Preemergents can be an effective tool in orchard weed management. When properly timed and applied, weed pressures can be significantly reduced during the early growing season.
There are around 12 different herbicides registered for preemergent use in almonds. They all differ in their rates, weed control spectrum, application methods, and time to incorporation. Kurt Hembree, farm advisor in Fresno County, has developed updated selectivity charts that will aid in the selection of an herbicide.
Preemergents can provide a long lasting control of escaped and herbicide resistant or tolerant weeds such as hairy fleabane, horseweed, and yellow nutsedge. Row strip applications made in the winter/early spring months can provide control for three to four months without the heavy use of a post-emergent herbicide. This often reduces the number of applications of post-emergent herbicides saving time, chemical costs, fuel, and wear and tear on equipment. It is not uncommon for growers to reduce their applications of postemergents by 50 percent or more when incorporating a preemergent into their postemergent only program.
Comments have been made regarding preemergents as "soil sterilants." Preemergents do not sterilize the soil and many are broken down by the soil microbial community. It is important to note, however, that they are not without environmental impacts. Before using an herbicide, ask about its volatility, water solubility, soil mobility, toxicities to bees, birds, fish, and aquatic insects, and ability to contaminate groundwater.
Not all of the preemergents are the same, while newer chemistries tend to be less hazardous to the environment than older chemistries. Keep in mind that newer chemistries will cost more than older, out of patent, chemistries. Questions regarding environmental and use regulations should be addressed with your local agricultural commissioner as restrictions vary by county.
A reasonable "first time" preemergent application for an orchard could look like:
• flumioxazin (Chateau - 12 oz) + pendimethalin (Prowl H20- 5 pts) + a post emergent to burn down weeds, or;
• rimsulfuron (Matrix - 4 oz) + pendimethalin (Prowl H20- 5 pts) + a post emergent to burn down weeds.
Either of these treatments should provide control for just about all (if not all) problematic orchard weeds, including hairy fleabane and horseweed. Please ensure registration of the product before crop use.
Please note that this is not an endorsement for any of the trade names listed, nor does the omission of specific trade names reflect the view of the author. Refer to your local chemical dealer or manufacturer for specific herbicide products available.