Optimal depth: 12 inches for most vegetables

A 12-inch soil depth accommodates the root systems of approximately 90% of common garden vegetables. This depth provides adequate space for medium-rooted crops (beans, peppers, cucumbers, brassicas) and sufficient moisture buffering to reduce watering frequency. For beds placed directly on the ground, roots can extend further into native soil below, making 12 inches effectively deeper. Two stacked 2x6 boards (each 5.5 inches actual) give 11 inches of depth, which is close enough. Two 2x8 boards give 14.5 inches, and one 2x12 gives 11.25 inches. For deep-rooted crops like carrots, parsnips, and large tomatoes, 16-18 inches is ideal, achieved by stacking three 2x6 boards or two 2x10 boards. Shallow beds of 6-8 inches work for lettuce, herbs, radishes, and green onions but limit your crop options. Deeper beds cost more to fill but require less frequent watering and support a wider range of crops. Always overfill by 10-15% as soil settles in the first season.