Journal of Applied Horticulture, 7(2): 90-94, July-December, 2005 Effect of Eucalyptus cladocalyx mulch on establishment of California sycamore ( Platanus racemosa ) A. James Downer and Ben Faber University of California Cooperative Extension, 669 County Square Drive, Suite 100 Ventura, CA, 93003, USA. E-mail: ajdowner@ucdavis.edu Abstract Mulches prepared from fresh and composted Eucalyptus cladocalyx prevented growth of annual weeds, increased soil moisture retention, reduced diffusive resistance of California sycamore (Platanus racemosa) and increased stem diameter compared to unmulched sycamores. Sycamore root lengths were greater in soil profiles under mulched trees than in soil under unmulched ones. Eucalyptus mulches reflected more photosynthetically active radiation and maintained lower surface temperatures than biosolids mulch or unmulched soils. Eucalyptus branches both freshly chopped and composted were effective in promoting growth of sycamore. Key words: Biosolids, pine bark, composting, Eucalyptus cladocalyx, Platanus racemosa, mulch Introduction Eucalyptus trees are commonly planted as windbreaks and amenity plantings in orchards and landscapes throughout subtropical and mediterranean regions of the world. Suppression zones of sparse understory vegetation are often associated with eucalyptus trees and are sometimes caused by allelopathic mechanisms (del Moral and Muller, 1969 & 1970; del Moral et al., 1978; Nishimura et al., 1984; Lamont, 1985; Molina et al., 1991). Reduction in growth of row-crops is also associated with eucalyptus allelochemicals (Rao and Reddy, 1984). Shredded or chopped leaves of some eucalyptus species can be toxic to seedlings (Baker, 1966; Nishimura et al.,1984; Igboanugo, 1986; Molina et al.,1991; Kohli and Singh, 1991). Molina et al. (1991) found that leachates from decomposing eucalyptus litter reduced germination of herbaceous annuals. However, May and Ash (1990) suggested that decomposition of leaf litter destroys the toxic effects found in living eucalyptus trees. Yet, Duryea et al. (1999) found that Eucalyptus grandis mulches contained phytotoxic residues three months after application to soil. Eucalyptus trees are common landscape trees that when trimmed or removed, become components of “green-waste” that is now frequently collected and recycled. Although eucalyptus allelopathy has been demonstrated, there is little information on the phytotoxic potential of eucalyptus mulches that might be used in landscapes. Because eucalyptus phytotoxins may harm desirable plants, there is concern that municipally collected “green-waste” could be contaminated with eucalyptus that will “poison” yardwaste compost products. The purpose of this study is to determine the suitability of mulches made from Eucalyptus cladocaylx trimmings for the establishment of young sycamore trees and whether composting reduces any phytotoxicity symptoms that might be associated with fresh Eucalyptus cladocalyx. Materials and methods Platanus racemosa Nutt. seedlings were planted from #1 (3.7L) containers at a site in Ojai, Ca. The soil type was an Ojai stony fine sandy loam (fine-loamy, mixed, thermic Mollic Haploxeralfs). Forty-eight trees were planted at 6m distance and allowed to grow for 3 months before mulch was applied. Branches of Eucalyptus cladocalyx F.J. Muell. (up to 8cm diameter) were pruned from mature landscape trees and chipped with a commercial brush chipper to produce 4-6 cm long chips. Approximately 4 m 3 of fresh chipped branches (including leaves, flowers, fruit and bark) were composted using the rapid composting method (Raabe, 1974). Ammonium sulfate was applied once to initiate breakdown of the compost (0.454kg (NH 4 ) 2 SO 4 ). m -3 of fresh eucalyptus). The compost was turned at seven-day intervals for 90 days and moisture added as needed to maintain compost heat. After the compost was stable (no longer heating), branches were again harvested from the same trees, and chipped to 4-6 cm long chips (large chips) or 1 cm chips (small chips). The following mulches were then applied ten cm deep around each tree: 1. Unmulched; 2. pine bark (Xerimulch ® , Kelloggs Supply Inc., Carson, CA); 3. composted biosolids and wood (Growmulch ® , Kelloggs Supply Inc., Carson, CA); 4. composted E. cladocalyx; 5. fresh E. cladocalyx (large chips); and, 6. fresh E . cladocalyx (small chips). The mulched zone around each tree was 2.5 by 2.5m. All treatments were applied in randomized complete block design with 8 replications. No fertilizer was applied before planting or during the study. Irrigations were by micro- sprinkler and applied so that water percolated through the mulch materials. Trees were irrigatied when soil moisture tension (at 15cm depth) exceeded 60Kpa. During planned dry downs, soil moisture tensions exceeded 100 Kpa. Soil moisture was monitored with gypsum blocks (Irrometer Company, Riverside, CA). Soil moisture content (% by volume) was determined by time domain reflectometry with a Trace ® TDR (Soil Moisture Equipment Corporation, Santa Barbara, CA), using 15cm waveguides.