YARDWASTE MULCHES IN AVOCADO AND CITRUS ORCHARDS
Ben A. Faber, A. James Downer, Nicholas J. Sakovich, Milton E. McGiffen, Dave Cudney and Sue A. Mills
University of California
Since A8 939 took effect in 1995, large amounts of yardwaste have been made available to growers. We have been studying the myriad effects that a mulch can have on plant and soil properties. Since 1990 we have evaluated the effects of different source, depths and particle sizes of mulches, and mulch effects on five different tree species (Platanus racemosa, Liqui damber styraciflua, Persea awricana, Citrus sinensis, and Citrus limon). Measured parameters have included plant growth and yield, soil and plant water parameters, .and plant health. Since eucalypt species are such a prominent part of the yardwaste stream, single source mulches derived from seven different species of eucalyptus have been studied along with more generic yardwaste mulches. With the possible inhibitory effect on annual seedling growth of Eucalyptus sideroxy1on as freshly‑chipped mulch, coarsely‑textured mulches have a beneficial effect on plant growth (Downer et a1, 1995). Mulched trees have lower soil moisture tensions and higher rates of stomatal conductance, either through reduced evaporative loss or competition with weeds (Downer et al, 199). Reduced disease severity has been measured in field grown avocado trees (Persea americana)(Faber, in press). No dramatic nutritional effects have been noted, although in the case of lemon (citrus 11mon) a reduction in toxic levels of boron have been measured (unpublished data). Other side effects of mulch that have been measured have been reduced snail activity and brown rot on lemon fruit (unpublished data).
Over the last few years, it has became very apparent that one of the most important mulch effects is weed suppression. Organic mulches can provide good weed control if applied to sufficient depth (Robinson, 1988). A sufficient mulch can provide control comparable to herbicides (Singh et al., 1985). Organic mulches are effective against annual weeds, but have little effect against established perennial weeds which can emerge through deep layers of applied organic mulches (Robinson, 1988). In some cases, combined mulch‑herbicide applications give excellent results (Robinson, 1988).
The objective of this report is to relate the effect of different thicknesses of a mixed‑source urban yardwaste on weed growth in an orchard.
A 6 year old Valencia orange on carrizo rootstock orchard in Ojai, CA was selected for the study site. Experimental plots of 5 trees each, received either 0, 1, 3 or 6 inches (0, 2.5, 7.5 or 15 cm) of mixed‑source urban yardwaste from the city of Ojai. The trees were planted on a 6m x 6m spacing. The mulch was applied in a band 6 feet wide down the tree row. Mulch was not applied within a 2 foot (0.6 m) radius of the tree trunk. The treatments were replicated 10 times. The trees were initially drip irrigated but converted to microsprinkler a year after the project was started. Mulch was first applied in August 1993 and material was reapplied each year to the original depths. After measurements were taken, weed growth was controlled with glyphosate as needed by the grower.
On a bi‑monthly basis, weed counts were made. The whole plot was assessed for percentage cover by the weeds. Then, along the transect of the polyethylene irrigation line, individual weed counts were made and the species recorded. It was not always possible to obtain weed counts every other month, because of field access problems. The first counts were made April 1994 and are continuing to this day.
In all, a total of 76 weedy species from 21 families were noted in the two years of data collection. The most commonly occurring species were in the Poaceae (16 species), Asteraceae (11) and Fabaceae (6) families. The Brassicaceae, Geraniaceae and Portulacaceae each had 3 representative species. There was a clear distinction in comparing mulched and unmulched plots, with a greater numbers of weeds growing in the unmulched tress (Figure 1). There were also greater numbers of grasses germinating in the unmulched plots, at approximately 3 times the frequency of broadleaf weeds (Figure 2).
Total weed diversity decreased with the depth of mulching.
In the mulched plots, scarlet pimpernel (Anagallis arvensis), purslane (Portulaca oleracea), spurge (Euphorbia maculata), horseweed (Conyza canadensis), yellow clover (Melilotus indica), tall fescue (Festuca arundinacea), and common groundsel (Seneclo vulgaris) either did not occur at all or at extremely low levels, while being common in the unmulched plots. Wild oat (Avena fatua) was an exception. It appeared in all plots, but at between 2 and 10 times the frequency in the mulched plots, regardless of mulch depth. California live oak (Quercus agrifolia) appeared only in the mulched plots and not in the unmulched. The acorns may have come with the mulch, although the orchard is surrounded by live oak and may have been planted by jays or other animals.. Three commonly imported tree seeds in mulch are palm, eucalyptus and pine species. Although we have found these species in other trial sites, these did not appear in any of the Ojai plots.
In evaluating mulch depth on weed suppression, it is clear that mulches have an inhibitory effect (Figure 3). Even a 1" depth has an effect, although weeds covered between 2 and 5 times the area with 1" versus 3" and 6", depending on the time of year. statistically, there is no difference between a 3” and 6” depth on percentage weed cover.
Selecting an application depth might be a function of availability of material: and how frequently applications would be made. At the Ojai site there is approximately 1" of decomposition occurring in the plots regardless of the depth. Decomposition rate will of course depend on the age, woody composition and particle size of the mulch. The growers we have worked with would prefer less frequent applications, such as once every 2 or 3 years rather than annually. A 6 " application should be effective at weed suppression for at least 3 years.
The use of yardwaste mulches in orchards will be determined by economics. Does the mulch reduce the amount of money and labor spent on weed suppression, disease suppression, water conservation and any number of other practices growers currently follow? There is now in Ventura County a contractor who will apply 6" depth by 6 ft wide mulch strips (approximately 100 tons of mulch). Depending on the conditions, he charges approximately $3.50 per tree ($400 per acre), with the grower obtaining the material. In Ventura, chipped yardwastes are available for as little as free up to $2 per ton. We are slowly learning what value mulch may have for a grower.
Downer. J., B. Faber and R. White. 1993. Green waste compost for landscaping ‑ a demonstration project. In: Proceedings 1993 Calif Plant and Soil Conf, Sacramento, CA.
Downer, J. and B. Faber. 1995. Determining the usefulness of Eucalyptus mulches in landscape plantings. In: 1993‑94 Research Report of the Elvinia M. Slosson Endowment, DANR.
Faber, B, J. Downer, J. Menge and H. Ohr. Mulch effects on avocado root rot. Acta Hort. in press.
Robinson, D.W. 1988. Mulches and herbicides in ornamental plantings. HortScience 23:547‑552.
Singh, Aparbal, Man Singh, D.V. Singh, A. Singh and M. Singh. 1985. Relative efficacy of organic mulch and herbicides for weed control in Cymbopogon species. Ann. Conf. Indian Soc. Weed Sci. p. 77 (abstract).
(Note: figures were not available for scanning and inclusion with this article. Contact Dr. Ben Faber for further information.)