Raised bed gardens

Raised bed gardening is a simple technique that can improve the health and productivity of your garden.

Reasons for growing in a raised bed

• Access: Raised beds enable you to have a garden in areas where a traditional garden might not be feasible (such as a patio, a very wet area, or in some homeowners’ associations that prohibit in-ground gardens).
• Accessibility: Raised beds bring the garden to a more comfortable working level.
• Contaminants: If the soil is contaminated with lead or other heavy metals, growing in raised beds can help reduce the risk. For more information, watch the Extension video about lead in gardens.
• Compaction: Soils that have been heavily disturbed, such as urban soils or those in housing developments, can become compacted, making gardening challenging. Raised beds may mitigate the challenges posed by growing in compact soils.
• Weeds: Stubborn perennial weeds can be less of a problem in raised beds than in other gardens.
• Planting earlier: Soil in raised beds warms more quickly in the spring.
• Aesthetics: Raised beds often look nice in a yard or garden landscape.

Like any garden, raised beds should be placed in an area with access to water and that gets the proper amount of light for the plants you want to grow.

• For plants like tomatoes, peppers, and eggplants that require full sun, locate your raised bed in an area that receives at least 6 to 8 hours of sunlight each day.
• If your raised bed receives less than a few hours of sunlight (part to full shade), you can still grow plants such as lettuce, baby kale, scallions, microgreens, radishes, and beet greens.

Containerized raised beds

When people think of raised beds, they typically imagine a containerized raised bed with sides made from wood, metal, or other materials, such as bricks, cinder blocks, plastic, or fabric.

Pros of gardening in containerized raised beds

These types of raised beds offer protection to plants in high-traffic areas and can improve physical accessibility in the garden. They also provide an easier space for growing plants in areas with heavily compacted or limited topsoil. 

Cons of gardening in containerized raised beds

Raised beds act as containers, and the soil dries out more quickly than soil in the rest of your yard or garden. The taller the raised bed, the more frequently you need to water. 

Choosing the right size for your raised beds

Height

For many gardeners, raising beds more than a few inches above the soil may not be necessary.

• If your raised bed is in a very hot area, such as on top of pavement, raising it by one or two feet can help to reduce heat.
• For wheelchair users, 27 inches is generally a comfortable height.
• If children will be using the garden, raised beds with lower sides may be more accessible.

Length and width

The reach of your arm is generally a good metric for determining the width of your raised bed. You should be able to comfortably reach the center of the bed to avoid needing to walk into the bed for garden maintenance, which can cause compaction. 

If you use boards for the sides of your raised bed, shorter boards (6 feet or less) are less likely to warp or break than longer boards.

Building multiple smaller beds vs. one long bed can also help with crop rotation, which reduces plant disease pressure. Turn boards “heartwood in” so that if they warp, they will curve slightly outward at the middle.

Choosing materials for your raised beds

Wood

Wood is the most common material for raised beds. It is relatively low-cost and easy to find. Hardwoods like cedar or hemlock are more expensive than softwoods like pine and fir, but they last much longer. 

There are a variety of chemicals and processes used for pressure treating wood, some of which pose greater risks than others. Decades ago, arsenic was used to treat wood. Avoid using old lumber in your garden to reduce risk.

Railroad ties and lumber treated with copper, chromium and arsenic (CCA) are also harmful to vegetable crops because some of the arsenic may leach out of the wood and into the plants. Newer wood treatment processes use copper-containing wood preservatives rather than arsenic.

If you plan to use pressure-treated wood in your garden for any purpose, it should be labeled as intended for ground contact. Consider mitigating the risk of copper leaching into soil by lining the edges of beds.

Read more about the risks of treated wood in the garden.

Metal

It is increasingly common to see raised beds with metal sides or animal feeding troughs used as garden beds. These materials are generally more expensive than wood but can also last longer. 

Galvanized steel is a popular alternative to traditional wood beds. It is virtually rust-resistant for upwards of 50 years and typically comes in panel sizes of 24 inches wide, providing a nice depth for raised bed gardens.

While it is possible that zinc from the coating on the inside of this type of container may leach into the bed in highly acidic soils (pH less than 5), most Minnesota soils are neutral to alkaline and won’t allow the degradation of the zinc coating. If zinc is a concern, a liner may be added, or you could switch to stainless steel.

Wood corners are typically used to easily connect the panels when constructing beds.

Take caution when purchasing metal raised-bed materials; some prefabricated beds sold online have been found to contain lead.

Bricks or pavers

Landscaping pavers or bricks may be more expensive than wood or steel, but some gardeners like to use them to match the overall aesthetic of their landscapes. Depending on the material, bricks can raise the pH of soil.

Fabric

Fabric grow bags can be as large as 4 feet across in diameter and resemble small raised beds.

These bags are sold at garden centers and are often included in community “garden in a box” initiatives due to their low cost.

The bag fabric is typically engineered from plastic. Limited research exists about the potential for microplastics to leach into the soil from the bags.

In general, grow bags should be managed more like containers than like raised beds. Read more about container gardening.

Filling your raised beds: soil, compost, and other materials

The ideal soil mixture for a raised bed is a mixture of around 2/3 to 1/2 topsoil and 1/2 to 1/3 plant-based compost. If the topsoil contains a lot of clay, you may also add some sand to create a more loamy soil texture.

High-quality topsoil can be purchased in bulk from most garden centers. Buying in bulk by the cubic yard is generally much cheaper than buying individual bags of soil.

Ideally, look for loam or sandy loam topsoil and avoid very sandy or clay soil. If you prefer to purchase bags of soil, look for soil mixtures labeled “raised bed mix” or something similar.

Too much compost creates issues with water retention: compost dries out quickly and can repel water if it becomes too dry, but a little bit of compost helps to aerate the soil and provides organic matter.

While potting soil is suitable for pots and seedlings, it will not work well in raised beds because it dries out too quickly. Manure and composted manure contribute high concentrations of phosphorus to the soil and should be used sparingly, only when a soil test indicates a need for more phosphorus.

Alternatives to topsoil and compost

Some gardeners seek alternatives to topsoil when growing in very tall raised beds (higher than 2 feet tall), usually to reduce costs. One common approach is to use lasagna-style layering of materials like sticks, leaves, compost, and soil.

A variation of this approach is the popular hugelkultur method. As these materials break down, you will need to continue to add soil to the raised bed.

Some sources recommend peat or coconut coir. Like potting soil, these materials may dry the soil and are better suited to pots than raised beds. Peat also has significant environmental concerns.

Barriers between soil and raised beds

In most cases, you will not need a barrier between your raised bed soil and the soil underneath. In fact, a barrier is likely to stunt root growth.

Instead, focus on killing the grass or other vegetation in the area where you want to put your raised bed.

• Aerate the soil with a tool like a broadfork or using a shovel to turn over the top layer of soil.
• Then place the raised bed and the new soil on top.
• If you use a method like solarization to kill the grass, remove the tarp or cardboard before placing your raised bed on top. Materials like cardboard can prevent water from infiltrating until they begin to break down.

Liners

Some gardeners prefer to use a liner material between the sides of their raised beds and the soil. Depending on the liner, this can prolong the lifespan of the raised bed materials (such as wooden sides) and may help prevent the bed material from leaching into the soil.

Place a barrier of hardware cloth at the bottom of your raised bed to reduce the chances of mice and chipmunks tunneling up into it.

When growing a raised bed on top of a non-soil surface such as a patio, using a liner along the bottom of the raised bed may also help keep soil in the bed. Choose a material that allows water to pass through, such as landscape fabric, rather than nonpermeable plastic.

Raised ground beds

The simplest form of raised beds is flat-topped mounds, usually six to eight inches high. They require no materials other than additional soil.

First, decide on the size of your raised ground bed. If you can reach only one side of the bed, the maximum width should be 2.5 feet. If you can access both sides, the bed can be up to five feet wide. The length and shape are entirely up to you.

There are two main approaches to forming raised ground beds: 

• You can bring in additional soil to form new mounded beds.
  • If you bring in additional soil, be sure that it does not come from an area with soil-borne plant pathogens or contaminants like lead and pesticides.
  • You may also use compost to form these beds. Till the compost into the soil below rather than leaving it on the surface. This helps to prevent the compost layer from drying out.
• You can dig soil from pathways and add it to your beds. Renting equipment like a potato hiller or a cultivator that can throw soil can help with this process. 

Usually, raised ground beds are treated as permanent beds in a garden rather than tilling the garden and remaking the beds every year. Avoid stepping in the raised bed as this will compact the soil. Use a hoe to reach weeds in the middle of the bed. Lean on the hoe to harvest from the center of the garden.

Try to keep the sides of the mounds intact so your raised bed does not slump out onto the pathways. As the season progresses, the soil will settle, but the mound will remain.

Maintaining your raised bed garden

Fertility and soil health

Once your raised beds are established, you can treat them much like the rest of your garden.

• Collect a soil test every 2 to 3 years.
• Apply compost, fertilizers, and other inputs as needed based on soil test results.
• Review Extension’s videos about taking a soil sample and interpreting your soil test.

Keep in mind that regular, heavy applications of compost can build up phosphorus and soluble salts in the soil, which can burn the roots of small seedlings. Composted manure tends to be higher in phosphorus than plant-based composts.

After establishing, raised beds should require minimal or no tillage. Raised ground beds need only minor reshaping with a rake at the start of each season.

Soil health practices like cover crops can be used in raised beds. Watch a video about cover crops in small gardens for more information.

Watering

Raised beds dry out more quickly than in-ground gardens and need to be watered more frequently. This can be done with a basic soaker hose or watering can. Try to water as close to the soil as possible to prevent water from hitting or splashing onto plant leaves, which helps prevent plant diseases.

For gardeners who want to save time with an automated system, some irrigation companies sell kits that allow gardeners to install drip irrigation in raised beds using a series of risers and connector pieces to bring the hoses up over the walls of the raised beds.

These kits work best when raised beds have low walls, as it can be difficult to push water up to a tall raised bed with the water pressure from a standard home faucet. The kits are also fairly expensive, but they can save significant time for people with large gardens.

A more low-tech option for keeping raised beds watered, especially if you plan to be away from your garden for a few days, is to use “ollas,” ceramic pots that are buried in the soil. These pots slowly release water as the soil around them dries.

Read more about watering the vegetable garden.

Diseases, weeds, and insects

Raised beds can be a great management strategy to manage diseases, weeds, and insects in a small gardening space. 

By building multiple, smaller raised beds rather than one large bed, you can create distinct gardening areas to allow for crop rotation. This will mostly help with disease management, but may also help with some insects that overwinter in the soil.

As a general rule, try to avoid planting plants from the same family in a bed for 3 to 4 years. So, if you grow tomatoes in one raised bed this year, do not plant anything from the tomato (solanaceous) family in that bed for 3 or 4 years. This includes peppers, eggplants, potatoes, tomatillos, and managu.

Raised beds also allow gardeners to easily add netting or fencing to keep out pests like rabbits.

Extending the growing season

Raised beds, especially containerized raised beds, allow gardeners to easily add hoops over the top with spun-bound row cover or plastic to extend the growing season.

Some gardeners also build cold frames over their raised beds using wood and polycarbonate panels. This creates a greenhouse effect, which traps extra heat in the raised bed, allowing you to grow cool-season frost-tolerant crops like spinach and lettuce earlier in the spring and later in the fall.

Learn more about extending the growing season in gardens.