Size a root ball by noting plant height or trunk size
After deciding when to dig, the next thing to consider is how big a root ball should be. For evergreen shrubs and trees, 1 foot of root ball to every 2 feet of height is a good rule of thumb. It’s better to err on the side of generosity, since digging a tree is sort of like cutting hair: Once you sever a root, you can’t go back.
For deciduous shrubs and trees, I calculate the size of its root ball by measuring the trunk’s diameter. Usually, there’s 1 foot of root ball for every inch of a trunk’s diameter. Industry standards dictate that diameter measurement be made at 6 inches above ground level. The best way to measure a trunk’s diameter is with a tree caliper. Your local garden center may offer an inexpensive model. If you cannot locate a caliper, you can measure the circumference of the tree with a piece of string (illustrations, above right), then divide that number by pi (3.14) to get the diameter. Let’s say the trunk’s circumference measures 6 inches. Divide 6 by 3.14 to come up with a diameter that equals 1.91 inches, which, when rounded up, means that the root ball should measure about 2 feet across.
Plants growing in a landscape tend to have fewer and longer roots than fieldgrown nursery stock. So if the perfectly shaped root ball you envision ends up looking more like the head of a golf club once it’s out of the ground, don’t worry. A root ball doesn’t have to be picturesque for a plant to thrive.