What if your back yard has an ideal spot for growing vegetables—open space, sunlight, protection from excessive winds, and a source of water—but the soil isnʻt ideal, or maybe the ground is covered by concrete or another hardscape?
Creating a raised bed over the surface is a great solution. In comparison with in-ground planting and pots, beds can be the best of both worlds. You can fill the bed with the specific soil of your choice, which might have better consistency and fewer weeds than the existing soil in your back yard. The high walls help deter outside grasses from creeping in. Some people even build their beds higher up on legs so they don’t have to bend or squat.
Materials
The options for constructing a raised bed are limited only by your imagination. Think outside the box! Find and reuse items around your yard. Iʻve used banana stumps, which aren’t very long lasting, but have an attractive tropical look—and finding a use for extra banana stems is a good way to practice sustainability.
You can also purchase boards, bricks, and many other materials. Personally, I find it easiest to use 2”x6” borate-treated lumber, which can be cut to the desired length at the store. Many people make 4’ wide beds, but I like 3’ because it's easier for me to reach the middle. If you have access from only one side, you might consider making them only 2’ wide.
The deeper your bed, the better, so the roots can have space. For most vegetables, 6” clearance over existing soil or 12” on top of concrete or hardscape is adequate. You’ll need more depth for daikon, carrots, gobo, and other vegetables with long roots.
A note on safety: Borate-treated lumber is considered safe for use in the garden. But please be careful of older materials, which may have been treated with chromium, arsenic, creosote, lead paint, or other toxic contaminants. Or you could go with untreated lumber, which will still last for some time but may host termites.
Irrigation
Raised beds may require less frequent watering than containerized plants, since there’s more soil volume. They also provide a structure where you can conveniently set up a simple irrigation system.
You can use an irrigation timer to reduce the amount of time you spend watering. An easy way is to add a hose-end timer connected to your hose bib, then with compression fittings connect to ½” flexible black plastic tubing. You then add spray or drip emitters, or drip tubing off the ½” line, with ¼” tubing to connect them all. As I write this, City Mill has all of those necessary supplies. A timer is especially helpful as we get into summer, since some plants (such as kale) might like a twice-daily watering to deal with the heat.
Soil
For raised beds, I prefer a mixture of clay topsoil and compost. Both are local products. Some gardeners have a prejudice against using clay, but it has excellent moisture- and nutrient-retaining qualities. Compost helps to improve drainage, aeration, and the physical qualities of the soil, while also improving its biological and chemical properties.
Topsoil and locally-produced compost can be purchased in bags from a garden shop. If you want greater quantities at excellent prices, try going directly to producers such as Hawaiian Earth Recycling or Island Topsoil. Look for a garden blend, which may be roughly 40% topsoil and 60% compost. This mix is ready to plant in. Over the months, the soil blend will shrink as the compost decomposes, so youʻll periodically need to dig in more compost (I like to do this before new plantings).
Of course, you could fill the raised bed entirely with potting mix instead, but I feel that if you’re going to use that many cubic yards’ worth, it’s more sustainable to use locally sourced clay topsoil and compost.
To add nutrients, I prefer to add a bag of composted chicken manure, up to 5% of the raised bed’s total volume. If you don’t want to deal with manure, simply add your preferred fertilizer, and you’re ready to plant.
Protecting our environment
When growing over a hardscape, make sure the drainage is directed straight toward your yard’s existing soil and landscape plants, rather than off your property and into the storm drain. Water leaching from your soil will carry plant nutrients like nitrogen and phosphorus, which pollute streams and the ocean by encouraging algae growth.
Happy gardening! I’m sure you’ll have some successes and failures, but learn from them and don’t give up.
Kalani Matsumura, Cooperative Extension Service and Master Gardener Program, UH College of Tropical Agriculture and Human Resources