There are many topics to consider in optimizing the use of herbicides in vegetable and small fruit plantings. These will vary depending on whether the herbicide is soil-applied or postemergence. Important issues include herbicide selection, rate selection, application parameters, proper timing, and herbicide activation.
Soil-applied herbicides
Herbicides should be selected based on their ability to control the weeds which are present in the field. Proper field scouting, usually during the summer or fall prior to planting, is essential to determine which weeds are problems. Herbicide labels or recommendations are then needed to be sure a given herbicide is registered for the crop to be grown. Also, for small fruits, the age of the planting is important. For example, grapes must be three years old before using simazine (Princep).
Selection of the proper herbicide rate is based on both soil type and soil organic matter percentage. All herbicide labels will provide specifics on rate selection. Generally, however, rate will increase as the soil clay content increases. Also rate will increase as the soil organic matter percentage increases. Be careful, though, when using large amounts of organic mulch as with sawdust in blueberries. Fresh organic matter will not tie up herbicides as will older and more decomposed organic matter. The lowest labeled herbicide rate may be needed on mulch or crop injury may result.
Herbicides used in most vegetables and strawberry plantings are generally broadcast applied while herbicides used in plasticulture and in blueberries, brambles, and grapes are generally banded. Banding drastically reduces (50% or more) the total amount of herbicides needed in a field. In the case of small fruits, the row middle can then be maintained with the use of cultivation or cover crops.
When spraying blueberries, brambles, and grapes, directed sprays are generally used. Offset nozzles provide the best spray pattern especially if a tractor is used to make the application. Directed sprays also minimize crop contact, sometimes increasing crop safety. Be careful not to overlap herbicide spray patterns in the crop row when spraying each row from both sides. A high clearance boom set to spray both sides of the same row at the same time would be the preferred method of application.
It is important to apply soil-applied grass herbicides before grasses and small-seeded broadleaf weeds emerge. There is no postemergence activity with these herbicides. Most soil-applied broadleaf herbicides, however, will control many newly emerged broadleaf weeds. If emerged weeds are a problem and the crop has not yet emerged (vegetables), it may be possible to use a burndown type herbicide.
Check the labels and recommendations before application and be sure to avoid any contact with the crop. For small fruit, check labels and weed species to determine the best season to apply herbicides. Some applications can be made in the spring or the fall and some can be applied at both times with either the full rate or half rates each time. If two timings are possible, growers should consider splitting the annual rate allowed and applying half of the herbicides at each application. This has many benefits. It reduces the possibility of crop injury if overlap occurs. It also can help to eliminate very low rates of residual herbicide in the soil between applications. Herbicides break down in the soil over time and annual applications may not provide the same level of weed control as semi-annual applications. This is especially true with the herbicide Princep.
Herbicide activation is essential to obtain good weed control. Activation moves the herbicide from the soil surface to a depth where the herbicide is most active. This also protects the herbicide from vapor loss or breakdown by sunlight. For incorporated herbicides, the mechanical incorporation process activates the herbicide. For surface-applied herbicides, rainfall or irrigation after application is usually required. Be sure to check the label for activation requirements of each herbicide.
Postemergence herbicides
To avoid duplication, this section will refer to the above section and make contrasts or additions as needed.
Postemergence herbicide selection is also based on weeds scouting and crop grown. Scouting here is during the growing season usually just prior to application. Additional factors which must be considered include the size of the crop and the time of the year.
Selection of the proper postemergence herbicide rate is based on the weed species and the weed size. Some species require higher rates and larger weeds usually require higher rates. All herbicide labels will provide specifics on rate selection.
Application parameters related to broadcast or banded applications are discussed above. Some postemergence herbicides require shielding to protect the crop. Especially in the case of glyphosate (Roundup) or sulfosate (Touchdown), any crop contact could result in severe crop injury or crop death. Other herbicides such as metribuzin (Sencor or Lexone) or sethoxydim (Poast) have specific temperature or other environmental requirements. Follow all label directions closely.
Always check labels to determine the best season to apply herbicides. Some applications can be made while the crop is in the field, while others must be made either before crop emergence, prior to spring budbreak, or after crop harvest or crop dormancy. In some cases, the reason for this is weed efficacy; in other cases the reason is crop safety. Read the label.
Be sure to obtain an updated copy of the latest New England Vegetable or Small Fruit Management Guides for more specifics on label requirements and timings. Finally, always read and follow all label directions before using any pesticide.
