August 28, 2010
There are many reasons for building raised beds in the garden. Raised beds give the garden a framework, they reduce soil compaction and they extend the growing season. Raised beds can also allow for less maintenance and easier access. All these benefits help today's busy gardener spend more quality time in the garden and see better results with less effort.
Marin's climate allows for a long, vegetable-growing season, arguably year-round for some crops, but the typical Marin gardener is short on space and time and can benefit from all the advantages that raised beds can offer. Making the investment of installing raised beds is well worth the expense.
Raised beds help organize the garden to allow for easier crop rotation programs, crop spacing, intercropping and companion planting. There are some easy rules of thumb for bed construction.
The bottom line is that they can be made in any shape or height, from a huge array of materials and in a multitude of arrangements. Using the tenets of bay-friendly gardening, one can choose first to use recycled and/or locally produced materials.
While materials on site may be the best option, consideration should be taken in terms of permanence and the effort necessary to rebuild the beds over time.
For this reason, pine and Douglas fir may not be the best option. Plywood falls apart quickly when exposed to moisture and soil. Materials less than 1-inch wide require more posts and deteriorate quicker. Treated wood contains arsenic and other poisons that should be avoided near food crops. Old lead paint is also a concern. The two options I've favored are either 2-inch rough redwood or rot-resistant composites that have become popular for decking.
Bed spacing and layout is determined by function. Sun orientation, as well as hose and wheelbarrow access, are the prime concerns. Eliminating soil compaction within a bed is achieved by designing them in such a way that there is never a reason to step into them. If they are up against a wall or fence, they should not be more than 3 feet wide. If they are out in the open, 4 feet is generally the best width. Major pathways should be 3 to 4 feet wide and should have a hose bib at one end and be on the path to a composting system.
Minor access paths should be at least 2 feet wide (30 inches). Unless beds are against a wall or fence, they should never exceed 12 feet long. If they are longer, the impatient gardener (we all have our moments!) will step through. Ideally, the beds would be arranged lengthwise east-west to take advantage of the sun.
Beds can be 6 inches to 2 feet tall. Beds 12 inches high are easy to build out of 2-by-12s and work really well. Beds 18 inches are best if you want to add a cap and use it as a seat. Two-foot-tall beds work well for wheelchair access or for those who have trouble bending over.
Avoid mitered joints as these rarely hold up over time. Posts 4-by-4 inches, extending a foot below the beds and spaced every 4 feet, will keep the beds strong for many years. Concrete is usually not necessary. Beds can also be stepped up a hillside, making great use of a slope.
Extension of the growing season is achieved with raised beds, because by their nature they raise the plants and soil up above the cold ground. This can be enhanced by the use of floating row covers, fitting the tops of the beds with cold frame lights, mulching them with black plastic or even making little temporary greenhouses with hoops and visqueen or polycarbonate greenhouse panels. Trellis components can also be added for vertical gardening to get more out of less space.
The material to use between the beds should be inexpensive, available locally and all right if it finds its way into the compost. Weed-free wood chips or rice straw works best. Filling the beds with high-quality, loamy, organic topsoil is worth the investment and will minimize weeds and other pathogens, as well as ensure high yields. Plan on regular additions of compost, meals, manures and guanos as well as regular layers of organic mulch for best results.
The best irrigation systems are automatic, incorporate 1/2-inch in-line, pressure-compensating, drip tubing and adjustable micro sprays. Even the best systems need occasional garden hose use.
By building strong, well-designed raised beds in a layout that makes functional sense, filling them with a rich loamy top soil and utilizing an automatic irrigation system, busy gardeners are freed from many routine tasks and allowed to concentrate on planting and harvesting nutritious vegetables for their friends and family for years to come.
IF YOU GO
What: "Winter Vegetable Gardens - Now is the Time"
When: 7 p.m. Sept. 2
Where: Livermore Pavilion, Marin Art & Garden Center, 30 Sir Francis Drake Blvd., Ross
Information: James Campbell, firstname.lastname@example.org; Gail Mason, email@example.com
The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 499-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato.