A chemical must be toxic to plants before it can be effective as an herbicide. An herbicide that is effective against most kinds of plants is called a nonselective chemical. Such chemicals are useful in areas where complete control of vegetation is needed. On the other hand, herbicides that are more toxic to some plants than others are called selective chemicals. Most herbicides for use on lawns and crops are selective; using the proper rate of these herbicides can remove susceptible weeds from tolerant plants. If excessive rates are used, however, the tolerance may be exceeded, resulting in injury to desired plants. Each kind of flower, fruit, and vegetable varies in its tolerance to a specific herbicide. In the small orchard, this means that not all herbicides can be used around all plants and that many plants do not have tolerances for any of the herbicides available on the market. A limited number of selective herbicides can be used in the home garden and around the home grounds.
Before selecting an herbicide, the fruit grower should consider the available weed-control alternatives. The weed should be identified and its growth habit--annual, biennial, or perennial—should be determined.
Additional consideration should be given to the location of the weeds and the intended use of the area after the weeds are removed. In a noncrop area, a certain herbicide might be very practical and effective, but in a crop area, that same material might not be suitable for controlling the same weed species. The potential problem of drift, or contamination of adjacent plants, is always present when using herbicides in small areas. In addition, contamination of adjacent properties might also result in legal problems for the applicator.
The formulation of the herbicide also should be considered in relation to the crop and area being treated and the type of equipment available. Herbicides come in different forms and are prepared for the following:
- Mixing with a liquid carrier--usually water--and applying with a sprayer. These herbicides include wettable powders, water-soluble liquids, emulsifiable concentrates, and water-dispersible liquids.
- Distribution with mechanical spreaders or by hand. These herbicides include granules or pellets that are applied dry.
- Injecting into the soil for vaporization or fumigation. These include liquids, liquefied gases, and granules. They usually require specialized equipment available only to individuals who have a commercial pesticide applicators license.
The area to be treated, the crops, the type of weeds, and the availability of application equipment all affect the selection of a suitable herbicide.
In light of changing government regulations concerning pesticide use, it is very difficult to make specific recommendations about the control of weeds on an individual's property. Far too many variables exist within a given property, not to mention among different properties, to safely recommend herbicides with a safety range wide enough to meet an individual's needs. In addition, each type of fruit varies in its tolerance to a specific herbicide, further compounding the difficulty of selecting the optimum material for a given situation. Even with today's restrictions and limitations on pesticide use, the interested individual can take several routes to obtain assistance in selecting suitable weed control. Once you have studied the site conditions and determined your needs, you will be able to select an herbicide that satisfies those needs without exceeding the limitations of the site or crop.
Herbicides generally are applied at different times, depending upon the emergence time of the weeds and upon the type of fruit plants.
Herbicides that are applied at specific times include the following:
- Preplant herbicides are used before the crop is planted to control germinating weed seeds, and are usually mixed into the top 2 to 3 inches of soil. No preplant herbicides are labeled for fruit plants.
- Preemergence herbicides are used after the crop has been planted, but before the weeds or crop emerges. Restrictions on the age of plants to be treated must be followed.
- Postemergence herbicides are used after the crop and/or weeds have emerged from the soil surface and are growing. The most common of these is Round-Up®, which can be purchased without a pesticide license.
Herbicides usually are more effective when temperatures before application have favored uniform germination and rapid weed growth. Rapidly growing weeds are easiest to kill. High temperatures at the time of application also tend to increase the activity of the herbicide but also increase the possibility of crop injury. Moderate temperatures between 70 and 85°F are the most favorable for spraying.
Wind can also be a factor in herbicide application. It can cause improper distribution over the weeds, reducing herbicide effectiveness while increasing the danger of drift onto desirable plants. Fewer problems occur if sprays are used when the wind velocity is low and the wind is blowing away from desirable plants.
The activity of herbicides applied to the soil is improved by moderate rain or irrigation shortly after application. The water helps to move the chemical into the weed zone and aids in the germination of the weed seeds. When herbicides are applied to the foliage, rain or irrigation should not occur until several hours after the material has been applied. After this time, most herbicides will have been taken into the foliage of the plant where they are not affected by rainfall.
Soil-applied herbicides are effective against germinating seeds or young seedlings.
To function, they must persist in the soil during the time when the weed seeds are germinating. After this point, they will have little effect on the plant.
As long as growing conditions are favorable for weed growth, you should expect good control with soil-applied herbicides. Herbicide activity in the soil is influenced by the soil texture, organic matter content, acidity, and moisture conditions, all of which are also factors necessary for optimum plant growth. High temperatures and moist soils also favor the decomposition of herbicides in the soil. Microorganisms and chemical reactions in the soil also aid in the decomposition process, which prevents herbicide buildup in the soil.
Some chemicals used for long-term weed control, however, may last for two or more years in the soil. When seasonal weed control is needed among desirable plants, chemicals that remain in the soil only a few days to a few weeks can be used. Some chemicals, however, may remain in the soil in sufficient amounts to affect susceptible crops the following season.
To be effective, herbicides applied to foliage must remain on the foliage long enough to penetrate into the leaf, move within the plant to an active site, and cause a toxic effect.
Foliar herbicides can either kill or burn the foliage on contact, or be translocated within the plant to its growing points. The nontranslocated herbicides may not completely eliminate the weed problem but will kill any aboveground parts. The translocated chemicals are effective in killing both aboveground and belowground parts of the plants. Such translocated herbicides are effective against many of the perennial weeds with vegetative reproductive parts below the soil surface.
Weed response to selective herbicides depends upon the retention, uptake, or translocation of the chemical through the plant's foliage, as well as upon the reactions of the herbicide within the plant. The selectivity of an herbicide can result from slight differences in the plant's structure, the quality of the foliage, the depth of the root system, or the location of the growing points.
Substantial time, effort, and money have been invested in the development of pesticides designed to do a specific job. Each pesticide label must specify the intended use of the product, as well as the pests controlled and on which plants it can be applied safely. Chemical weed control, in its simplest form, involves matching a specific problem with the herbicide that will safely solve that problem.