Building a raised bed is a great fall project. We’ll help you get started with our step-by-step guide on how to build a simple raised bed from scratch. No special DIY skills are required! Find out what kind of wood or material to use, and how large a raised bed should be.
What Is a Raised Garden Bed?
But let’s start from the beginning and a definition. When we say “raised garden bed” or simply “raised bed,” we’re referring to a freestanding box or frame—traditionally with no bottom or top—that sits aboveground in a sunny spot and is filled with good-quality soil. Raised beds are usually open on the bottom so that the plant roots can access soil nutrients below ground level.
Of course, a raised bed can be even more simple than that: you could build a raised bed without a frame, and simply mound the soil 6 to 8 inches high and flatten the top. This requires no additional materials (beyond soil).
Benefits of Raised Garden Beds
- They drain well and help prevent erosion.
- They warm up early in the spring and give you a longer growing season since the soil raised above the ground warms up more quickly.
- Raised beds give you control over the soil you put in them making it possible to plant intensively; plants grown close together in raised beds mature faster.
- They keep weeds from taking over because the beds are elevated away from surrounding weeds and filled with the disease- and weed-free soil.
- Since you are not walking in the bed, the soil doesn’t get compacted and stays loose without the need for backbreaking digging every spring. Raised beds help to keep things organized and in check.
- Garden chores are made easier and more comfortable thanks to less bending and kneeling. Save your knees and back from the strain and pain of tending the garden!
- Raised beds are ideal for small spaces where a conventional row garden might be too wild and unwieldy.
- It is easier to separate and rotate crops each year.
- Raised beds allow for easier square-foot gardening and companion planting.
- And, finally, raised beds are attractive!
Choosing the Right Material for Raised Beds
- Untreated wood Pine is the cheapest but is going to rot after a few years, as will many untreated kinds of wood. Hemlock will last a little longer. Rot-resistant woods like cedar, redwood, or locust will last much longer; they are expensive. Cedar is the top choice because it’s both rot-resistant and durable, lasting 10 to 15 years. It is also insect-resistant because of the oils in the wood. Recycled wood made from plastic bottles is also a bit pricey but will last indefinitely. Another option is to simply choose much thicker boards of untreated wood. For example, a 2-inch-thick board of larch wood should last a decade without treatment.
- Modern treated wood has chemicals to prevent rotting. Unlike in the past, however, studies have shown that any compounds that leach out are well within safe levels established by the EPA. Some gardeners still feel uncomfortable with treated wood. One option for those concerned is to line the inside of the bed walls with polyethylene.
- Railroad ties (treated) are easy because you can simply lay them on the ground and drive in iron spikes. Old railroad ties treated with creosote do not appear to pose any health problems because most of the creosote has leached away.
- Pallets can be a cheap source of garden bed materials, as long as you know where they came from. Pallets are developed for shipping materials. Avoid pallets that are also treated with a chemical called methyl bromide, a known endocrine disruptive chemical that can impact your reproductive health. Most pallet producers stopped using the chemical in 2005, but many old pallets are still out there. Look for a stamp on the pallet that says “HT” or heat treated. If there is no stamp or you can’t verify an HT on the surface, don’t use the pallet in your garden.
- Concrete or brick can be used. However, keep in mind that concrete will increase the soil pH over time, and you may have to amend the soil.
- Composite wood is a newer product made from both recycled plastic and wood fibers. It’s rot-resistant and long-lasting, but also very expensive.
- Cinder blocks: The extra gathered heat from concrete is perfect for Mediterranean-type herbs such as rosemary and lavender. Their holes can be filled with soil mixed and planted with herbs or strawberries. Each block is 16 inches long by 8 inches high; the price at big box stores is most reasonable.
- Rocks and stones are plentiful in some areas and make great free edging. You can build the bed in place around the soil mound you already have started. Once that is enclosed you can fill the sides with more soil, and add compost, shredded leaves, manure, etc. Rake the top smooth and let it sit until next spring when it will be ready for planting. The two beds below were built using Trex lumber from the “seconds” pile at a local lumber yard. It was too warped for building a deck but worked just fine for a garden bed. The bottom is lined with 1/4 inch hardware cloth screening to keep the voles from eating precious bulbs.
How Wide Should Your Raised Bed Be?
Garden beds should be no wider than 4 feet so that you can access the garden without stepping into the bed. Fortunately, lumber is often cut in 4-foot increments.
- Stepping into bed is a no-no. It compacts the soil, making it harder for plant roots to get the oxygen they need. Making the bed too wide will also make it difficult to reach the middle, which makes weeding and harvesting difficult, too.
- If your raised bed is being built against a wall or fence, we recommend making it narrower than 4 feet (2 to 3 feet wide), since you’ll only be able to access the garden from one side.
Length is not as important. You can make a bed 4×4 or 4×8 or 4×12. It can be as long as you wish but I find it easier to make several shorter beds than to have one really long one. Also, many crop families are best in separate beds.
How Deep Should a Raised Bed Be?
Usually, lumber (such as cedar) will come in a standard size that’s 6 inches in height. In other words, the sizing is 2 inches x 6 inches x 8 feet. (Note that boards bought at a lumber yard are actually 1.5″ thick x 5.5” high.)
You could also stack the two boards. The height of two stacked “2 x 6″ boards is 12 inches (technically 11 inches).
You can certainly go taller (18 inches, 24 inches, 36 inches), but note that the weight of the added soil will add pressure to the sides. You’ll need to add cross-supports to any bed over 12 inches high.
Consider what you may grow. The depth of the soil itself is very important and depends on how much soil depth the crop needs belowground. For example:
- Deep-rooted crops such as carrots, parsnips, potatoes, tomatoes, and squash need a minimum soil depth of 12 to 18 inches. If plants don’t have loose soil to this depth, the roots will not be able to go down deep enough to access nutrients.
- Shallow-rooted crops (such as lettuce, greens, and onions) need a minimum soil depth of 6 inches.
To place it safely, you could just ensure your beds have a depth of 12 to 18 inches. Whatever height you choose for your frame, you’ll need to loosen the soil below the ground accordingly. For example, if you have a bed that’s 6 inches high, we recommend loosening the soil below the ground about 6 to 9 inches if you wish to grow root vegetables. If you are only growing shallow-rooted crops, there’s no need.
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