What’s the secret to growing a healthy, vigorous plant this season?
Hint: It doesn’t start with what you see above ground.
Root depth is a topic that isn’t often considered when we think about growing in containers, building raised beds, or planning an irrigation system for our garden.
But knowing how deep the roots of your plants reach is one of the most important pieces of the puzzle, especially if you’re working with limited space.
We tend to visualize our plants growing up or out, but before we transplant that first seedling, we need to know how deep they’ll go beneath the surface as well.
Why does root depth and soil depth matter?
Most plants will grow within the space you allow them. They’ll survive with a minimum of soil depth, but they’ll thrive if you give them as much room as possible for their roots to branch out and breathe.
In fact, if you’re a container gardener, this plant study found that simply doubling the pot size allowed plants to grow 43 percent larger.
So forget those 5-gallon buckets for your indeterminate tomato vines — you want them in half-barrel planters (at least 15 gallons) or larger for a good harvest.
Just like the saying goes — Feed the soil, not the plants — you want to focus on building strong roots, not just healthy stems and leaves.
Roots that grow deep down in the soil are better able to anchor plants in the ground, maximize their water uptake, and pull in more nutrients and trace minerals.
Knowing the root depth of vegetables can help you plan your garden better
If you’re getting a garden bed ready for planting, knowing the root depth of the plants you want to grow can help you determine how extensively to prepare the soil.
For example, shallow-rooted plants like lettuce may do better in soil that’s high in clay and doesn’t drain well.
This may seem to go against common gardening wisdom, but after dealing with heavy clay soil for nearly a decade in my old garden, I’ve realized there’s a time and place for it.
Related: Lettuce… Even in Summer
Since the roots of lettuce and other leafy greens stay close to the surface, they like the extra moisture, tolerate less frequent watering, and only need nutrients in the top 2 to 3 inches of soil, reducing the amount of soil prep you have to do every season — a simple top dressing will work for these types of plants.
On the other hand, heavy-feeding, deep-rooted tomatoes require rich, loamy, well-draining soil, so they benefit from aged compost and plenty of amendments dug down at least 12 inches where the bulk of their root mass is concentrated.
Root depth can help determine the best height for raised garden beds
If you install raised garden beds over concrete or gravel, you’ll want to know the root depth of the vegetables going in, since they will guide how high you need to build the sides of your beds.
Raised beds built over grass or dirt, however, typically don’t need to be more than the standard 8 to 12 inches in height because the roots can sink into the subsoil (assuming your beds are open on the bottom).
Gardeners in dry climates can use root depth to help with hydrozoning
Root depth is also helpful for hydrozoning, especially if you live in a drought-prone area.
Hydrozoning is the practice of grouping plants with similar water needs together in order to conserve moisture and irrigate more efficiently.
This means you’ll cluster all your shallow-rooted plants in the same bed so you don’t inadvertently overwater them, and group deep-rooted plants together in another bed (on their own irrigation line) so you don’t underwater them.
Hydrozoning also comes in handy if you like to interplant your crops by growing beneficial flowers and herbs among your vegetables, or planting quick-growing crops (like radishes) between rows of long-season crops (like broccoli).
It’s not just how deep, but how wide
While we usually think of roots as growing downward, it’s important to remember that roots grow laterally as well, and to account for that in our garden beds and containers.
For example, a cucumber plant sends down a single tap root 3 to 4 feet deep. The majority of its roots, however, extend outward about 2 feet and are concentrated just below the soil surface.
You’ll also want to factor in the final height of the mature plants, as insufficient soil volume in a container can make them too top-heavy.
Always follow the spacing recommendations on your seed packets and plant tags to ensure you give your plants plenty of room to grow.
Soil depth requirements for common garden vegetables, fruits, and herbs
Some vegetables, like spinach and spring radishes, have very shallow roots and don’t need more than 4 to 6 inches of soil to grow successfully. But the smaller the planter, the more often you’ll need to water.
Just keep in mind there’s no need to plant these varieties in containers deeper than 12 inches, as you’ll only be wasting soil and space.
On the other hand, plants like lemongrass (which clump and multiply rapidly) and tomatoes (which grow adventitious roots along their stems) will grow as large as you let them, so giving their roots more room to roam will produce a more robust harvest.
Use the chart below to help you find the root depth of common vegetables and herbs, plan and prep your garden, and determine how deep your raised garden beds or planters should be.
Garden Betty’s Vegetable Root Depth Chart
(12 to 18 Inches)Medium Rooting
(18 to 24 Inches)Deep Rooting
(24 to 36+ Inches)ArugulaBeans (fava)ArtichokesBasilBeans (pole)AsparagusBlueberriesBeans (snap)Beans (lima)Bok choyBeetsBurdock root (gobo)BroccoliCantaloupesCardoonBrussels sproutsCarrotsOkraCabbageChardParsnipsCauliflowerCucumbersPumpkinsCeleriacDaikonRhubarbCeleryEggplantSquash (winter)ChivesMuskmelonsSweet potatoesCilantroPeas (shelling)TomatoesCollard greensPeas (snap)WatermelonsCornPeas (snow) EndivePeppers (hot) FennelPeppers (sweet) GarlicRosemary GingerRutabagas Jerusalem artichokesSage KaleSquash (summer) Kohlrabi Turnips Leeks Lemongrass Lettuce Mint Mustard greens Onions Oregano Parsley Potatoes Radishes (spring) Radishes (summer) Radishes (winter) Scallions Shallots Spinach Strawberries Tarragon Thyme Turmeric Download printable chart
More ways to prepare for planting this season:
This post updated from an article that originally appeared on February 27, 2019.