For tomatoes, planter boxes from a planter box factory should generally be at least 18 inches deep, with 20–24 inches being the safer target for full-size indeterminate varieties. Tomatoes are often described as deep-rooted, but most active feeder roots sit in the upper 12–18 inches of soil. Even so, extra depth gives the plant more buffer against heat, drought, and fluctuating moisture. A box that is only 10–12 inches deep can work for compact or dwarf tomatoes with very careful drip irrigation and rich, well-drained soil, but it leaves little margin for error and usually limits fruit load.

Depth should be matched to the vegetable and the growing strategy. Herbs and leafy greens often perform well in 6–8 inches; lettuce and chard prefer about 8–12 inches; peppers and eggplant are usually happier at 12–16 inches; tomatoes are best at 18–24 inches; and root crops such as carrots need 12 inches or more of loose, stone-free mix. For a planter box factory order, ask for internal depth rather than external height, because soil depth is what roots actually use.

A practical tomato box should also offer strong drainage, a breathable soil mix, and enough width for support stakes or a cage. One full-size tomato plant generally needs 15–20 gallons of soil volume or more, so a deep but narrow box may still dry out too quickly. If the planter box is on a patio, line it with landscape fabric, use a free-draining raised-bed mix, mulch the surface, and water deeply rather than lightly. In short, choose 18 inches as the minimum for tomatoes and 20–24 inches for better resilience, especially if you want large plants, heavy yields, or greater tolerance for hot, windy weather.