How to Garden in Raised Beds
Gardeners who are cursed with poor garden soil will find the raised-bed gardening method to be their salvation. The basic idea is that instead of battling against poor soil conditions, you build above ground, where you have absolute control over the soil texture and ingredients. In a region with extreme soil (very sandy, very clayey, or very infertile), gardeners can tailor the soil in a raised bed, so it is perfect for growing whatever plants they choose.
There are other notable advantages to raised bed gardening. They are excellent for older gardeners or those with physical limitations, for whom tending plants at ground level can be painful or impractical. A garden bed raised even nine to 12 inches above the ground can be much easier to tend, and you can even build elevated, waist-level raised beds that make it possible to garden without ever stooping over at all.
And because the soil in raised beds is exposed on four sides, it warms up earlier in the spring, making for a slightly extended growing season.
When to Garden in Raised Beds
While your actual gardening work will begin at whatever time is appropriate in your region (based on USDA hardiness zone, etc.), the preliminary work can begin months in advance, as you can spend the winter months planning, ordering seeds and other plants, and even cutting lumber in anticipation of speeding along the construction of your raised bed when the weather becomes favorable. In general, though, raised bed gardening begins in earnest about the time the last frost of winter has passed and soil temperatures have reached 60 degrees Fahrenheit or higher. For zone 3 gardeners, this may mean you don't actively begin gardening until June, while zone 9 to 10 gardeners may be able to garden year-round.
In a larger sense, the time to begin raised bed gardening is whenever the advantages of a more accessible garden bed, filled with good-quality, tailor-made soil, become evident to you.
Before Getting Started
A raised bed garden can be best understood as a large, stationary container garden raised up above ground level, sometimes incorporating native soil, sometimes not. Many gardeners choose to fill the entire raised bed with same kind of soilless potting mixture used for portable containers. But if buying large quantities of potting mix (or the ingredients to make your own) is too expensive, you can also fill your raised bed with a mixture of purchased topsoil and organic amendments, such as compost, manure, or peat moss.
Most raised beds include some form of sidewalls to contain the soil. But a raised bed can also be more free-form, with soil and amendments merely piled high over existing soil to make an elevated planting berm.
Contained raised beds are great for vegetable and herb gardens, as well as flower gardens. Fruits, such as strawberries, grapes, blueberries, and raspberries, also do very well in this type of bed. They can even be used to make shrub islands. There are very few plants that won't grow better in a raised bed than they do in an ordinary garden bed.
Choosing a Garden Site
If you know that you'll be growing vegetables or herbs, or sun-loving flowers in your new garden, try to select a site that gets at least eight hours of sun per day—six hours is a bare minimum. A flat, level area is important, and you should also make sure that the area has easy access to water sources, as well as room for you to work.
Planning the Raised Bed Design
When choosing a shape and size for your raised bed, make sure that you'll be able access all parts of the garden without stepping into the bed. One of the main advantages of a raised bed is that the soil doesn't get compacted the way it does in a conventional bed, because it is designed so that you don't have to walk through it.
It is a good idea to keep the garden no more than 4 feet wide, because this way you can access the middle of the bed from either side. If you're placing your bed against a wall or fence that limits access from the back side, then your raised bed should be no more than 3 feet wide. The raised bed can be any length you choose.
Depth for your raised bed should be dictated by the quality of the underlying soil, as well as the plants you are intending to grow. At a minimum, a raised bed should be at least 6 or 8 inches deep—a depth that will support many vegetables, herbs, and bedding plants. At this shallow depth, though, you are counting on underlying soil being of sufficient quality to allow plant roots to extend down below the bed.
If you have decent subsoil (not too clayey or rocky) you can simply loosen the soil with a garden fork and build a 6- to 8-inch deep raised bed. If you have poor subsoil, then a deeper raised bed is a better idea. Many crops, such as carrots, parsnips, or potatoes will do better in beds that are at least 12 inches deep. Deeper beds are also well suited for many woody shrubs and perennials.
Choosing Building Materials
While it's possible to build a raised gardening area by simply heaping additional prepared soil onto the garden site, most raised beds are constructed with some kind of rigid walls for the frame.
You can choose from a variety of materials to construct your frame. Wood is a very popular choice, because it is inexpensive and easy to work with. One advantage of wood is that you can base your raised bed dimensions on standard lengths of framing lumber, which minimizes the amount of cutting required. (Home centers generally sell framing lumber in 8-, 10-, 12-, and 16-foot lengths.)
Concrete blocks, natural stone, or brick are also nice options. Retaining wall blocks, for example, are easy to assemble into secure raised bed walls. Stone and concrete are more expensive options, though, and moving them around requires no small amount of effort.
If you will be using your raised bed to grow edibles, it's best to avoid wood that has been pressure-treated with chemicals to protect it from insect damage. While newer forms of pressure-treated lumber no longer use arsenic, which was a known health hazard, some experts still caution against using the newer forms of copper-based pressure-treated lumber, as the chemicals can still leach out into the soil. Cedar makes a good alternative to pressure-treated lumber for outdoor beds that will grow edibles.
Some gardeners go the ultra-simple route and simply place bales of hay or straw in whatever configuration they desire, then fill it with good soil and compost and plant it up. This solution will only give you a year or so of use because the straw will decompose. But it's worth trying if you don't mind replacing the bales yearly, or if you're still developing a more permanent solution.
Our demonstration shows how to build a raised bed from standard cedar dimension lumber.
What You'll Need
Equipment / Tools
- Garden fork or tiller
- Circular saw or jig saw
- Screw gun
- 3 2 x 10 or 2 x 12 boards, 8 feet long (cedar or pressure-treated)
- 4 4 x 4 posts, 18 inches long
- Corrosion-resistant utility screws
- Organic soil additives (compost, peat moss, manure, etc.)
- Plants or seeds
Prepare the Site
Once you know the size and shape of your bed, you can get to work prepping the site. How much prep you'll require is determined by the depth of the bed you're planning, as well as by the plants you're planning to grow there.
To ensure that your plant's roots have plenty of room to grow, it is a good idea to dig out the existing sod and loosen the soil with a shovel or garden fork to a depth of 8 to 12 inches. You can even add compost or other amendments to the subsoil before the raised bed is constructed, which will make for a very deep growing medium. This can be essential if your raised bed will be growing shrubs or other deep-rooted plants, where the raised bed alone doesn't provide enough depth.
Less site preparation is needed for raised beds that will be used for herbs or other shallow-rooted plants. Here, a mere 6-inch deep raised bed placed right on the subsoil can be sufficient. To save yourself some labor, you can use newspaper, landscape fabric, or cardboard to cover and smother the grass, then put your soil and amendments right on top.
Construct the Frame
If you are building the raised bed frame from lumber, begin by cutting the frame pieces. A very easy method is to use 2 x 10 or 2 x 12 lumber to build a 4 x 8-foot frame. This design will require just three 8-foot cedar boards, plus four 18-inch-long 4 x 4 posts used to anchor the corners of the frame.
To build the frame, cut the pieces to length, so you have two 8-foot sides and two 4-foot ends, then attach them together to make a simple rectangular frame with side and end pieces secured to 4 x 4 corner posts with corrosion-resistant utility screws. The end grain of the side pieces should be covered by the end pieces, and the top of the frame should be flush with the tops of the posts. (The posts will be partly embedded in the subsoil to give the frame stability.)
If you wish, the corner joints of the frame can be reinforced with metal L-brackets or some other hardware for extra strength.
Anchor the Frame
Position the frame on the garden site, digging down where necessary to embed the corner posts in the subsoil. Use a carpenter's level to adjust the frame, so it is level side to side and back to front. If necessary, you can remove additional soil or shim under the frame to hold it in a level position as you fill the frame with soil.
Create the Soil Mix
The whole point of a raised bed garden is that it gives you the opportunity to garden in perfect soil. While it is possible to simply have a load of local topsoil delivered to fill your raised bed, most gardeners will have better results by creating a tailored soil mix. You can also fill the bed with large bags of ordinary commercial potting mix, though this can be expensive
One common DIY "recipe" is to mix equal parts quality topsoil, compost, and well-decomposed manure. Some gardeners also like to blend in a slow-release fertilizer with the other ingredients. If you will be growing plants that have specific soil needs, you can tailor the mix to their needs. For example, an azalea shrub island requires a more acidic soil, while a raised bed used to grow ornamental succulents may call for a sandier, more porous soil mix.
Another home recipe is to create a "soilless" medium similar to that used in ordinary potting soils. This mixture is comprised of equal parts peat moss, compost, and vermiculite. This type of mix, recommended for square foot gardening beds, will gradually decompose, however, and will require yearly replenishment.
Whatever soil recipe you choose, it's easiest to thoroughly blend the ingredients on a large tarp or on a concrete slab before shoveling the mix into the raised bed. When the raised bed is filled, smooth the soil out with a rake.
Plant the Garden
Raised beds are equally well suited for direct-sowing seeds and growing just about any potted nursery starts you want. Remember that the soil in raised beds warms up faster in the spring, but will also cool off faster at night, so this may affect when you plant in the spring. Many people choose to plant a raised bed earlier in the spring, but guard against nighttime chills by covering it with a tent of plastic at night. Covering the bed can also extend the growing season in the fall, when nights begin to get chilly.
The soil around the edges of a raised bed tend to get hotter during the day and colder at night than in the center of the bed, so this should be kept in mind when choosing the location of plants within the raised bed. Temperature-sensitive plants may not do well around the perimeter of a raised bed.
Tend the Plants
Caring for plants in a raised bed can be slightly different. Soil warms up, cools off, and dries out faster, because all four sides are exposed. This effect will be more noticeable with smaller raised beds. You may need to water slightly more often than with traditional in-ground gardens. And with more frequent watering comes more frequent feeding for certain plants.
Other than this, care for a raised bed garden is usually easier than for in-ground gardens. The hand-mixed soil is usually free of weed seeds and pathogens that are plentiful in traditional gardens, and those weeds that do appear are easier to remove when there is good access around all sides. As with any garden, mulching the top of the soil will help retain moisture and keep weeds down. Moisture retention is important because raised beds tend to drain faster than conventional beds.
Each spring or fall, it's a good idea to top dress with fresh compost and manure, or, if your bed holds plants for only part of the year, go ahead and dig the compost or manure into the top several inches of soil before planting in the spring.
At the end of the growing season as the weather grows cold, remove annuals and cut back perennials to ground level. Heavy mulching of perennial roots can be a good idea in a raised bed, as the soil can get somewhat colder than for in-ground gardens.
Shour, Mark. Toxicity Concerns about Raised Bed Construction Materials. Iowa State University Extension and