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An Alternative Weed Control - Mulching

By Nick Sakovich

Weeds have always been a menace to growers. But, for about the last 50 years, chemical weed control has proven to be a good method for keeping orchards clean of harmful weeds. For various reasons however, many growers would like to avoid the use of soil residual herbicides. Contamination of the groundwater, growth of young trees planted back into herbicide active soils and crop phytotoxicity (especially when the herbicide is used at high rates in sandy soils) are all potential problems growers may face when dealing with long residual soil herbicides. Although we need to preserve the use of herbicides, in order to give growers as many options as possible, researchers have been investigating alternative methods for weed control.

Covercrops have recently been reintroduced. Not only can they improve water infiltration, add nitrogen to the soil, provide a food source for beneficials and reduce soil erosion, they can also provide good weed suppression. Ducks have been used in some orchards for snail control, but they also provide some degree of weed control. Geese are natural born grass eaters. They can even do a good job in eliminating bermudagrass and Johnsongrass from a field, but they will leave most broadleaves alone. About 4 to 8 geese are needed per acre. They require a lot of care, so they may not be for everyone. The initial cost and maintenance does exceed the cost of herbicides. However, they do offer an alternative, especially for the organic grower or for the small scale farm.

Various companies have introduced fabric mulches. Weed germination does occur under this cloth, but the seedlings are unable to grow through the mulch, and subsequently die. The fabrics are expensive, but at least in some cases, have lasted five years now. This would bring the cost to be equivalent to herbicide applications over the same period of time.

Recent research has shown that the use of mulches in orchards also provides good weed control.

They have already shown to improve soil structure, keep soil cooler during high summer temperatures, conserve water and even aid in the suppression of root rotting fungi. But perhaps the most apparent quality mulches have, are their effect on the suppression of weeds. Weed control using a sufficient depth of the applied mulch can provide control which is comparable to the application of herbicides. Mulches have a lesser effect against established perennial weeds which can emerge through deep layers of the applied mulch. However, a combination of mulch and herbicide application will often give excellent control.

In 1990, the California Legislator passed AB 939, which states that the stream of waste into landfills must be reduced by 50%, by the year 2000. The most easily reduced component of this stream is yardwaste, which will make large amounts of this material available to the grower.

In 1993, a mulch trial was establish in Ventura county, by Farm Advisors B. Faber, J. Downer and N. Sakovich, and Specialist M. McGiffen. In addition to weed control, this research will eventually give us good information about mulching with respect to plant nutrition, soil moisture and pest control (snails, ants, nematodes and phytophthora).

In a citrus orchard, a mixed-source, chipped urban yardwaste (mulch) was applied to the orchard floor at the depths of 1, 3 and 6 inches, in a band 6 feet wide down the tree row. So as not to encourage gummosis, no mulch was applied within a 2 foot radius of the trunk. The trees were initially drip irrigated, but were converted to microsprinklers a year after the project began. Mulch was applied on a yearly basis to maintain the original depths. Once established, weed counts were made on a bimonthly basis. The plots were assessed for percent coverage by weeds in addition to individual weed counts.

A total of 76 weed species from 21 plant families were recorded growing in the plots. There was a definite and clear distinction between the mulched and unmulched plots, with the greater number of weeds growing in the unmulched areas. Weed species such as spurge, groundsel, tall fescue, horseweed, purslane and scarlet pimpernel either did not occur at all or at extremely low levels in the mulched areas, but were very common in the unmulched plots. Wild oats, an exception, appeared in all plots, but at a greater frequency in the mulched plots, regardless of mulch depth.

In evaluating depth of mulch on weed suppression, it is clear that mulches at all depths have an inhibitory effect on weed germination. Even at one inch there was an effect, although the weeds covered between 2 and 5 times the area compared to the 3 and 6 inch mulch depth. Statistically, there was no difference between the 3 and the 6 inches of mulch on percent of weedcover. But in general, more weeds were observed in the 3 inch mulch plots.

Decomposition of the mulch material will depend on environmental factors, particle size, age and composition. In this study, approximately one inch of decomposition occurred each year, regardless of mulch depth. Since growers seem to prefer less frequent applications, it would seem that one application of 6-8 inches of mulch every 3 years would give adequate weed suppression.

Applying yardwaste to the orchard is an excellent means of weed suppression. Water conservation, soil improvement and disease suppression are other benefits of mulching. However, it does seem that as in most situations, the amount of mulching a grower does will depend upon the economics. Even if the material is free, there would still be costs involved for associated operations such as hauling and spreading.