Timothy J. Smith, WSU Extension, 400 Washington, Wenatchee, WA 98801
Orchard Weed Control—a Quick Overview:
Orchards are a system of compromises, developed over a period of years, and one aspect of management is usually interwoven with many others. The management of the orchard soil cover is vital to the over-all orchard health and productivity. While we continue to look for a better system of cover management, the mowed grass cover between rows with a four to eight foot wide herbicide-treated band under the trees has proven over the past fifty years to be the best compromise in Pacific Northwest orchards.
During the early years of orcharding in the Northwest, clean cultivation, mud, stuck tractors (if you didn’t use horses) and soil erosion were common in the region’s orchards, as rill irrigation and mechanical weed control were the only tools available. Trees were grown on 32 foot centers, to better allow cross-cultivation for weed control.
The development of herbicides for orchard use in the 1950’s allowed the development of sprinkler irrigation and grass cover crops. Trees could be planted closer, increasing yields and increasing the efficiency of labor. The mowed grass cover provided a permanent soil cover to prevent soil erosion, and provide a firmer surface for farm machinery. Very little, if any, soil leaves the modern orchard during heavy precipitation or irrigation.
The area under the tree row can not be properly mowed, and would become a thick tangle of annual and perennial weeds if left to grow. Various methods have been tried over the years to cut down this growth, including mechanical tillage, mulches, and flaming. Each one of these alternatives work, but often are very time consuming, expensive, and are carried out, usually, by people intentionally trying to avoid the use of synthetic pesticides. These methods are not free of problems: see a picture of flame damage to the lower portion of
some pear trees.
Most producers opt for the simpler, and often, lower over-all impact on the environment, use of herbicides to reduce weeds under the trees.
Why control weeds in orchards?
Reduced competition: trees may be much larger than most weeds, but they have root systems that do not compete well with other plants. Where cover crop or weeds grow, the bulk of tree roots form in the second and third foot of soil. If competition is reduced, the trees form the highest percentage of their roots in the much more biologically active first two feet of soil depth. In areas with poor quality soils, the orchardist should not give the best foot of soil to the weeds.
Nutrient Management: Weeds can greatly out-compete the trees for nutrients, especially nitrogen. This complicates the growers attempts to create an efficient nutrient balance in the trees, as it is never certain from one application to the next what percentage of the
applied nutrient will enter the trees, or when it will get there. Trying to compensate for weed growth by applying higher rates of nitrogen fertilizer may increase the nutrient in the tree, but more often leads to greatly increased weed growth.
Irrigation and Water Management: Weeds use water, which would have been much more beneficial to maintain fruit quality during the hot portion of the Summer. Perhaps even more important, weeds block the sprinkler pattern, which may greatly decrease the efficiency of water application. Blocked sprinklers over-water some areas of the orchard, and under-irrigate others, leading to leaching and drought stress in the same orchard block. Weeds also have the aggravating ability to lean over and tangle the mechanisms of sprinkler heads, preventing their rotation.
Rodent Management: Short-tailed meadow mice can cause great damage to an orchard. During snowy winters, they sometimes chew the bark off of the lower portions of trunks, especially on younger trees. While growers try to save these damaged trees with approach grafts or bridge grafts, these methods are very slow and expensive,
and do not always work well. There are no mouse poisons on the market which will economically control large populations of mice in the Fall (as Endrin once did). Mouse control is a season-long effort, with reduction of mouse cover as the key component. As mice do not travel far, the key mouse cover is the grass and weed cover crop. Well mowed grass and a fairly clean weed strip is the most effective mouse management program.
Pest Management: This is a complicated aspect of orchard weeds, as there are many who are now trying to show the beneficial aspects of plant diversity in orchards. The theory, poorly demonstrated at this point, is that beneficial insects need alternative sources (other than trees, various weeds and grass) of nutrients, alternate prey and habitat during the growing season. By intentionally planting, or allowing to grow, various flowering plants, the “good guys” will build up in the orchard, then spread to control the “bad guys” in the trees. This is a great theory, and everybody hopes it works out well. Everybody but the grower, who would rather be working with the crop, and who wonders how to manage all of those potentially weed-like plants in the orchard system, taking into account all of the reasons for weed control listed above in this article.
There are some known pest that build up on weedy hosts, then spread to damage the trees or fruit. Some true bugs, such as Lygus, like plants in the legume and mustard family, much more than they like young fruit. They may build up, at times, on these weeds, then spread as adults to damage the fruit. More commonly, spider mites may build up on weeds under the trees during the Spring, then spread to the trees when the large weeds are sprayed in late Spring. (Suggestion to prevent this from happening: do not allow weeds to become large, then spray them in the late Spring!) At times, cutworms that are generalist feeders (not Lacanobia) will infest large weeds during the Spring or Summer, then move to the tree limbs that hang into the weeds. (To prevent this, follow the suggestion about weeds and mites.)
How to Manage Weeds in the Orchard
There are a wide number of choices of products and application timings to mix and match, but none of them are as safe and effective unless they are properly applied. Take the time to set up and check the mechanics of the weed sprayer. Boring suggestion, huh? It is, however, often brought up because it is very often neglected. Every time a sprayer is brought out for use, it needs to be checked, both for level application across the boom and rate per sprayed acre. There are many ways to calibrate orchard sprayers, use the one you are most comfortable using. If you do not have a good one, try the procedure outlined at the end of this article.
Most orchards are sprayed with a single sided boom sprayer, with three or four flat fan nozzles placed about a foot apart, starting at the distant tip of the boom. The boom is adjusted so that the spray from each nozzle over-laps about 1/3 with its’ neighbor on both sides at the level of the target. The “target” can be either weed growth if you are spraying contact herbicides, or the soil surface, if you are applying soil residual products. Some growers use single “flood-jet” style nozzles to apply contact materials on both sides of a tree row with a single pass by the sprayer. This is somewhat effective, but not without problems. There are few single nozzle band applications wider than a foot or two that evenly apply herbicides. They should not be used with most soil residual materials, or products that may injure the young tree if applied to the trunk.
Possible Product Choices
There are plenty of effective weed control materials registered for use in orchards, and a few new ones that will be registered soon. None are always harmless to the trees and completely effective, so the trick to chose the proper product or combinations to fit the situation. Special attention should be paid to the following:
- Species/variety of tree: Apples and pears are generally treated as a group, have the greatest choices of herbicides registered, and are usually the more tolerant of mild exposure to herbicides. This is not always true. Some fruits and varieties are more sensitive than others. For instance, Gala and Golden Delicious apples and all pears seem to be more sensitive than other fruits to 2,4-D leached into the root zone. In these examples, you should take care not to apply, then irrigate the product into the root zone within 10-14 days after application. This is one of the reasons that 2,4-D is easiest to use postharvest, in the Fall after irrigation is not needed, and before heavy rains are likely.Cherries and other “stone fruits” are generally less tolerant of soil active herbicides, and have many fewer safe, registered product choices. For instance, simizine is registered for use on many stone fruits in the Eastern half of the USA. Do not mistake this as an oversight in the label. Simizine can cause serious symptoms on stone fruits when applied in many Western orchard soil and irrigation situations.
- Age of the tree. Young trees have tender, green bark that can be damaged or penetrated by contact herbicides, both systemic and non-systemic. Damage to a high value, perennial crop can cost more than your job is worth. Pay close attention to the development of corky, dead outer bark on the portion of the trunk that will be contacted by a contact herbicide. Well applied paint or mouse guards will help if they completely block the spray from contacting the green bark. The most common and serious damage occurs on young fruit trees when unprotected bark is contacted by concentrated doses of glyphosate products. This is does not mean that these products can not be used in young orchards, you just have to be certain to avoid significant exposure of the trees. Other non-systemic products can also damage the young tree, usually by burning a dead area into the trunk within about a foot of the soil surface. This can occur if the product is overly concentrated in the spray solution, and/or mixed with higher rates of liquid nitrogen fertilizers.Young trees have shallow root systems, and most of their roots are under the herbicide treated area. They may become highly exposed to root active herbicides that leach into the upper foot of soil. A similar exposure of an older tree may not cause symptoms, as most of the trees root would be below or outside of the herbicide strip, and the product may be broken down in the tree tissue without causing damage. Use root active products with special care in young orchards. Consider if the product or rate is safe, taking into account the factors listed below:
- Soil Factors: These are the same well-known considerations used with many herbicides and crops. Organic matter and increased binding sites that come with finer soil texture are major soil qualities that hold potentially mobile herbicides in the upper 2-4 inches of soil, controlling weeds rather than affecting the fruit tree. If the product suggests on the label that you take these factors into consideration, do so. Remember, orchards often have bands of lighter, shallower or gravelly soils running through them. Ask the grower, they will know about these poor soil areas. Use products and rates that are safe on the weakest soils, not the average.
Product characteristics: There is a wide range of products, but they tend to fall into loose groupings (the following is not a complete list of registered products):
- Soil residual, potentially mobile, and actively taken up by roots. These tend to be the older, cheaper, and highly effective products that are often the foundation of a weed control combination. Examples include simizine (Princep), diuron (Karmex), terbacil (Sinbar), and, to a lesser extent, norflurazon (Solicam) and dichlobenil (Casoron). All very good and useful herbicides, but they may be slightly to highly hazardous to fruit trees. Read labels and follow directions to the letter.
- Soil residual, not very mobile, and not likely to be transported in significant levels into the tree. Examples of these include indaziflam (Alion), rimsulfuron (Matrix), oryzalin (Surflan), pendimethalin (Prowl H2O), oxyfluorfen (Goal, Goaltender), and pronamide (Kerb). These products are more or less effective, but often need to be used in combinations to increase the weed control spectrum. They are more likely to fail if application directions are not followed very carefully. They also tend to be much more safe for use in young orchards or on “weaker” sandy and/or rocky soils in mature orchards.
- Contact, systemic, not soil active. The usual list: glyphosate (Roundup, many other names). These tend to be an important part of the mature orchard weed control program, and very useful for the suppression of tough perennial weeds, such as field bindweed (a.k.a., morning glory) and quackgrass.
- Contact systemic, somewhat soil active: 2,4-D (Weedar 64, Saber), clopyralid (Stinger). Some apple and pear cultivars react badly to mid-growing season applications of this product followed by irrigation.
- Contact, not very systemic, not soil active. Paraquat (Gramoxone Inteon, Firestorm), gluphosinate-ammonium (Rely), and pyraflufen-ethyl (Venue) which are used as “chemical hoes” in young orchards. Apply and repeat as necessary before weed growth gets in the way of effective applications. Perennial weeds anjd larger annual weeds usually rebound in a few weeks, at best, as these products have no residual activity and need to contact most of the weed’s surface for more complete control. Use a wetting agent to improve plant surface coverage.
Herbicides are often applied in combinations of residuals and contacts in the late fall or early Spring. These combinations are usually of products that cover the others’ weaknesses. For example, simizine controls a wide range of weeds, but is weak on summer grasses, Solicam is weak on some important broadleaf weeds, but is an especially effective residual summer grass control product. Neither product will control deep rooted perennials, so glyphosate is usually added to the mixture. Other more modern combinations that have provided excellent include Matrix + Prowl H2O and Alion + simizine.
The normal weed control program would include a combination soil residual plus contact mixture, best if glyphosate is applied in the Fall, followed up by a spring applied residual tank mix, and possibly, you may be able to skip a mid-Summer contact or contact systemic.
Other options include mulches, mechanical weeders, flaming, hoes, or just letting weeds go. There are special circumstances where these options are the best way to control weeds, especially if the orchard is in organic production. Otherwise, herbicides are far cheaper, effective, and many have less impact on the environment than some of the non-herbicide methods listed above.
No matter the option on herbicide, none are safe and effective when mis-applied. Please take care to apply them under low-wind conditions, and with properly set up equipment.