Conventional orchard practices typically include a weed-free strip under trees and a weed-free sod strip between rows.

There are many products suitable for chemical control of orchard weeds, whether for brambles, vineyards or tree fruit.

Dave Myers, Extension educator and a member of the Southern Maryland Fruit Team, University of Maryland Extension, presented on weed management at the recent 2020 Bay Area Fruit School. When using herbicides, “there’s too many ‘rights’ that have to occur for good weed control – the right material, the right timing, the right placement and then the right weather,” he said.

Myers explained herbicides have different modes of action, including burn down, pre-emergent and post-emergent control. Growers need to understand how products work in order to see good results.

He reminded growers that a lot of the data they need to make the best product decisions are not on the label, but rather contained in the Material Safety Data Sheet (MODS). Information such as the average half-life, the K-coefficient and water solubility are listed on the MODS.

The K-coefficient is a measure of the affinity to carbon and tells users how tightly the product binds to the soil. A high K-coefficient means the product does not move very well in the soil and is less likely to be dispersed during heavy rainfall. “We can get some of these products with a fairly high water solubility, and a low K-coefficient, and they’ll move. Sometimes you’re watching the weather and you do need to be a little bit more flexible with what you do,” Myers said.

This is important, as pre-emergent herbicides need to be active in a specific zone in the soil to combat the germination of weed seeds. With less frequent rainfall, products with a lower K-coefficient (so they can move to reach the germination target zone) would be more productive.

“It’s nice to know how long these products can be in the soil,” Myers said. But just because the product remains in the soil doesn’t mean it’s active the entire time.

Organic certified, OMRI-approved products are also available. They can often be substituted for conventional products, providing the same mode of action. They are burn down products, and can be quite effective on annual weeds.

In high tunnels, Myers recommends products applied to the soil are not active for more than a short period, cautioning growers to utilize those with a short average half-life and low residual activity.

“I really like the pelargonic acids when we get into some of these interior environments, in high tunnels and greenhouses,” Myers said, as these burn down products do not persist in the soil, instead acting via foliar uptake.

Common Herbicide Modes of Action

Pelargonic acids are cell membrane disruptors. These are post-emergents and have no soil uptake or activity. They are non-selective, non-systemic contact herbicides which are rapidly taken up foliarly by the plant. Concentrated pelargonic acids need to be used in relatively large quantities to get to the percent spray solution needed.

“You have to be careful with these products, even if they are organically approved,” Myers said, as some of the pelargonic acids products have an LD50 value in the warning category, while most herbicides are in the caution category. The LD50 value measures the amount needed to cause a lethal dose to 50% of test subjects. It is measured in milligrams of chemical per kilogram of body weight. LD50 values of less than 50 are the most dangerous category; values of 50 – 500 are in the warning category; and values of 500 and above are in the caution category.

Organophosphates include glyphosate, a is post-emergent that kills on contact. There is no soil uptake, it is non-selective and it’s not used with a surfactant. When using these types of products, care must be taken to avoid any spray being directed onto the trunks of orchard trees, Myers emphasized.

Another post-emergent, systemic organophosphate herbicide is glufosinate, which acts as a glutamine synthesis inhibitor. These degrade rapidly in the soil – in about seven days – by soil microbial activity. There is the potential for cambium layer death if the product comes in contact with tree trunks, Myers said.

“There could be disaster potential” if organophosphates contact the trees, Myers said. He recommends avoiding glufosinate or glyphosate during the first year of tree establishment, painting the trunks with white latex paint and using horticultural tape before applying these products. These products should be mixed with ammonium sulfate; do not use any surfactants.

Dinitroanilines are merisystemic root inhibitors, acting as pre-emergents. They do not leach into the soil, but form an herbicide barrier (particularly in clay soil) and target small-seeded broadleaf weeds and grasses. These products do not translocate in plants. Orchards have to be established, and there are pre-harvest intervals, Myers said.

Mobile photosynthetic inhibitors are the ureas and uracils. These are absorbed, primarily via roots, and travel through the xylem. Do not use these products in soils where the pH is above 6.8, Myers said, and use these only during the dormant period.

Effective Herbicide Use

Following the labels, paying attention to rain-fastness and knowing what needs to be added to the products – do you need to add a crop oil concentrate or ammonium sulfate or a seed oil? – are important, Myers emphasized. The pH is critical, and water needs to be slightly acidified, as most herbicides are weak acids. Alkaline water will act as an antagonist to most herbicides. For example, when mixing with hard water, free cations will react with glyphosate and make the product inert. While it will still get into the plant, it won’t be activated.

“Tank mixing is very important. There’s nothing worse than an unsprayable mixture,” he said.

The chemistry of herbicides in the soil as well as the interactions between the product’s chemistry and soil microbes is important, he added. Continuing to spray the same products routinely will inure microbes to the product, making continual use of the product ineffective. Product rotation is critical. Soil type and organic matter also make a difference in product effectiveness.

“[There are] just some really unique things you can do to make these chemicals work – getting all those ‘rights’ and understanding the chemistry of it” are the keys to success when applying herbicides in the orchard, Myers said.