An Alternative Weed Control - Mulching
By
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
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
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.