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Weed Control - From Herbicides to Mulches

By Nick Sakovich

The UC IPM Pest Management Guidelines are offered for over 40 crops, including citrus and avocado. This very useful handbook contains information for controlling insects, diseases, nematodes and weeds. Newly revised in May of ‘99, the citrus guidelines contain two weed susceptibility charts; one showing winter weeds, the other summer weeds. They are a valuable tool in determining which herbicide to use. I have printed these charts below. Your first step, of course, is to identify the problem weeds in your orchard, then to choose the proper herbicide. For weed identification, you might be interested in UC publication 4030, Grower’s Weed Identification Handbook. See below for ordering.

Surveying the orchard twice a year (late winter and summer) will give you valuable information as to what weed species you are dealing with; especially note any perennial species. Include in your observations the perimeters of the orchard to evaluate sources of reinfestation. And remember, if it is at all possible, never let weeds go to seed. This is not only true for weeds in the orchard, but also for those around the perimeter and in nearby adjacent fields.

Weeds compete with the orchard plants for nutrients, water and light. Controlling them today is even more important, since a high percentage of our orchards are under low-volume irrigation, making the trees’ roots now more concentrated.

Herbicides can provide effective control for most weed species. However, if a particular herbicide is repeatedly used, the species that it does not control will have an opportunity to thrive in the absence of any competition. At times, circumstances may dictate that a clean orchard floor is not the best situation. Problems with erosion, compaction and penetration (silty surface layer) may be helped by the addition of a mulch or covercrop, either planted or natural.

It has been shown that a 4-inch layer of wood- chipped mulch can provide adequate weed control. In addition, mulches help to conserve water, improve soil structure and add valuable organic matter to the soil. Tests are presently ongoing to see the effect that mulches have on phytophthora and nematode control, and the nutrient status of the trees.

A covercrop can also be a valuable asset to an orchard. In addition to erosion control and helping to increase water penetration, covercrops contribute towards good weed control, put valuable organic matter back into the soil, control dust, add nitrogen to the soil (legumes) and provide a good environment and food source for beneficial insects.

In days gone by, a clean orchard floor was a sign of good management. Parallel to this was the thought that the soil was just an anchor for the roots, and that we can produce and supply whatever the tree needs. And for many years, production certainly did increase. However today, we are learning more and more about a ‘living soil’. Not too many years ago, this term was foreign to UC researchers. But today, it is used quite often, not by the organic aficionados only, but by the UC researchers too. Many growers are turning away from preemergent herbicides and looking to alternative methods, knowing that some weed growth, or controlled weed growth, can be a good thing. As an example, growers have spent millions of dollars on malva control, also known as cheeseweed, but long ago and now again today, malva is sometimes used as a winter covercrop.

Whether choosing the chemical route or finding alternatives, growers should definitely not 1. let weeds grower taller than their trees, 2. allow vines to entangle the canopies, 3. allow perennials, such as bermudagrass, to run rampant and 4. you should not have uncontrolled weed growth under the canopies and around the trunks.