EFFECTS OF AMENDMENTS, SOIL ADDITIVES, AND IRRIGATION ON TREE SURVIVAL AND GROWTH

Journal of Arboriculture, Sep 2004 by Gilman, Edward F

Root systems on five trees for each treatment combination (7 amendments × 2 irrigation treatments × 5 replicates = 70 trees) were partially excavated in December 1996 (2 years after planting). All roots were collected from two one-eighth circumference soil volumes (one on the north side and one on the south side of the tree) as deep as the root ball [40.6 cm (16 in).] beginning at the edge of the root ball out to 12.7 cm (5 in.) away. All collected roots were in the backfill soil. Soil was washed from the roots and roots dried for 7 days in an oven at 70°C (158°F).

Data were analyzed using ANOVA and regression in SAS. Means were considered significant at P

RESULTS AND DISCUSSION

Two weeks after planting, trees amended with Slockosorb, Drought Releaf, and Profile that were not watered for 3 days were significantly more stressed (more negative xylem potential) than trees in nonamended backfill (data not shown); there was no difference in stress among soil amendments for trees irrigated frequently. Five (Figure 1*), ten (data not shown), and fifteen weeks (Figure 2) after planting, xylem potential in trees was similar for all backfill amendments, and frequently irrigated trees were less stressed than those receiving less frequent irrigation.

During the first and second year after transplanting, there was no apparent benefit from adding amendments to the backfill soil around newly planted live oaks nor was there a benefit from applying liquid additives over the root ball. Top growth in the first 2 years after planting was not enhanced on trees with amendments added to the backfill soil compared to the control (Figures 3 and 4). The controls received only the native soil present at the site as backfill. Irrigation did not interact with soil amendment treatment, meaning that amendments did not affect survivability or growth of live oaks when irrigation was either minimal or plentiful after transplanting. This finding contradicts promotional literature distributed by manufacturers of many amendments but supports nearly all published studies on trees (Pellet 1971; Schulte and Whitcomb 1975; Ingram et al. 1981; Corley 1984; Smalley and Wood 1995; Ferrini and Nicese 2002).

Not surprisingly, in the first growing season (March 1995 through April 1996) after planting, trees irrigated frequently after planting (3 limes per week for 38 weeks) grew about twice as fast (caliper and height increase) as those irrigated periodically only during the first 3 months after planting (Figures 3 and 4). Growth rate in the second growing season (April 1996 through December 1996) was similar for irrigated and nonirrigated trees (slopes were equal at P

Trees treated with Profile 2SC grew slower (caliper and height) than all other treatments because of the growth-regulating effect of paclobutrazol. This growth regulator has been used to slow growth of trees near power lines and in other urban situations. Despite slower top growth, there was no evidence that roots grew any slower than other treatments. This suggests a change in the root:shoot ratio observed by other researchers (Watson 1996). For this reason, it has been suggested for use on newly planted trees to help them overcome transplant shock. However, reduction of transplant shock did not occur in the present study.