When you buy planter boxes from a planter box factory, there is no single universal depth that fits every shrub. The right depth depends on the shrub’s mature size, the size of its root ball, and whether the box will be used as a permanent home or a temporary display. However, factories and landscape professionals often work with a few standard ranges.

For most small to medium shrubs, the internal soil depth should be at least 18 to 24 inches (45–60 cm). This range accommodates a typical nursery root ball, which is often 8 to 16 inches deep, while leaving room for drainage and fresh soil beneath and around the roots. If the shrub is sold in a 3-gallon, 5-gallon, or 7-gallon container, its root ball may be roughly 10 to 14 inches deep, so an 18- to 20-inch-deep box is usually the minimum practical size. For a larger shrub, such as a 10- to 15-gallon specimen, the root ball can be 14 to 20 inches deep. In that case, a 24- to 30-inch-deep planter box is safer. Very large shrubs, screening plants, or shrubs with deep taproots may need 30 to 36 inches of internal depth.

Factory-standard planter boxes often come in depths such as 12, 16, 20, 24, 30, and 36 inches. The shallow options—12 to 16 inches—are better for annuals, herbs, and low perennials. They are generally not deep enough for a shrub root ball unless the shrub is very small and the box is unusually wide, which allows roots to spread horizontally. A 20-inch box can work for compact shrubs, but 24 inches is often considered the starting point for reliable shrub growth. If the box is 24 inches deep, the actual soil depth may be less after accounting for drainage material, a false bottom, or wheel clearance. Always check the internal depth, not the outside height.

Depth alone is not the whole story. A shrub root ball needs enough soil volume, oxygen, and drainage. A deep but narrow box can still fail because roots have nowhere to spread. As a rule, the planter box’s internal width and length should be at least two to three times the width of the root ball. For a shrub with a 12-inch-wide root ball, look for a box that is at least 24 to 30 inches wide and long. For a 20-inch-wide root ball, a 40- to 60-inch-wide box is more appropriate. The box should also have drainage holes, and it should be raised slightly off the ground or patio so excess water can escape.

Another factor is overwintering. In cold climates, a shrub in a planter box is more exposed to freezing than one planted in the ground. A deeper box—ideally 24 to 30 inches—provides more insulation and protects more of the root ball from rapid temperature swings. If winters are severe, choose the deeper end of the standard range or wrap the box for extra protection.

So, what is the standard depth from a planter box factory that is enough for a shrub root ball? For many shrubs, the practical answer is 18 to 24 inches of internal soil depth. For larger or long-term shrubs, choose 24 to 30 inches. For very large specimens, 30 to 36 inches is best. If you are ordering from a factory, ask for the internal depth and confirm the root ball size of your shrub before you buy. Matching the box depth to the root ball—plus giving roots room to spread—will give your shrub a much better chance to establish and thrive.