After planting

If you do not use the killed sod method described above, then herbicide applications are recommended to control weeds around young plantings. There is a critical period in crop development where weed competition hurts yields. For new orchards, this is usually the first 2 to 3 months of the planting. If weeds are controlled early in this first season, trees grow better and start cropping earlier and heavier. For bearing trees, this period runs from prebloom until 30 days after bloom. Weeds will rob the most growth and fruit size from your trees during the months of May and June. If weeds are controlled during this critical time, yields will not be reduced. Weed growth later in the season primarily affects the water status of the tree and may not be a problem in most years. Weak weed growth under your trees from August onward can actually be a good thing—as long as it does not hamper harvest—because it may help the tree go dormant faster.

Herbicides labeled for use on newly planted trees include clethodim, Fusilade DX, Solicam, Surflan, oxyfluorfen, Prowl, Poast, and paraquat (see Tables 4-1 and 4-2 in Part IV). The rates and timings are listed in the tables in Part IV. Be careful when applying herbicides to newly planted trees. Read and follow label instructions completely. No herbicides should be applied until after the ground has settled around the tree and there are no cracks in the soil. Avoid using paraquat on newly planted trees unless you shield the tree trunks from the spray.

As trees age and become established, the range of herbicides that can be used increases. Diuron, simazine, and 2,4-D can be applied to orchards established at least 1 year. Once trees have been in the ground for 2 years, a combination of Sinbar plus diuron can be used, as well as glyphosate. Sinbar can be used alone at higher rates once the trees have been in the orchard for 3 years.

Herbicides should be rotated to prevent the buildup of resistant biotypes and a gradual shift in weed population. Weed scientists have confirmed that certain species of weeds that were once susceptible and easily controlled by herbicides no longer are controlled. Herbicide resistance most likely occurs when a particular population of weeds is continuously exposed to a particular family of herbicides. The most common example is the development of triazine-resistant pigweed and lambsquarters. The only triazine material used in orchards is simazine, which is one of our most commonly used materials. Rotation away from a continual usage of simazine will help reduce the chance of resistance developing.

Some scientists believe that to be effective in preventing herbicide resistance you need to not only rotate specific herbicides, but also rotate between similar modes of action. Mode of action refers to how the herbicide will kill the plant once it is absorbed. Some herbicide modes of actions are very specific and only work on one site in the plant. Others may work on several sites. As with fungicides and insecticides, the more sites a herbicide affects, the less likely resistance is to develop. In the case of simazine, it acts by inhibiting photosynthesis. The same is true of norfluazon, diuron, and terbacil.

Continual use of one particular residual herbicide can also lead to a gradual shift in the weed species present. Knowing which herbicides affect which weed species can tell you if you are causing that population shift. One of the best things you can do during the supervision of your harvest crews is to notice the weed population in your various orchard blocks. You should note where the weed problems are the worst, how dense the weed growth is, and the weed species present. Follow up by making a physical map of the weeds to prepare to control the problem.