and not the grass family. Yellow nutsedge reproduces by underground tubers.
MULCHES
Mulches can be divided into organic, such as grass clippings, and inorganic, such as black plastic. Mulches can be the easiest and most effective way to control annual weeds in the garden. Mulches may also suppress perennial weeds. Mulches control weeds by preventing sunlight from reaching the soil surface. Light is required for the germination of certain weeds, and light is required for the growth of all green plants.
Organic Mulches: Organic mulches include grass clippings, pine bark, straw or similar materials (see table, page 3). Organic mulches cool the soil surface, which is beneficial during hot summer days, but may reduce crop growth in the spring. Do not use grass clippings from a lawn that has been treated with a broadleaf herbicide such as 2,4-D. Tomatoes, peppers and most other vegetables are very sensitive to 2,4-D and could absorb residues of the compound from the treated grass clippings.
Avoid overmulching, which can reduce oxygen levels in the soil. Crop roots require oxygen for growth, so limit the mulch layer to a maximum of about 3 inches. Organic mulches provide good control of annual weeds, but perennial weeds may be able to push through the mulch layer. Also, annual weeds may germinate and grow in the mulch layer.
When using an organic mulch, make sure that the source is not contaminated with weed seed, rhizomes or tubers. Weeds are often spread by contaminated mulch, making weed control harder than if no mulch was used.
Inorganic Mulches: Inorganic mulches are synthetic and can be divided into black plastic and the newer geotextiles. Black plastic, a solid sheet of polyethylene, effectively controls annual weeds. The disadvantage of black plastic is that water and oxygen cannot pass through this material. The soil should be moist prior to laying this synthetic material. Check the soil under black plastic during the growing season to ensure that the soil contains adequate moisture. Black plastic warms the soil, which is an advantage in the spring but can be harmful in the summer.
Clear plastic, which increases soil temperature more than black plastic, will not control weeds since sunlight can reach the soil surface. Perennial weeds may be suppressed by black plastic, but plants like yellow nutsedge will push through the material in places.
The new geotextiles, also called weed barriers, are woven or spun-bonded fabrics containing polypropylene or polyester and may come in black or white. These fabrics are more expensive than black plastic, but they allow water and gases to pass through the fabric. Research generally indicates good control of annual weeds, but annual weeds may be able to germinate above the fabric and send their roots through the fabric. Certain weeds, such as large crabgrass, are able to germinate below these fabrics and push their shoots through holes in the material. Perennial weeds may also push through these fabrics.
PHYSICAL CONTROL
Since mature weeds remove large quantities of moisture and nutrients from the soil, removing the weeds when they are young is important. Hand-pulling works for small gardens and raised beds, but a hoe is critical for larger gardens. Cut off weeds just below the soil surface with a sharp hoe. Hoeing must be shallow because most vegetable roots are near the soil surface and can be easily damaged.
Manual-powered rotary cultivators do a good job on long rows and pathways, provided that the soil is not too wet or dry and the weeds are small. In large gardens with widely spaced rows, a rotary tiller of appropriate size makes the work easy and fast. Manual and powered rotary cultivators are usually unable to turn under weeds close to vegetable plants without damaging the vegetables. Hand-pulling or hoeing are best for removing weeds near vegetable plants. Deep cultivation with any instrument is likely to damage roots or stems of crop plants.
While cultivating, try to move as little soil as possible to limit the amount of weed seed brought to the soil surface that can then germinate. It is best to cultivate when the soil is moist but not wet. After removing weeds from the garden, apply mulch to control later germinating weeds.
CROP COMPETITION
Once vegetable plants become established, their foliage will begin to shade the ground. This shading effect reduces the amount of light available for weed germination and development. Close spacing of the vegetables will decrease the amount of time required for the crop canopy to significantly shade the soil. If weeds are controlled initially by other means, crop competition will reduce weed growth once the vegetables become well-established.
CHEMICAL CONTROL
For most gardens, annual weeds can be controlled using a combination of mulches and hand-weeding, and herbicides are not needed. If the size of the garden warrants herbicide use, check Weed Control In Home Vegetable Gardens in the Georgia Pest Management Handbook at: Herbicides may temporarily reduce early vigor and growth of some vegetable plants, particularly under cool, wet conditions which often occur in early spring. Timing of the herbicide application and seed planting sequence may vary and can determine whether or not a temporary reduction of plant vigor will occur.
The greatest potential for vigor reduction occurs when cool, wet weather follows an application of either a preemergence or preplant incorporated herbicide. Severe injury can also occur when: label directions for rate and application timing are not followed; herbicides are used on a species of vegetable not shown on the herbicide label; or incorrectly calibrated application equipment is used. Vigor reduction is minimized when a labeled herbicide is used according to label directions.
Herbicides are not universally labeled for use on all vegetable crops, because (among other reasons) different crops have differences in tolerance, just as weeds may vary in tolerance.
Hard-to-kill perennial weeds, such as bermudagrass or nutsedge, may be controlled when the garden plot is fallow (not planted in a crop) with glyphosate (Roundup®) in combination with cultivation.
COMBINATION OF METHODS
A combination of two or more of the methods listed above may be required for weed management in the garden. A combination of mulching and hand-weeding will probably be the best system for most home gardeners. Herbicides could be used for special situations, such as for the control of perennial weeds. Early-season weed control will greatly reduce the time required for hand-weeding later in the growing season. Once vegetable plants are established, they will shade the soil if closely spaced together. This shading of the soil will suppress weed growth.
Some gardeners experiment with various types of no-till gardening to prevent erosion and moisture loss. One method is to plant a fall cover crop and then kill it with a postemergence herbicide, such as glyphosate. Vegetables can then be planted in the killed sod after a waiting period. Another alternative is the use of a mowed, living sod.
The use of cover crops over several seasons can reduce weed problems. However, this method requires leaving that part of the garden uncultivated, which reduces growing space. Use of cover crops and no-till gardening should be tried in small sections of the garden prior to extensive use.
Mulching materials for vegetable gardens
Organic
|
Synthetic
|
| Straw |
Plastic (polyethylene) |
| Peanut hulls |
Wax-coated paper |
| Chopped cornstalks |
Aluminum foil |
| Ground corncobs |
Kraft paper-polyethylene combination |
| Pine needles |
|
| Broomsedge |
|
| Sudangrass |
|
| Grass clippings |
|
| Leaf mold |
|
| Compost |
|
| Newspapers |
|
| Sawdust |
|
| Bark |
|
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Composting is controlling the natural decay of organic matter by providing the right conditions for
composting critters to convert yard trimmings into aproduct that can be returned to your landscape and
garden. Tiny organisms (mainly bacteria, fungi and protozoa) break down garden and landscape
trimmings in a moist, aerobic (oxygen-demanding) environment. The final product is a dark, crumbly
form of decomposed organic matter. Compost improves your soil. When added to soil,
compost breaks up heavy clay soils, helps sandy soils retain water and nutrients, and releases
essential nutrients. Compost also contains beneficial microscopic organisms that build up the soil and
make nutrients available to plants. Improving your soil is the first step towards growing healthy plants.
WHAT CAN I COMPOST?
Most plant material can be used for compost. Organic trimmings in your landscape, such as fallen
leaves, pine needles, grass clippings, flowers and the remains of garden plants make excellent compost.
Compost made from grass clippings treated with herbicides and pesticides is not recommended for
use in vegetable gardens. Kitchen scraps, such as fruit and vegetable peels and trimmings, crushed
eggshells, tea bags, and coffee grounds and filters can also be composted. Woody yard trimmings can
be run through a shredder before adding to the compost pile. Sawdust may be added in moderate
amounts if additional nitrogen is applied. Add a pound of actual nitrogen per 100 pounds of dry
sawdust.
WHAT MATERIALS SHOULD AVOID ADDING TO MY COMPOST PILE?
Organic materials that should not be added to your compost pile include meat, bones and fatty foods
(such as cheese, salad dressing and leftover cooking oil). Do not add pet or human wastes to a compost
pile. Weeds that have not gone to seed can be added to the compost pile. Weeds with large storage roots
like nutsedge, Florida betony or greenbriar should be left out and dried in the sun before composting to
reduce their chances of survival. The high levels of heat produced in the center of the
compost pile can kill many pests, such as weeds with seeds and diseased or insect-infested plants.
However, it is very difficult to mix the contents thoroughly enough to bring all the wastes to the
center, so some disease organisms may be returned to the garden with the compost.
“ESSENTIALS” OF COMPOSTING
Organic materials for composting all contain nutrients that provide energy and growth for
microorganisms. These organic materials each have their own ratio of carbon to nitrogen (C:N) in their
tissues. These C:N ratios are important because the tiny decomposers need about 1 part of
nitrogen for every 30 parts of carbon in the organic material. If the ratio is greater than 30:1, nitrogen
will be lacking and materials will decompose more slowly. Leaves, straw and sawdust are high in carbon, while
grass clippings, manure and vegetable scraps are higher in nitrogen. It helps to think of these
materials as greens and browns. Greens, such as grass clippings, are high in nitrogen. Browns, such
as leaves or sawdust, contain high amounts of carbon. Be aware that anything organic will decay (as long
as it is organic, the critters will eat it); however, it may take a long time to make compost when the
C:N ratio is too high. For example, a pile made solely of sawdust will take years to decay. Adding
more greens, such as grass clippings or vegetable scraps, will speed up decay and produce compost in
less time. Experiment to find the right combination of materials for your compost pile.
Average carbon to nitrogen ratios for organic materials.
Greens:
Pig manure 5-7:1
Coffee grounds 20:1
Vegetable scraps 12-20:1
Grass clippings 12-25:1
Cow manure 20:1
Poultry manure (fresh) 10:1
Horse manure 25:1
Alfalfa / sweet clover hay 12:1
Poultry manure w/litter 13-18:1
Horse manure w/ litter 30-60:1
Browns:
Leaves 30-80:1
Cornstalks 60:1
Straw 40-100:1
Bark 100-130:1
Paper 150-200:1
Sawdust 400:1
Wood chips 800:1
Surface Area and Size of the Compost Pile: The more surface area the microorganisms have to work
on, the faster the materials will decompose. You can increase the surface area of your yard trimmings
by chopping them up with a shovel or running them through a shredding machine or lawnmower.
A large compost pile will insulate itself and hold in the heat created by the tiny organisms. Piles smaller
than 3 feet x 3 feet x 3 feet have trouble holding this heat, while piles larger than 5 feet x 5 feet x 5 feet
prevent enough air from reaching the center of the pile and the microbes. In addition, turning a large
pile is a chore. If your pile is large, you will have to turn it more often. If the pile is small, you will get a
good batch of compost during warm months.
Moisture and Aeration: The microbes in your compost pile need a certain amount of water and air
to survive. Microbes function best when the materials are about as moist as a wrung-out sponge
and are provided with plenty of air. Too much moisture will force out the air and suffocate the
microorganisms. Too little moisture will slow down decay. Whenever you add water, be sure to mix the
material to distribute the moisture evenly. Turning the materials in your pile supplies oxygen to
the composting critters. A lack of oxygen in a compost pile can lead to an odor problem due to the
production of ammonia and methane gases. Decomposition without oxygen also causes the
production of chemical compounds that are toxic to plants. Organic matter that has been allowed to
decompose without oxygen (for example, “composting” in closed garbage bags) should be
exposed to air for several days to complete the composting process and to destroy any plant-toxic
compounds.
Temperature and Time: As a result of the decomposition process, the interior temperature of
the pile should peak between 90 and 140 °F or higher. A hotbed (or long-stemmed) thermometer
can be used to check the interior temperature of the pile at least 12 inches from the surface. The
intensity of the process depends on the amount of nitrogen in the materials. The time required to
produce compost depends on the kind and coarseness of the materials, volume of the pile, and
availability of moisture and air. It can take a month, a year or longer.
SOME COMPOSTING METHODS
“Fast” Compost Recipe: This method can produce compost in a couple of months or less but is labor
intensive and requires frequent turning. Start your pile with a layer of browns, and then add a layer of
greens. If the greens are not fresh, sprinkle in some blood meal or cottonseed meal, poultry manure, or
other nitrogen source. Mix well and add water if necessary to moisten. Adding a layer of garden soil,
old compost or manure to each brown-green layer will introduce more critters to speed up the process.
Continue adding and mixing layers of greens and browns until you either fill the bin or run out of
materials. Slant the top of the pile to the center to catch rainfall. You may want to cover the pile with
a plastic covering or tarp to regulate the amount of moisture entering your pile. The cover should not
rest on the pile because it may cut off oxygen. Periodically, check the moisture content of your pile.
The compost should feel damp. Check the interior temperature of your pile and when the temperature
reaches 140 °F or begins to fall, it is time to turn the pile. You will need to turn your pile every three to
five days. Once your turning causes no rise in temperature, and the material appears dark and
crumbly, your compost is ready.
“Slow” Compost Recipe: Slow composting is the least labor-and time-consuming way to compost; it is
ideal for people who do not have a large amount of yard trimmings to compost all at once. This method
can take from six months to two years or longer to produce compost, so be patient.
The ingredients are the same as those for a “fast” compost. Add greens and browns to your pile
whenever they become available. Turn the pile occasionally to mix the materials together to prevent
the materials from clumping together and to avoid anaerobic decomposition. You will know that your
materials are decaying without oxygen by the foul odor: a telltale sign for you to turn the pile. Look
for ready-to-use compost near the bottom of the pile.
COMPOST STRUCTURES
Composting structures can be made from a wide variety of materials or purchased through local
garden centers or mail order catalogues. There are no set rules when building a compost bin. The sides
should be loose enough to provide some air movement and one side should open for easy
turning and compost removal. If you choose not to use a container, cover the heap with a layer of yard
trimmings or soil to prevent moisture loss.
Circular Bins: A circular bin is very useful for composting larger quantities of organic materials,
and can be easily made from a length of closely spaced woven wire fencing held together with chain
snaps. The bin should be 3 to 5 feet in diameter and at least 4 feet high. It is easiest to turn the
composting material by simply unsnapping the wire, moving the cylinder a few feet, and turning the
compost back into it. Simple bins can be made of old wooden pallets
stood on their ends in a square or open square and nailed or tied together. A chicken wire cage
supported by three or four wooden stakes will also work well. A standard-sized garbage can with eight
or more slots in the sides of the can for ventilation and five in the bottom for drainage can also be used.
Three-Chambered Unit: A three-chambered bin works on the assembly line principle, and is an
efficient structure for “fast” composting. Three batches of compost are in various stages of
decomposition at all times. The composting process is started in the first bin for three to five days. This
material is then turned into the middle bin for another four to seven days and a new batch is
started in the first bin. Next, the middle bin is turned into the third bin as nearly finished compost. Each
bin should be at least three to five feet in each dimension and should be made with rot-resistant
wood (redwood) or wood treated with an environmentally safe preservative.
Barrel or Drum: The barrel or drum composter is an excellent choice if you have limited space in your
yard. A barrel of at least 55-gallon capacity with a secure lid is required. To provide good air
circulation and drainage, drill six to nine rows of half-inch holes over the length of the barrel, and
place the barrel upright on blocks. Fill the barrel three-quarters full with organic wastes and add
water to lightly moisten. Mix the compost every few days by turning the drum on its side and rolling
it around the yard. The compost should be ready in two to four months.
USING COMPOST
Think of compost as a soil amendment and not as a fertilizer, since the nutrient level of compost is low
and released over time. Mix compost with soil to enrich the flower and vegetable garden. It can be
used to improve the soil around trees and shrubs, as a top-dressing for lawns, or as a mulch. Screen
compost by separating the larger particles and any uncomposted materials from the finer ones and add
it to the potting mix for houseplants. No more than one-quarter to one-third by volume of the potting
mix should be compost. Soaking compost in a burlap or cheesecloth sack steeped in water can
make compost “tea.” The weak nutrient solution can be given to young plants.
Troubleshooting guide for efficient composting.
Symptoms Problems Solutions
Rotten odor – Not enough air; pile too wet Turn pile; add coarse, dry materials (straw, corn stalks, etc.)
Ammonia odor – Too many greens (excessive nitrogen/lack of carbon) Add browns (straw, paper or sawdust)
Low pile temperature -Too small; not enough air or moisture; few greens; or cold weather Make pile larger;
add water while turning the pile; mix in nitrogen sources (grass clippings, manure, or a synthetic fertilizer, such as
10-10-10); or insulate the pile with a layer of straw or plastic
High pile temperature – Too large; not enough air Reduce pile size; turn pile Pests, such as rats, raccoons or
insects Meat or fatty food scraps in pile Remove meat and fatty foods from pile; cover with a layer of
soil or sawdust; build an animal proof compost bin.
All recommendations are for South Carolina conditions and may not apply to other areas. Use
pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at
the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies.
Follow all directions, precautions and restrictions that are listed.
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