Soil Depth Requirements for Popular Vegetables

Be Sure There Is Adequate Depth for the Roots of the Crops You Plant

In most gardens, the top 6″ of soil contains the most nutrients needed for plant growth. This is because most root growth in vegetable gardens occurs in this relatively shallow depth. Nutrients such as compost and fertilizers are added to the bed from above and lightly tilled in. Mulches are also ‘top-dressed’ throughout the growing season, and gradually decompose into the top layer of soil adding additional nutrients.

Taproots will travel deeper into the soil if nutrients and water are available, and this also brings more trace minerals to the plant. Larger vegetable plants will send down deeper roots. When plants are able to send their roots deeper, they are less likely to fall over in windy conditions or if the ground becomes too wet. Large-leafed, shallow-rooted plants such as broccoli, Brussels sprouts and cauliflower will still require staking to ensure they stay upright as they grow to maturity.

Knowing the average root depth for your garden vegetables will help you decide where to plant each crop and how deeply to prepare your soil.

When preparing a garden bed for planting, it is helpful to know the root depth of vegetable crops since this has a bearing on where you may decide to plant certain crops and how deeply the soil is prepared. For example, in our garden we may plant shallow rooted crops like lettuce in beds where the subsoil has more clay and does not drain well. The deeper clay does not affect the shallow roots, and they benefit from the added moisture. Plants with deeper roots, such as tomatoes, would not do well in this soil depth.

Some raised beds are set on cement patios or on gravel surfaces, which prevent roots from going deeper than the height of bed sides. In these cases, it is especially helpful to know the soil depth requirements of different vegetable crops. Gardeners can compensate by building the raised beds higher to allow for more root space. While raised beds are commonly 8″ – 12″ tall, some raised beds have sides which are 3′ or higher. These taller beds enable deeper rooted crops to be planted even if there is no soil beneath the bed, but drainage must be provided by blocking the bed up an inch or so, or drilling drain holes near the bottom of the bed sides.

Soil Depth Requirements for Common Garden Vegetables

Shallow Rooting

12" - 18" Medium Rooting

18" - 24" Deep Rooting

24" - 36"+ Arugula Beans, dry Artichokes Broccoli Beans, pole Asparagus Brussel sprouts Beans, snap Beans, lima Cabbage Beets Okra Cauliflower Cantaloupe Parsnips Celery Carrots Pumpkins Chinese cabbage Chard Rhubarb Corn Cucumber Squash, winter Endive Eggplant Sweet potatoes Garlic Kale Tomatoes Kohlrabi, Bok Choy Peas Watermelon Lettuce Peppers Onions, Leeks, Chives Rutabagas Potatoes Squash, summer Radishes Turnips Spinach Strawberries

Root Growth Pattern

For most vegetables, the bulk of the root mass is within the top six inches of soil. The soil should be light and well aerated to enable roots to access available nutrients. Deeper soil provides additional nutrients and trace minerals, which further facilitate plant growth. Plants will send some roots deeper if the soil conditions permit. When preparing soil for raised beds, ‘double-digging’ the soil will aerate this deeper soil and clear it of rocks and debris.