What is a Weed?
A weed may be broadly defined as any unwanted plant. A corn plant, for example, is a weed if it is in a strawberry planting. Most weeds are plants that are generally considered undesirable by gardeners. They are neither eaten nor considered attractive. They are frequently invasive and difficult to control.
How do weeds harm gardens?
Weeds compete with crop plants for moisture, nutrients and light. They may also harbor insects and diseases, promote other diseases by increasing humidity, decrease vegetable quality and make harvests difficult. Vegetable plants grown under weedy conditions will have reduced yields or they may not survive to produce at all. A weedy garden or flower bed is also unattractive.
Why are weeds hard to control?
Weeds are highly adapted to where they grow. They have large, efficient root systems, grow rapidly and frequently produce tremendous amounts of seeds. Most weeds tolerate drought and low fertility. Little of their energy is used producing lush foliage, large seed or fruit; thus, they often thrive where vegetables struggle to produce. Many spread rapidly by vegetative structures as well as by seed. The principal methods of weed control fall into three categories: (1) mechanical, (2) cultural and (3) chemical.
Mechanical weed control
Mechanical weed control involves removing weeds while they are small and preventing them from producing mature seed. It consists principally of mowing, plowing, rototilling, hoeing and hand pulling. These procedures give immediate results, are chemically safe and require little, if any, specialized equipment. Their main disadvantage is that they work only on growing weeds and may need to be repeated frequently.
Remove weeds while they are small because it is quicker, easier and does less damage to crop plants. Hoe or till shallowly (less than 2 inches deep) to avoid damage to desirable plants and to minimize moisture loss from the soil. The deeper the soil is disturbed, the more weed seed will be brought to the surface where it will grow. Weed seed may survive many, many years in the soil.
Gardeners often quit weeding as a crop matures. Weeds do not affect crop yields as much at this stage, but they will produce seed. A single weed of some species can produce hundreds of thousands of seed. Weed removal should continue until the vegetable crop is completely harvested. Gardens may then be kept tilled, mowed or heavily mulched while cops are not growing to prevent weeds from propagating. Frequent tillage has the additional advantages of turning organic material under where it will add to soil organic matter and mechanically destroying any insects present.
Cultural weed control
Most cultural methods of weed control emphasize prevention. These include mulching, solarization and multiple or wide-row planting.
Mulches
Any layer of material spread over the soil surface may be considered
to be a mulch. There are two classes of mulch, organic and inorganic.
Both reduce weed growth, retain soil moisture, improve soil structure and
moderate soil temperature.
Hay, straw, sawdust, ground bark, leaves, compost and even newspapers
are among the many substances used as organic mulches. All reduce
weed emergence and make it easier to pull those weeds that do emerge.
Some weeds such as nutsedge will grow through mulch, even through plastic
mulch.
Inorganic mulches include black and other opaque plastic, as well as landscape fabric. Landscape fabric is much more expensive than plastic mulch but has the advantage of allowing water to pass through into the soil. Black plastic is the most commonly used inorganic mulch. Clear or translucent plastic should not be used as a mulch because weed seeds will grow under it. Black plastic eliminates growth of most, but not all weeds except through the holes cut for vegetable growth.
Chemical weed control
Chemicals (herbicides) are only occasionally used by home gardeners to control weeds. The main reason why herbicides are not used as much as mechanical or cultural weed control methods in home gardens is that suitable herbicides are seldom legally available in small economical amounts.
Several other difficulties also exist with the use of herbicides by home gardeners. Many herbicides are non-selective and will kill both vegetables and weeds. Others are selective and can be used only with certain vegetables or control only certain weeds. Home gardens generally contain many vegetable species and weeds, which makes using selective herbicides difficult. Some herbicides may also damage nearby vegetables or remain in the soil and damage future plantings.
Even if available and effective, herbicides may not be legal for use on a specific vegetable or at the time when they are needed. The herbicide application rate may be very low and extreme accuracy in application may be absolutely essential. Overlapping applications may kill vegetable crops and, if areas are skipped, weeds will not be controlled. Required waiting periods between application and harvest can be lengthy and must be observed. Herbicides may also be effective only for a short period or produce results slowly.
Despite all these problems, there are occasions when herbicides may be successfully used in home gardens. The following suggestions will assist home gardeners in effectively using herbicides.
1. Understand the difference between pre-emergence and postemergence
herbicides. Pre-emergence herbicides are effective only before weeds
germinate. Postemergence herbicides work on weeds that are actively
growing.
2. Follow all instructions on the label. This is extremely
important. Failure to follow the label instructions precisely may
result in harm to the applicator or the environment. Pre-emergence
herbicides require weed-free soils without lumps or clods. Most are
best applied to moist soil and shallowly incorporated by tilling, irrigation
or rainfall. They must be applied uniformly and at the proper rate
to be safe and effective. Postemergence herbicides may be applied
over the top of the growing crop and weeds.