Maximum tree size at planting depends on several criteria

1) Irrigation. The maximum size of the nursery stock recommended for planting at a particular site should be governed not only by the budget for the job, but by the irrigation capabilities after planting, and climate. Also be sure you are able to physically handle the tree at the job site without causing damage to it. If irrigation cannot be provided for the recommended period after planting, choose smaller nursery stock to ensure survival. This technique will help avoid the now common practice of planting nursery stock that is too large for the irrigation capabilities at the site. Planting trees too large for irrigation capabilities leads to increased mortality (Struve 1999). Small nursery stock (2-inch caliper) could grow to be as big as larger nursery stock (4-inch caliper) several years after planting (Gilman et al. 1998).

2) Site drainage also should govern maximum tree size at planting. Large-sized nursery trees (more than 2 inches in trunk diameter) may be poorly suited for wet sites because roots in the bottom portion of the root ball could become submerged in water. This can stress the tree by killing the deeper roots in the root ball. Smaller trees have a shallower root ball and so they are better suited for planting on wet sites. If large-sized nursery stock is absolutely necessary for a poorly-drained site, purchase trees with a shallow root ball such as those grown in low-profile containers, or plant on a mound to keep roots out of the water.

3) Weed control and mulch management. If weed growth is not controlled chemically or with mulch application around the tree during the establishment period, small nursery stock may be the best choice. While large trees are trying to establish, weeds often compete with the limited water supplied which can extend the establishment period. Small trees establish quickly and therefore are able to better compete with weeds (Gilman et al. 1998).

4) Storm history. Small nursery stock establishes faster than large sized nursery stock. This may allow small nursery stock to become wind-firm sooner than larger sized nursery stock. Some tree managers believe they saw this occur in hurricanes along street rights of way. More research is needed to test this theory.

Table 1. Comparing small-sized with large-sized nursery stock.