Weed Management in Small Fruit Crops

Weeds compete with crop plants for water, light, space and plant nutrients. Land preparation, cover crops, mulches and other management practices can be used to control many of these weeds. Herbicides are available that can be used effectively in a weed control program; however, no single herbicide controls all weeds. Herbicides should not be depended upon as the only resource for weed control, but should be part of an integrated pest management program.

 

Land Preparation

With all small fruit crops, soil should be deep and well drained. Land preparation should begin, and soil samples should be taken, the year before crop establishment. All perennial weeds such as Johnsongrass, bermudagrass and wild blackberries should be killed one year before crop planting. Systemic herbicides such as glyphosate are effective in control of these weeds, but application needs to be made while the plants are actively growing. Two or more herbicide applications may be required for total control, especially on wild blackberries. After the weeds have been killed, any needed lime and fertilizer applications can be tilled into the soil. These operations should be completed no later than the fall season before the crops are to be planted.

 

Cover Crops

After soil operations have been completed, the soil should be planted with a cover crop. The soil should never be bare during the winter, and a partial cover crop can be maintained throughout the year with the woody small fruit crops (blueberries, brambles and grapes). Cover crops will reduce rainfall runoff, control erosion, reduce soil temperatures, add organic matter to the soil and improve soil tilth and root penetration. Cover crops will also provide some control of annual and perennial weeds. A low-growing, mowable cover such as ryegrass or fescue can be maintained between the rows of woody plants throughout the year. This will provide a clean, firm surface for field operations and for customers in pick-your-own operations.

 

Some commonly used cover crops are fescue, annual ryegrass, rye, wheat and oats. Fescue is a perennial and does not need to be established annually. It has a bunch-type growth habit preventing it from creeping into unwanted areas. The cereal grains will provide a quick soil cover, and substantial amounts of organic matter, but will need to be replanted each year. Legumes such as clover and vetch will add nitrogen to the soil, but are more difficult to establish and provide little growth during cold weather. All the species mentioned here are best seeded in the early fall so they can become established before freezing weather begins. If they are planted in September or early October, complete soil cover should be obtained before winter. If they are planted in late October or early November, some growth will occur before winter, but much of the soil will remain unprotected throughout the winter. In either case, substantial growth should occur during March and April of the following year. Depending on the crop and the cultural management system used, the cover crop can be killed with a herbicide, mowed or allowed to grow until maturity

 

Another possible cover crop is naturally occurring annual vegetation on the site. Native plants may be left between rows of woody small fruit crops, but should not be allowed to grow into the rows. These natural cover plants should be kept mowed to about four inches in height throughout their growing season.

 

Tillage between the rows is often desirable during straw-berry renovation. This eliminates weeds between the rows, including extra strawberry runners. Tillage is normally not effective on perennial grasses, as regrowth from their root segments can occur following tillage. Tillage can be used to control annual weeds, but cultivation should be very shallow to reduce the possibility of damaging crop roots.

 

Tillage of other small fruit crops is not recommended. Some crops, such as blueberries and grapes, are shallow rooted, and any tillage near the plant may damage the roots. With blackberries or raspberries, tillage that cuts the berry roots may initiate new shoot development from the severed root segments. This may result in abundant, unwanted cane development, which can be difficult to manage. Often, mulches may be the best solution for weed control within woody crop rows, while cover crops or native plants are used between rows.

 

Mulches

Mulches used for weed control include straw, sawdust, wood or bark chips, plastic sheeting, woven plastic and other materials. The mulch is placed around the crop plants thickly enough to shade out emerging weeds. Many mulches also provide a physical barrier to weed emergence. Mulches are undesirable for matted-row strawberry production because they prevent the rooting of daughter plants.

 

Straw is inexpensive, but only moderately effective for weed control. It provides little physical resistance to weeds, and may blow away. Weed seeds may even be brought in with the straw. Straw mulch usually only lasts a few months before it must be replenished.

 

Sawdust is relatively inexpensive and, when applied thickly enough (four to six inches deep), provides good weed control. Wood chips and bark mulches are usually more expensive, but work just as well as sawdust. All these materials have a disadvantage because they tie up nitrogen in which they come into contact. However, the nitrogen is released after woody material decays. Woody mulches usually need to be replenished yearly, depending on how rapidly they decay under local conditions and how much fertilizer is applied to them. If the fertilizer is applied through a drip irrigation system, less of the nitrogen comes into contact with the mulch, causing less mulch decay and allowing more fertilizer to reach the crop plant immediately.

 

Plastic mulches may be either solid sheets or woven material. Solid sheet plastic comes in several colors, but black and white are the most common. Black plastic is usually used in small fruit production because it is readily available and less expensive than other colors. It also can help warm root systems during the spring, and may cause earlier budding of the crop. However, sheet plastics are not very durable and will usually last only one growing season or less, unless they are covered by another mulch (straw or a wood product) to prevent sun and wind from damaging them. If they are used with another mulch, they provide excellent weed control in most situations, although some perennial weeds may penetrate these mulches. Sheet plastic covered with a wood mulch may still need to be replaced in a year or two. Considerably less woody mulch is needed for this system than if only woody mulch is used. Sheet plastics are difficult to install in existing plantings.

 

Woven plastic mulches have high initial costs, but may last up to ten years if they are protected from the sun by a woody mulch or straw. They provide excellent barriers to weed penetration, but should not be used in brambles because they must renew their canes from the roots each year. This process could be inhibited by the woven mulch. The long-term cost of these mulches may be very attractive if they are used in blueberries or grapes.

 

Other Management Practices

Other management practices, including fertilization, irrigation and sanitation affect weed growth. Weeds will be less of a problem if fertilizers are applied in mulched rows or through a drip irrigation system rather than broadcast throughout the field. Drip irrigation systems, which deliver water only to crop rows provide less opportunity for weed growth between rows. Sanitation of field equipment, such as hosing off equipment and tires when moving from a weedy field to another field, can help prevent the spread of weed seeds.

 

Herbicides

Some weed problems may not be adequately controlled by the methods discussed above, or conditions may be such that these methods are undesirable. In these situations, herbicide use is a viable alternative.

 

The number of herbicides registered for weed control on small fruits is limited. If the registered herbicides available are used correctly, they can be very beneficial. Each herbicide is used differently and for a specific purpose; therefore, it is very important to know what the herbicide will do, when to use it and how to properly apply it. 

 

The suggestions contained herein are based on the assumptions that the crop plants are healthy and proper procedures for soil preparation have been followed. Plants weakened by improper management may be susceptible to herbicide injury that would not have occurred on healthy, vigorous plants.

 

Federal and state laws and regulations pertaining to the use and application of herbicides are frequently revised. Always check on the status of label clearances for herbicides before use. Labels on the container give information on application restrictions, common rates, timing, directions for use and other facts that will allow for the most efficient use of these herbicides. Remember the label is the law. Always apply herbicides as the label instructs. This protects consumers, growers, the environment and the health of the planting.

 

Principles in Using Herbicides

The following basic principles are important in using herbicides for weed control: 

1. Identify the weed before choosing the herbicide. The susceptibility of weeds to different herbicides varies with the weed species. For help with weed identification, contact your local county Extension educator.
2. Read the label for registration approval, precautions, limitations and directions for use. The rate varies with crop, target weed, soil type, etc. Only use a herbicide that has been registered for use on the specific crop to be grown. Call your county Extension educator or chemical supplier for help in determining proper rates of application.
3. If the herbicide is new, try it on a small area the first time. Even though research has shown the herbicide to be effective, field use by growers on small areas is suggested before the herbicide is used on a large acreage. This gives the grower a chance to learn how to properly use the herbicide and to determine if there are any adverse effects from use of the chemical.
4. Time of application is very important in herbicide usage. Check the label to determine when the herbicide should be used in relation to crop growth, fruiting and weed growth.
5. Calibrating the sprayer annually is necessary to apply the herbicide accurately and at a uniform rate. Care should be used to apply the herbicide so drift is minimized.

 

Preemergence Herbicides

Preemergence herbicides are applied to the soil surface and must be activated by rainfall. They must be applied to a weed-free soil before the weed seeds germinate or be applied with a postemergence herbicide that kills existing weeds. When the preemergence herbicides are activated by rainfall, they are taken up from the soil and kill weed seeds as they germinate.

 

Preemergence herbicide rates are based on soil type. Clay soils or soils high in organic matter require more herbicide to control weeds than sandy soils. Labels usually specify the appropriate rate for each soil type. Do not exceed the rate specified on the label or plants may be injured. Some herbicides cannot be used on sandy soils, so be sure to consult the label before applying these chemicals.

 

No one preemergence herbicide will control all weed species, as each herbicide differs in the species it will control. Complete reliance on only one material year after year will result in a buildup of the weeds resistant to that material. It is important to rotate herbicides from application to application to avoid this weed buildup, and to prevent herbicide accumulation in the soil. Rotate herbicides with different mode of action numbers to help prevent developing resistances.

 

Two preemergence herbicides are often applied as a tank mix to broaden the range of weed species controlled. In general, any herbicide may be legally used in a tank mix, as long as the timing, rates, soil conditions, etc., do not violate the label instructions for each of the materials in the tank mix. However, the user assumes all risks associated with tank mixes not specifically mentioned on the labels for the materials in the mix.

 

Postemergence Herbicides

Postemergence herbicides are effective after the weeds have germinated and started to grow. The chemical must contact the leaf of the target plant.

 

There are two basic types of postemergent herbicides – systemic and contact. Systemic herbicides like glyphosate (RoundUp^®) are applied to weed foliage and are translocated throughout the plant. It is necessary for the weeds to be growing at the time of application for effective control. Addition of ammonium sulfate to the spray solution improves effect on perennial grasses. If the herbicide inadvertently gets to the cambium layer of the crop plant, it will translocate and injure it. Some systemic herbicides only affect broadleaf weeds and others only grasses. 2,4-D formulations are available for post-emergence control of broadleaf weeds. Extreme care should be exercised to avoid damage to the small fruit plants. Follow label directions carefully.

 

Poast^® and Fusilade^® are translocated postemergence grass herbicides. Like glyphosate, they will kill roots as well as top growth. In many respects they can be considered ‘reverse 2,4-D’ since they will kill grasses but leave broadleaved plants unharmed.

 

Contact herbicides such as paraquat* work best when applied at relatively high temperatures and in large gallonage per acre so that good coverage of the weeds is obtained. A non-ionic surfactant should be added to get maximum results. It is important not to allow the spray to contact green stems, fruit or foliage of the fruit plants. Paraquat* kills by contact and should be used on small weeds for best results. It kills the top growth, but does not affect the roots, so repeat applications on perennial weeds are required for season-long control.

 

Tank Mixes

The preemergence herbicides in a tank mix may be used each at full rate, but many growers get good control by using 1/2 to 3/4 of the recommended rate of each. While this can reduce costs, the user assumes all risks for reduction in weed control from reducing application rates. Combining preemergent herbicides targeting grasses with another preemergent broadleaf herbicide is a popular tank mix.

 

Additional References

• HLA-6005 Mulching garden soils
• CR-6252 Commercial grape insect & disease control
• CR-6221 Commercial blackberry, strawberry & blueberry insect
  & disease control
• HLA-6222 Home Fruit Planting Guide
• EPP-7450 Safe use of pesticides in the home and garden