Effects of Cupric Hydroxide-Treated Containers on Growth of Four Southwestern Desert Landscape Trees

Materials and Methods

We potted 10-month old seedling liners of four southwestern desert landscape trees [Acacia smallii (sweet acacia), Acacia stenophylla (shoestring acacia), Cercidium praecox (palo brea), and Prosopis chilensis (Chilean mesquite)] on June 1, 1993 from 3-liter polyethylene containers into 19-liter black polyethylene containers filled a 70% ponderosa pine forest mulch, 15% sand, and 15% silt (by vol.) rooting substrate that was amended with controlled release fertilizer (20 N - 3.9 P - 7.5 K) at the rate of 3.0 kg/m³ plus micronutrients at the rate of 0.9 kg/m³. Just prior to potting, we painted the inner wall of one-half of all containers with a latex paint containing Cu(OH)₂ formulated as Spin Out® (Griffin Corp, Valdosta, GA). Remaining containers were left unpainted as a control. Tree seedlings were then grown in an outdoor containerproduction nursery ata spacing distance of 0.5 m for five months. During this time, all trees were irrigated to container capacity daily with a spray-stake micro-irrigation system and fertigated every two weeks with a soluble fertilizer (20 N - 8.7 P - 16.6 K) at 200 ppm N.

Tree height and number and length of primary shoot lateral branches were measured at the start of the experiment and at harvest. After five months of growth in the nursery, all trees were harvested by severing shoots from the root ball at the rooting substrate surface. We next carefully separated the rooting substrate from each root system by floating the root ball in water. Then, five of the most developed, primary lateral root branch sections were excised at 30, second-order branching points away from a root terminal meristem for analysis of root branching frequency. For each first-order lateral root branch section, we counted the total number of terminal meristems and measured the length with the aid of a digital root imaging system (AgVision, Decagon Devices, Pullman, Wash.). Specific root branching was then calculated as the ratio of root length to the number of terminal meristems.

Finally, shoots and roots of all trees were placed into an oven dryer for 72 hr at 65°C and dry weights were recorded. The experiment was a four species by two cupric hydroxide painted-container factorial arranged in a completely randomized design with five single tree replications. General linear models were used to test for significant responses of the variables.

Results and Discussion

All measured variables were significantly affected by an interaction of species with Cu(OH)₂ treatments so data are presented by species after statistical analysis. No foliar copper toxicity symptoms were observed on any species during the study, similar to reports of others (2,10). Also, no evidence of root circling or matting at the rooting substrate-container wall interface was found for any species grown in Cu-treated containers except palo brea, which showed only minor evidence of root circling. All trees in unpainted containers had roots which circled around the rooting substratecontainer wall interface and matted at the bottom of the container.

Copper hydroxide-treated containers suppressed shoot growth of both acacia species. Shoot extension and the number of primary shoot lateral branches of sweet acacia and shoestring acacia were less in Cu-treated containers than for those in unpainted containers (Table 1). Final height of shoestring acacia, a species with an strong excurrent growth habit especially when young, was also less in Cu-treated containers than for those in unpainted containers. This was consistent with reduced height growth of bald cypress, another species with an excurrent growth habit, grown in Cu-treated containers (9). In contrast, final height of sweet acacia, a species with a decurrent growth habit, was not affected by Cu-treated containers. Specific root branching of both acacia species was decreased in response to Cu-treated containers compared with those in unpainted containers (Table 2). Reduced specific root branching indicated that Cu-treated containers increased the frequency of acacia root branching and subsequent number of new root tips as roots came in contact with the copper-painted container wall, and was in agreement with an earlier report on increased root branching by apple and green ash in response to Cu-painted containers (4).

Chilean mesquite grew taller and larger with increased shoot and root dry weights in Cu-treated containers in comparison with those in unpainted containers (Tables 1 & 2) and was similar to a report by Ruter (9) on the stimulatory effects of Cu-painted containers on growth of river birch. Stimulated growth of Chilean mesquite by Cu-treated containers was not related to changes in root branching, although Cu-treated containers controlled root circling and matting of Chilean mesquite. Growth and specific root branching of palo brea were not affected by Cu-treated containers (Tables 1 & 2). This was likely due to species specific high experimental error which resulted from seed propagation, currently the standard way of propagating palo brea in the southwestern nursery industry. In addition, the shoot-to-root ratios of all four tree species were not affected by Cu-treated containers which contrasts with the report of increased shoot to root ratio of green ash and red oak in response to Cu-treated containers (2).

In conclusion, we found that the growth responses of four southwestern desert landscape trees to Cu-treated containers varied among species. These findings are consistent with growth responses among temperate tree species.