Roots provide plants with water and nutrients taken up from the soil, physically anchor plants in place, and release substances that attract beneficial microorganisms to help the plants prosper. Beans, peas, and root crops like potatoes use their roots as food storage facilities, but no two species have exactly the same type or extent of roots. Small plants like chives and radishes have shallow root systems, which makes them suitable for troughs and containers, which would never work with deeply rooted pumpkins. Indeed, the rooting depths of vegetable crops have important implications for site selection, soil preparation, spacing, and fertilization practices.
Here I have sorted popular vegetables into groups based on rooting depth, and then paired them with gardening methods that can help roots and gardeners do their work more efficiently.
Basil, chives, cilantro, radicchio, lettuce, parsley, radish, scallions, spinach, strawberries, most culinary herbs
The roots of this group of veggies reach only 12 to 18 inches (30 to 46 cm) deep when grown in open soil, and most will accept much shallower conditions when their needs for sun, food, water and spacing are met. Containers or small beds often work well provided plants are properly spaced.
Shallow-rooted vegetables are usually fast growers that don’t ask for much in terms of vertical root space, but they are sensitive to crowding. Thin as needed to make sure plants have ample elbow room. When preparing to plant, simply mix compost and plant food into the top few inches of soil. Deep digging is not necessary for shallow-rooted vegetables.
Cabbage, carrots, chard, corn, okra, peas, beans, beets, broccoli, collards, cauliflower, cucumber, eggplant, kale, leek, kohlrabi, peppers, potatoes, rutabaga, compact tomato, sweet potato
Much depends on crop and climate, but most vegetables in this group root to around 18 inches (46 cm) deep. These vegetables also have wide roots, which spiral out from the central root mass to form a circular mat 4 to 14 inches (10 to 35 cm) below the surface. Variety makes a huge difference here. Petite varieties of tomato, cucumber, or chard bred for containers have smaller root systems compared to their full-size counterparts.
Medium-rooted vegetables tend to be heavy feeders that grow best in rich, fertile soil. In addition to enriching soil with compost, plan ahead to mix a balanced organic fertilizer into the soil a week or two before planting.
Vegetables that mature in midsummer benefit from mulch to help keep the soil surface cool. When supplemental water is needed, choose deep soakings over surface waterings for these crops.
Artichokes, melons, lima beans, parsnips, pumpkins, winter squash, large indeterminate tomatoes, watermelon
The longer a vegetable takes to mature, the more extensive its root system is likely to be. These and other deep-rooted vegetables need at least 24 inches (61 cm) of root space, so they often run into trouble when grown in shallow beds or containers. In addition to developing long, ropy roots that need room to roam, the most remote roots of these crops work like subterranean air conditioners in hot weather. When temperatures are scorching at the surface, watermelons and pumpkins use 6-foot (2 m) deep roots as built-in geothermal systems to help them keep cool.
Rain and gravity move nitrogen left from previous crops into the subsoil, where they can provide supplemental nutrition for deep-rooted crops. When possible, plant deep rooters where the subsoil is likely to contain leftover nitrogen, for example after sweet corn or cauliflower. Use a broadfork or digging fork to mix fertilizer as deeply as possible, and don’t skimp on compost and other amendments for these long-season crops.
For this vegetable group, the double digging or French Intensive method may be worthwhile in sites with compacted subsoil. The topsoil is dug out and the subsoil enriched with organic matter, which is a lot of work but gives lasting results. Deep-rooted vegetables also benefit greatly from nitrogen-fixing cover crops, which extend their roots far below the surface, leaving nutrients where deep-rooted crops can find them.