how deep should soil be to plant vegetables in a planter box?
For most plants, a 6-to 8-inch-deep planter box is sufficient. The depth may vary for some vegetables, however. Turnips, cucumbers, broccoli, beets, lettuce and green onions can all grow well in a planter box at that depth, but other vegetables, like cabbage, need a deeper depth of at least 10 inches.arge bulbs require a planting depth of 8 inches, which means the planter box should allow for the 6 inches needed at the bottom, as discussed earlier, making the depth at least 14 inches.