236-Weed Management in Wild Blueberry Fields
Fact Sheet No. 236, UMaine Extension No. 2193
A wide variety of woody and herbaceous species native to Maine naturally occur in Maine’s wild blueberry fields. Any plant occurring in a field other than a wild blueberry is considered a weed. The wild blueberry competes with weed species for space, water and nutrients. This competition usually results in a reduction in crop yields and prevents the blueberry from spreading. Weeds may also contribute distasteful fruit such as bunchberries and chokeberries, which reduce the quality of the processed pack. Weeds hinder harvest and reduce the quality of the fruit. When harvested by rakes, crushing and cutting of the fruit occurs. Weeds may also harbor pests or act as alternate hosts for diseases. However, weeds can give shelter to beneficial insects and help to reduce erosion on slopes. Yet, when weeds are controlled, blueberry yields increase. This increase may be attributed to reduced competition, which enables blueberries to increase plant stand, flower bud set, and fruit size.
Because blueberries are pruned every other year and fields may not be tilled or rotated into other crops, the types of weeds and the methods of control are different than cultivated crops. Many of the major weeds are perennial in habit and may regenerate like the wild blueberry, from underground rhizomes or rootstocks. Herbaceous plants can produce millions of seeds, which infest the soil and will germinate over a number of years. You need to identify the weed species in your field and their growth habits before you can develop ways to control them.
The use of one particular method, whether chemical or cultural, will reduce some species while encouraging others. It is therefore necessary to develop an integrated approach, combining both chemical and cultural methods.
Integrated Weed Management
The three main objectives of integrated weed management are prevention, control, and eradication. Prevention involves keeping weeds from being introduced in uninfested areas. Control is the suppression of a weed to the point that its economic impact is minimized. Eradication is the elimination of all weed plants and plant parts from an area. It is important to remember that a single treatment of any weed control method will not control all the weeds that are present. A successful weed management program incorporates both chemical and cultural methods to achieve objectives
Prevention involves preventing the spread of weed seed or vegetative parts and stopping emergence of weed seed present in the soil. Cleaning field equipment such as mowers, winnow machines, and boxes before moving them into other fields will reduce the spread of seed from one field to the next.
The use of mulches to exclude light will prevent some weed seeds from sprouting and starve out other weeds. The mulch must be heavy enough so that resprouting weeds cannot penetrate through it. Perennial weeds with vigorous underground reproductive systems will break through mulches. The mulching material can be any substance that is opaque or thick enough to exclude light and that is reasonably easy to handle. Useful materials may include composted sludge, wood chips, sawdust, bark, peat, and sand. The use of mulch will also serve to retain moisture, moderate soil temperature, and create a more favorable environment for the spread of blueberry rhizomes or underground stems. Refer to the Wild Blueberry Fact Sheet No. 228, Mulching for Improved Plant Cover, for more information.
Maintaining a low soil pH, reducing it to 4.0 will help reduce competition from certain weeds and promote blueberry growth. A soil test will provide the information needed. If the pH is too high, sulfur is needed (see Wild Blueberry Fact Sheet No.254 Cultural Management pH).
A good cover of blueberries in the field will reduce the opportunity for weeds to establish themselves. Open spaces among clones provide openings for weeds to establish themselves. Interplanting young, fast-growing blueberry plants in these open areas will decrease the time required for blueberries to fill in the field. The increase in plant stand will also result in greater yields. See Wild Blueberry Fact Sheets, Filling Bare Spots in Blueberry Fields and Mulching For Improved Plant Cover.
Control refers to suppressing a weed once it has become established in the field either by seed or vegetative parts. There are two categories of control: cultural control, which refers to mechanical means of suppressing weeds, and chemical control, which addresses the proper use of herbicides registered for blueberries.
Cutting clumps of woody weeds such as birch, maple, and willow to ground level will suppress their growth, but because most weeds will regenerate from roots or rhizomes, cutting will not give lasting control. You must re-cut the plants several times each season to ensure suppression. The initial cut will result in a greater number of smaller stems, but if these are re-cut, the plant will eventually use up its stored reserves and die. For lasting suppression, cut in June, July, and August of the prune year.
Mowing weeds at a height above the blueberry plants may be used for species that grow throughout the field such as bracken fern, sweet fern, poplar, and others. As with cutting, it’s important to mow several times in the season. Mowing may be useful to cut the tops off herbaceous weeds, such as St. Johnswort and dogbane, to prevent them from going to seed and compounding the weed problem in future years. However, you should expect some injury to the wild blueberries from the tractor and mower tires.
Pruning, although not intended for weed control, will influence weed populations. Pruning with fire will reduce small coniferous trees and some of the weeds that spread by seed. Seeds on the weed stalks and on the soil surface may be burned, especially in the fall, thereby reducing the number of plants that could be produced in the following years.
Weed seed included in straw or hay used for fuel for fire pruning may also contribute to future infestations. Avoid using weedy hay. Clean oat straw or salt marsh grass will provide a good burn without introducing new weeds into your field.
Certain herbicides must be applied preemergence, that is, after pruning but before the emergence of both blueberries and weeds. Other herbicides are applied post-emergence, after blueberries and weeds have emerged. The preemergence herbicides registered for blueberries are selective in that they will suppress weeds without undue injury to blueberries. These herbicides are selective only if they are applied at the proper time (before blueberry emergence) and at the proper rates (those described on the product label). The postemergence herbicides available for use for woody weed control in wild blueberries will kill blueberries if contact is made, so the herbicides must be applied selectively with a wiper. A discussion of the herbicides registered for use in wild blueberry fields and factors influencing their effectiveness to follow.
Trade names are used for identification purposes only. No product endorsement is implied nor is discrimination intended against similar materials. This information is meant only as a guide. Refer to the product label for detailed instructions and precautions.
Factors Influencing Effectiveness: Proper calibration and maintenance of the sprayer will ensure that herbicides are applied accurately and will maximize the benefits. Calibration involves the proper choice, spacing, height, and maintenance of spray nozzles as well as checking the output to ensure that the proper amount is flowing from each nozzle. For information on calibration of sprayers, refer to NRAES Bulletin No. 19, Boom Sprayers. Once the tractor and sprayer have been calibrated, determine the proper rate to be applied from the product label and use it. Too much herbicide will cause crop injury and could result in a reduction of the blueberry stand and yield; too little herbicide will result in inadequate weed suppression and also result in yield loss.
Preemergence herbicides must be applied before blueberries emerge or blueberry injury can occur. Any benefits gained from the control of weeds will be lost if blueberries are injured by the herbicide.
Other factors that influence herbicide efficacy include precipitation, soil type, and organic matter. Too little rainfall will result in the herbicide staying on the soil surface and not getting into the soil to inhibit weed growth. Too much rainfall can result in the herbicide running off or washing through the soil so that the chemical does not stay at the proper level. The soil type and organic matter may result in more or less herbicide being bound or tied up. Application rates may have to be increased for soils with more organic matter or sand and decreased for soils with less organic matter or loam.
Factors Influencing Effectiveness
Non-selective herbicides: The herbicide mixture used, adequacy of coverage, the timing of rainfall, the timing of herbicide application, and the species of weed treated all influence the effectiveness of these herbicides. Since some postemergence herbicides will kill wild blueberries, they must be selectively applied to weeds taller than the blueberries. Extreme caution must be used to prevent contact with wild blueberries.
Read the product label and mix the recommended strength of solution needed for the type of wiper you are using. Too strong a solution can burn the foliage and result in less uptake and poor results. Mechanical wipers have been designed and are being used for this purpose, in addition to a hand-held wiper with a sponge or paint roller type head. Wick wipers, using a rope wick to apply the herbicide, may be used, but the concentration of the herbicide must be higher because less is applied. Good coverage of the foliage is needed. Effectiveness may be increased if two applications are made in opposite directions.
Rainfall occurring too soon after an application may decrease effectiveness because the herbicide will get washed off the leaves before it can be taken up. Good control is obtained by treating perennials in the spring at the bud or regrowth stage, but sufficient foliage must be present to get a good application, and the plants must be taller than the blueberries. Additionally, fall is a time when herbicides will reach the underground parts through the natural translocation of the plant. Perennial weeds are most susceptible to herbicides in the fall before frost.
Selective Herbicides: Selective herbicides which will control only grass may be applied to blueberries after emergence without injury to blueberry plants. Do not apply to grass that is stressed due to moisture, temperature, low soil fertility, or mechanical or chemical injury, or if rain is expected within one hour.
Eradication is the elimination of all weed plants and plant parts from an area. This is not practical if the weed is well established or if many of its seeds are in the soil. It is feasible if a species has just invaded a field or is found only sporadically throughout the field.
Cutting, pulling, or using a wiper or spot treatment to eliminate a few plants is much easier than trying to control weeds once they have become established. Periodic checking of your field for new weeds and eliminating them early can prevent serious weed problems.
Successful weed management requires the integrated use of chemical and cultural methods. Knowing the weeds present, the proper method and timing of controls, and following labeled herbicide rates will decrease weed competition and increase the productivity of your field.
Successful weed management does not mean a completely weed-free field. However, adequate weed suppression will increase blueberry yields and maximize returns. A higher rate of an herbicide would produce a cleaner field but may not result in higher yields. If the rate is too high it may actually result in a decreased blueberry yield from injury to the blueberries. Concentrating efforts on fast-spreading weeds and those that are highly competitive will produce the highest return for efforts expended.
Resources available for weed management must be used wisely to get the best long-term effect. Mulching a blueberry field requires a high short-term cost, but the long-term effects of weed suppression and increase in blueberry spread over a number of years will result in greater productivity.
Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.
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