The ideal raised bed depth vegetables need falls between 10-18 inches for most crops. This range gives roots enough room to spread while keeping your soil and lumber costs under control. Deeper beds help root vegetables and tomatoes while shorter ones work fine for greens and herbs.
I built beds at three different heights to test what works best in my own garden. The 6-inch beds grew great lettuce and herbs but choked out my carrots. My 12-inch beds handled most vegetables well. The 18-inch beds produced the biggest tomatoes and longest carrots I have ever grown at home.
West Virginia University Extension backs up the 10-18 inch range for raised bed soil depth in most gardens. This depth matches how deep most vegetable roots want to grow. You waste money on soil above 18 inches for all but the deepest rooted crops.
Lettuce, spinach, and most salad greens need just 6-8 inches of soil to thrive. Their roots stay near the surface where water and nutrients gather. You can grow these crops in shorter beds and save money on fill while still getting great harvests all season long.
Knowing how deep raised garden bed soil should go depends on what you plan to grow. Peppers and bush beans do well with 10-12 inches of depth. These plants send roots down about a foot before spreading sideways. Most backyard gardens grow these crops without any issues at standard heights.
Root vegetables demand more space below the surface than other crops. Carrots need 12-18 inches to reach full length without hitting hard soil or barriers. Beets and parsnips have similar needs. Short beds force these crops to grow stubby or forked rather than long and straight.
The vegetable root depth for tomatoes runs deeper than many gardeners expect. These plants send roots down 2-3 feet when they can reach that far. Deeper beds let tomato roots tap into more water and nutrients, which means bigger fruits and better drought survival during hot spells.
Your native soil beneath the bed affects how deep you need to build. Good ground lets roots grow past your bed frame into the earth below. Hard clay or rock stops roots at the frame bottom. Test your native soil before you decide on bed height.
Building deeper costs more in both lumber and soil fill. A 12-inch bed needs twice the soil of a 6-inch bed for the same footprint. Plan your budget around what you want to grow rather than building all beds to max depth. Mix bed heights based on crops you plant most.
Start with 12-inch beds if you grow a mix of vegetables and want one height that works for most things. Add one deeper bed for root crops and tomatoes if space and budget allow. Shorter beds near the house work great for herbs you pick often.
Read the full article: Raised Garden Beds: From Setup to First Harvest
