How deep should a raised garden bed be?

The short answer

For most vegetable gardens, a bed about twelve inches deep sitting on loosened native soil works well. Roots grow through the frame into the ground below, so the bed itself does not have to hold every inch of root space. Go deeper when the ground underneath is poor, paved or contaminated, or when you want to garden without bending.

Depth by crop

These figures are root depth, not frame height. If the native soil below is loose and healthy, a shallower frame still gives deep-rooted crops room to grow.

Preparing the ground underneath

Before setting the frame, loosen the soil underneath with a garden fork. Breaking up the surface lets roots and water move between the bed and the ground, which keeps the bed from drying out as quickly. If grass or weeds are growing there, a layer of plain cardboard under the mix smothers them and breaks down within a season.

When to go taller

Beds of twenty-four inches or more save your back and knees and are easier to reach from a chair or wheelchair. A tall bed also deters most rabbits. The tradeoffs are more mix to fill and slightly faster drying, so mulch the surface and water deeply rather than often. The soil mix guide covers ways to fill a tall bed affordably.

Beds on patios and hard surfaces

Where there is no soil underneath, every inch of root space has to be inside the bed. Build at least twelve inches deep, use the lighter end of the soil mix so the bed is not too heavy for a deck, and make sure water can drain out of the base. Check that a deck or balcony can carry the weight of a filled and watered bed before you build one there.

Width and length matter too

Depth gets the attention, but width causes more everyday frustration. Keep beds to about four feet wide, or three feet if you can only reach from one side, so you never need to step into the soil. Length can be whatever fits your space, though very long beds are easier to work if you leave a gap to walk through partway along.

Deep beds and watering

Depth changes how a bed holds water. A shallow bed on a patio has only a few inches of mix to draw from, so in hot weather it can dry out in a day. A deep bed holds far more water in reserve, and the lower layers stay moist long after the surface looks dry. The flip side is that a deep bed takes longer to wet all the way through once it has dried out completely, so water slowly and check a few inches down with your finger rather than judging by the surface.

On loosened native ground, a twelve-inch bed draws moisture up from the soil below during dry spells, which is one more reason to break up the ground before you build.

Depth and soil temperature

Soil in a raised bed warms faster in spring than the ground around it, and the shallower the bed, the faster it warms and cools. That head start helps early crops like peas and lettuce, and lets you plant warm-season crops a little sooner. In midsummer, the same effect can stress roots in a shallow bed in full sun. Deeper beds even out these swings because the center of a large volume of soil changes temperature slowly.

A quick way to decide

• Good soil underneath, mixed vegetables: about twelve inches.
• Poor, rocky or compacted ground: eighteen inches, with the ground loosened first.
• Patio, deck or concrete: at least twelve inches of mix, eighteen for tomatoes and root crops.
• Back or knee trouble, or gardening from a chair: twenty-four to thirty inches.

Whatever depth you choose, the frame needs to be strong enough to hold it. Tall beds push hard on their sides, so taller wooden beds need corner posts and sometimes a cross brace in the middle of long sides. Metal beds with corrugated panels resist bowing well at most heights.