Graham FisherBy Graham Fisher

Updated November 2026

Raised garden soil depth decides how well your vegetables root, how much water they hold, and how often you will need to fill the bed again. For most vegetables, 10 to 12 inches of soil is enough. Deep-rooted crops such as tomatoes, carrots and winter squash do better with 18 inches or more. This guide shows how to match depth to what you plan to grow.

Match Depth to the Crops You Plan to Grow

Root depth is the single best guide. Shallow-rooted crops need less soil, while taproot and sprawling crops need more room to spread.

Shopping for one? See our guide: 7 Best Soil Depth Raised Garden Beds for Vegetables (2026). For a deeper look, How to Make a Soil Mix for Raised Garden Beds covers this in detail.

  • 6 to 8 inches: Lettuce, spinach, radishes, herbs such as basil and parsley, and most microgreens.
  • 10 to 12 inches: Peppers, bush beans, kale, chard, onions and most leafy vegetables.
  • 18 to 24 inches: Tomatoes, eggplant, carrots, parsnips, potatoes and winter squash.

If a bed will hold mixed crops, build it to 12 inches and plant the deep-rooted vegetables in the center where soil is deepest after settling.

Measure Usable Depth, Not Just Bed Height

The height of the frame is not the same as the depth of soil you can grow in. Once you add the bottom layer and allow for settling, the usable depth is usually smaller than the board height. Plan on soil settling about 10 to 20 percent in the first season, so a 12-inch frame may hold closer to 10 inches of loose soil after the first few waterings.

To check, dig a hole in the middle of the bed and measure from the top of the soil to the bottom of the native ground or liner. If you want 12 inches of growing depth, fill the bed slightly above the rim the first time and top up after the soil settles.

Build the Soil Depth Step by Step

Whether you are building a new bed or deepening an old one, work in this order so the root zone stays loose and drains well.

  1. Set the frame on level ground and remove grass or weeds from the footprint.
  2. Lay cardboard or landscape fabric on the bottom if you want to block weeds, but skip heavy plastic that traps water.
  3. Add coarse material such as small logs or straw for the lower third if you are building a deep bed (over 18 inches). This saves soil and breaks down slowly over several years.
  4. Fill the rest with a well-draining raised bed mix, adding layers of compost as you go.
  5. Water thoroughly, wait a day, and top up any low spots before planting.

Pay attention to drainage. A bed filled to 24 inches with dense clay will stay soggy, so keep the mix loose even when you go deep.

How Soil Depth Affects Watering and Feeding

Deeper soil holds more water and gives roots more room to find moisture between waterings. A 12-inch bed dries out much faster in summer heat than an 18-inch bed of the same width, so shallow beds need daily checks during hot weather. Nutrients also stay closer to the roots in deep soil, which means you can feed less often if the mix starts with good compost.

Check moisture by pushing a finger into the soil to the second knuckle. If the soil is dry at that depth, water deeply rather than lightly.

Common Mistakes

  • Building a 6-inch bed for tomatoes or squash, which limits root growth and drought tolerance.
  • Forgetting that soil settles, so the bed looks full but ends up shallow after the first season.
  • Filling deep beds entirely with heavy topsoil, which compacts and drains poorly.
  • Placing a bed on compacted ground without loosening the native soil underneath, which blocks root expansion.
  • Adding fresh manure in the root zone at planting, which can burn young roots.

A few tools make depth checks and filling much easier, especially for deeper beds.

  • A long-handled soil probe or trowel: Helps you check depth at the center of the bed before and after settling. View on Amazon
  • A bed-fill measuring stick: A marked stick lets you confirm depth quickly as you add each layer. View on Amazon
  • A moisture meter: Shows whether moisture reaches the root zone, which matters most in deeper beds. View on Amazon

FAQ

Is 6 inches of soil enough for a raised bed?

Six inches works for lettuce, radishes, herbs and other shallow crops. It is not enough for tomatoes, carrots or squash, which need deeper roots to stay productive.

How deep should a raised bed be for tomatoes?

Aim for at least 18 inches of growing depth. Deeper soil holds more water and supports the larger root system tomatoes develop over a season.

Does soil settle in a raised bed?

Yes. Expect about 10 to 20 percent settling during the first season, so fill the bed a little above the rim and top it up after the first few waterings.

Conclusion

Choose raised garden soil depth based on your crops: 6 to 8 inches for greens and herbs, 10 to 12 inches for most vegetables, and 18 inches or more for tomatoes and root crops. Account for settling and drainage when you fill the bed. A little planning now saves refilling and watering trouble later.

Ready to choose one?

Graham Fisher

About Graham Fisher

Graham Fisher is part of our editorial team and reviews and edits our vegetable & edible gardening guides, checking product details against manufacturer specifications and owner feedback.