Certified Professional Horticulturist (CPH) Practice Exam

What is the best diameter for a root ball when root pruning a tree/shrub with a stem diameter of 3 inches?

1. 48 inches

2. 30 inches

3. 24 inches

4. 15 inches

The correct answer is: 30 inches

When determining the appropriate diameter for a root ball during root pruning, a commonly accepted guideline is to aim for a root ball diameter that is a certain multiple of the stem diameter of the tree or shrub. In this case, for a stem diameter of 3 inches, the best practice suggests that the root ball should be around 10 times the stem diameter. This guidance helps ensure that an adequate amount of roots is preserved, promoting the health and stability of the plant once it is transplanted. Therefore, a 30-inch diameter root ball aligns well with this principle, as it is large enough to contain a sufficient root system while being manageable for handling during transplanting. The other choices either exceed or fall short of the ideal size, which can affect the overall success of the transplantation. A 48-inch diameter may be unnecessarily large for a 3-inch stem and could make handling difficult. Options of 24 inches and 15 inches would not provide enough root area, potentially compromising the tree or shrub's health after replanting.