A raised bed can rise to meet some common vegetable gardening challenges: infertile garden soil, weeds and poor drainage. Although a well-designed, properly installed and maintained raised bed benefits all garden vegetables, some vegetable plants are particularly responsive to being elevated above the natural grade of garden soil.
Annual vegetables grown for their roots, such as carrots (Daucus carota var. sativus), turnips (Brassica rapa) and beets (Beta vulgaris), prosper in the loose soil a raised bed offers. If you grow these root crops directly in compacted garden soil, the roots may be stunted or deformed.
Prepare raised beds to a depth of 12 inches to accommodate the long taproots of some carrot varieties.
Plants With Shallow Roots
Cole crops, such as broccoli, also known as Italian broccoli (Brassica oleracea), Brussels sprouts (Brassica oleracea) and spinach (Spinacia oleracea), are notoriously shallow-rooted. Beans (Phaseolus vulgaris) also have shallow and weak root systems. Whether you remove the weeds around shallow-rooted plants by hand-pulling or cultivating with a hoe or tiller, you may damage their roots, which can compromise plant health and harvests. Because raised beds allow these vegetables to be grown above ground level, it's harder for weeds to encroach.
Plants That Like Warm Soil
Some warm-season vegetables simply cannot be coaxed to grow if the soil is too cool. So if you like to come out of the gardening starting gate a bit earlier than is suitable for these plants -- or if you live in a short-season climate -- a raised bed may be the solution. The soil in a raised bed warms earlier in spring than the soil at ground level.
Use a soil thermometer to eliminate the guesswork of a raised bed's soil temperature when planting these and other warm-season crops:
- Tomato (Lycopersicon esculentum) is the quintessential warm-season vegetable. Although it's technically a perennial in U.S. Department of Agriculture plant hardiness zones 10 through 11, it's typically grown as an annual. A soil temperature below 50 degrees Fahrenheit can stunt this plant's growth, but it prospers when the soil temperature is at least 60 F.
- Corn (Zea mays) needs a soil temperature of at least 60 F to germinate and grow. Plant corn in blocks within raised beds instead of long rows for better pollination.
- Okra seedlings (Abelmoschus esculentus) don't transplant well because of their fragile roots, so this vegetable is best started by sowing seeds directly into the garden. But you'll need to wait until the soil temperature has warmed up to a toasty 75 F before the seeds germinate.
As a long-lived perennial vegetable in USDA zones 4 through 8, asparagus (Asparagus officinalis) may produce edible spears for up to 15 years. Because asparagus crowns rot in soggy soil, planting them in a raised bed offers sharper drainage than planting at ground level.