Basics of weed control

Establishing a weed control program for orchards begins with site preparation. Prior to planting, any persistent perennial weeds should be treated and removed. Most of the herbicides labeled for tree fruit work well against annual weeds, but only a few can control perennial weeds. Therefore, you should clean up any problem perennial weeds before planting. Ideally, the site to be planted should be either fallow or row cropped for at least two years before planting. The most common practice is to rent out the land to a local farmer to grow crops such as corn or wheat. Both of these crops are cereal grains that do not support tomato ringspot virus.

Care should be taken in selecting herbicides for the rotational crops. Many common corn and wheat herbicides can persist and injure subsequent crops. Persistent herbicides can kill young trees, reduce growth, or injure root systems. Root system injury may not show up until later in the growing season, when the plants are under stress. Typical signs of root injury include the sudden collapse of the tree.

Generally, three major classes of herbicides used in row crops are persistent and can cause damage to new plantings: the sulfonylureas, the imidazilinones, and the triazines. Examples of the sulfonylureas include the brand names Accent, Escort, and Pinnacle; examples of the imidazilinones include Pursuit, Raptor, and Steel. Atrazine is the most common triazine used in field crops, but it is also closely related to simazine, which is a common herbicide used in tree fruit. Injury from these two classes of herbicides is typified by chlorosis of the growing points and new growth, along with root growth inhibition. Atrazine inhibits photosynthesis, which may cause the older leaves of plants to turn yellow.

Herbicide persistence is dependent on soil and environmental factors. Breakdown of herbicides in the soil occurs either by microbial degradation or chemical hydrolysis. Both factors require that the soil be moist and temperatures warm. Thus, cool temperatures and dry conditions slow herbicide degradation. Soil pH and organic matter are also important for herbicide degradation. For example, degradation of some herbicides is slowed considerably when the soil’s pH is above 6.5. We also know that if the soil pH is 6.0 or lower, microbial breakdown of herbicides slows down. Because of these soil and environmental influences on herbicide degradation and persistence, rotation restrictions should be used as minimum guidelines.

If you are unsure whether there is a herbicide residue, conduct a bioassay. Collect small samples of soil and place in a small growing flat. Sow either grass seed or oats and watch the germination. If there is still a residue, germination and growth will be poor. If the germination test shows no residue then the field should be planted to a suitable grass sod. Research by the USDA Appalachian Fruit Station has shown that establishing an orchard-wide ground cover of grass and then killing off the rows where the trees will be planted with glyphosate has resulted in better initial tree growth. The grass residue improved soil moisture and organic matter and usually persisted for about 3 years after planting.