Raised beds and containers: growing food anywhere

Why grow up

Sometimes the ground you have isn't the ground you'd choose: compacted, contaminated, paved, or rented. A raised bed or a container gives you soil you can trust, wherever you are.

There's a safety reason too. The EPA calls raised beds filled with clean soil one of the best ways to limit exposure to lead in contaminated soil. It recommends testing your soil first, through an extension or state university lab, and placing beds away from old painted buildings and busy roads.

Size a bed

Build for your reach. University of Minnesota Extension suggests keeping a bed no more than about 2.5 feet wide if you can reach only one side, and up to 5 feet if you can reach both. About 27 inches tall is comfortable for gardeners using a wheelchair.

Depth depends on the crop. University of Maryland Extension notes that beds on hard surfaces need at least 8 inches of soil for leafy greens, beans, and cucumbers, and 12 to 24 inches for peppers, tomatoes, and squash.

What to fill it with

University of Minnesota Extension suggests a mix of roughly half to two-thirds topsoil with a third to a half compost, and warns that bagged potting soil dries out too fast for raised beds. For beds on pavement, University of Maryland Extension suggests compost and soilless mix in equal parts. Raised beds dry out faster than the ground, so plan to water more often.

Choose safe materials

Stone, block, and untreated rot-resistant wood are all good alternatives.

• Rot-resistant wood: cedar and hemlock cost more than pine or fir but last much longer, University of Minnesota Extension notes. Avoid old lumber and railroad ties, since older wood treatments used arsenic.
• Modern treated lumber: University of Maryland Extension notes that arsenic-based CCA treated wood was discontinued for home use in 2004, and that today's common copper-based treated lumber is considered low in toxicity.
• If you use treated wood: Oregon State University found that copper from copper azole treated beds rose only within about an inch of the wood and that vegetables showed no increase. It advises using lumber rated for ground contact and avoiding wood more than 20 years old.
• Metal kits: University of Minnesota Extension warns that some prefabricated metal bed kits sold online have been found to contain lead.

If your soil has lead

The EPA's gardening guidance advises caution as soil lead approaches 200 parts per million. Above 400, it recommends not using the existing soil for food and growing instead in at least 16 inches of clean soil, set on a visible barrier such as landscape fabric, or in containers. Keep the soil pH near neutral, peel root crops, and discard the outer leaves of greens.

Container sizes

University of Maryland Extension gives minimum container sizes for common vegetables:

• Half a gallon: leaf lettuce and beets.
• 2 gallons: carrots, Swiss chard, cherry tomatoes, and bush beans.
• 5 gallons: standard tomatoes, peppers, eggplant, cucumbers, and cabbage.
• 10 gallons: summer squash.
• Herbs: 2 to 5 gallons for small herbs, 5 or more for rosemary, sage, and dill.

Container care

Every container needs drainage holes, and University of Maryland Extension notes that a layer of gravel in the bottom won't improve drainage. Use a soilless potting mix, not garden soil, which can weigh around 75 pounds per cubic foot against about 10 for potting mix.

Containers dry out fast. The same guide says to check them daily in summer rather than watering on a schedule, and suggests a balanced fertilizer, with a light feeding every couple of weeks for long-season crops.