What You Need Before You Start
A raised flower bed is a wooden or composite box that sits on top of your ground and holds soil for plants. You build it by assembling four sides, placing them where you want the bed, and filling it with soil. The whole project takes two to four hours depending on the size and whether you use pre-cut boards or cut your own.
The most common approach is to buy untreated cedar or composite boards, cut them to length if needed, and join them at the corners with screws or nails. Cedar resists rot naturally and lasts eight to twelve years outdoors. Composite boards cost more but last twenty years or longer. Pressure-treated lumber is cheaper but contains chemicals that leach into soil over time, so most gardeners avoid it for vegetable beds.
You will also need a drill or screwdriver, a saw (if cutting boards yourself), a level, landscape fabric, and soil. A tape measure, pencil, and square help you get corners true. If you are building anything taller than two feet, you may want to add internal bracing to prevent the sides from bowing outward under the weight of wet soil.
Key Takeaways
- Cedar and composite boards are the best materials because they resist rot; pressure-treated lumber leaches chemicals into soil.
- Assemble the four sides with screws or nails at the corners, then place the frame on level ground and check that all corners are square.
- Line the bottom with landscape fabric to block weeds, then fill with soil to within two inches of the top.
- Beds wider than four feet are harder to reach into, so plan your width based on how far you can comfortably lean across.
- A bed two feet by four feet by twelve inches deep holds enough soil for most flower gardens and costs between forty and eighty dollars in materials.
Choosing Board Size and Bed Dimensions
Standard boards come in widths of six, eight, ten, and twelve inches. A six-inch board gives you shallow depth, good for herbs or shallow-rooted flowers. Eight inches is the most common choice for general flower beds. Ten and twelve inches work for deeper-rooted plants like peonies or roses, and they also look more substantial visually.
Length and width depend on your space and how you plan to use the bed. A bed four feet by eight feet is large enough to grow a variety of plants but still lets you reach the center from either side. A bed two feet by six feet fits against a fence or wall. Avoid making beds wider than four feet if you need to reach the center from only one side, because you will strain your back leaning across.
Depth matters for what you can grow. Flowers and herbs need eight to twelve inches. Vegetables need twelve to eighteen inches. If you are building on top of lawn, you can go shallower because the existing soil underneath adds depth, but you will get better results if you remove the grass first and add landscape fabric to block weeds from growing up through the bottom.
Cutting and Assembling the Frame
If you buy boards pre-cut to length, you skip this step. If not, measure twice and cut once. A four-by-eight-foot bed needs two boards eight feet long and two boards four feet long. Use a circular saw or hand saw to cut straight lines. Sand the cut ends lightly so they do not splinter.
Lay the four boards on a flat surface and arrange them into a rectangle. Drill pilot holes at each corner to prevent the wood from splitting, then drive three-inch exterior screws through the end of one board into the end of the next. Use at least two screws per corner, one near the top and one near the bottom. If you prefer nails, use galvanized or stainless steel to prevent rust.
Flip the frame over and check that all four corners are square by measuring diagonally from corner to corner. Both diagonal measurements should be equal. If they are not, push the frame gently until they match. This step takes a minute and prevents the frame from looking crooked once it is filled with soil.
Positioning and Leveling Your Bed
Choose a spot that gets the light your plants need. Most flowers want six to eight hours of direct sun. Move the frame to that location and set it down. Do not worry if the ground underneath is not perfectly level—you will adjust the frame itself.
Place a level across the top of the frame in both directions (lengthwise and widthwise). If one side is higher than the other, dig out soil from the high side and add soil to the low side until the frame sits level. This prevents water from pooling in one corner and keeps soil from settling unevenly as it settles over the first season.
If your ground is very uneven, you can dig out the entire footprint to a depth of two to three inches and level it before placing the frame. This takes longer but gives you a more stable base. For most home gardens, straightforward leveling the frame itself is enough.
Lining the Bottom and Adding Soil
Lay landscape fabric across the bottom of the frame. This blocks weeds from growing up through the bottom while still letting water drain. Overlap the edges by six inches and staple or pin the fabric to the inside of the frame so it does not shift when you add soil.
If you are building on top of lawn and want extra weed protection, you can lay cardboard under the landscape fabric. Cardboard breaks down over two to three years and adds organic matter to the soil as it decays. Avoid glossy cardboard from appliance boxes; use plain brown cardboard instead.
Fill the frame with soil to within two inches of the top. This leaves room for mulch and prevents soil from washing out when you water. Use a mix of topsoil and compost—roughly half and half—rather than pure topsoil, because compost improves drainage and adds nutrients. A four-by-eight-foot bed that is twelve inches deep needs about thirty-two cubic feet of soil, which is roughly one cubic yard.
Reinforcing Tall or Large Beds
If your bed is taller than eighteen inches or wider than four feet, the weight of wet soil pushes outward on the sides. Add internal bracing to prevent the boards from bowing. The simplest method is to install a crossbar down the middle of the bed, running lengthwise, and support it with vertical posts at each end.
Cut a two-by-four board to run the length of the bed and attach it to the inside of the two long sides with L-brackets. This creates a spine that holds the sides in place. For very large beds, add a second crossbar perpendicular to the first, creating a grid. This is overkill for most home gardens but necessary if you are building something four feet wide or deeper than twenty inches.
Another option is to use composite boards instead of wood. Composite does not rot and is stiffer than wood of the same thickness, so it resists bowing better. The trade-off is cost—composite boards run two to three times the price of cedar.
Finishing Touches and Maintenance
Once the bed is filled with soil, water it thoroughly. The soil will settle over the next few weeks. You may need to add more soil after the first watering to bring it back to the two-inch margin below the rim.
Add two to three inches of mulch on top of the soil. Mulch keeps moisture in, suppresses weeds, and breaks down over time to improve soil structure. Use wood chips, shredded bark, or straw. Avoid dyed mulch, which contains chemicals, and avoid fresh wood chips, which tie up nitrogen as they decay.
Cedar boards last eight to twelve years before they start to rot at the bottom. Composite boards last twenty years or longer. When a wooden bed starts to fail, you can replace just the damaged boards rather than rebuilding the whole thing. Drain the soil into buckets, remove the old boards, install new ones, and refill.
Frequently Asked Questions
Do I need to remove the grass underneath before building?
No, but you will get better results if you do. Grass and weeds underneath will compete with your plants for water and nutrients. The easiest method is to lay cardboard over the grass, then place the frame on top. The cardboard blocks the grass and breaks down over two to three years.
What is the best wood for a raised bed?
Cedar and composite are the best choices. Cedar resists rot naturally and costs less than composite. Composite lasts longer and does not need staining or sealing. Pressure-treated lumber is cheaper but contains chemicals that leach into soil, so most gardeners avoid it for vegetable beds.
How deep does a raised bed need to be?
Eight to twelve inches works for flowers and herbs. Vegetables need twelve to eighteen inches. Deeper beds hold more soil and give roots more room to grow, but they also cost more and are heavier to move. Start with twelve inches if you are unsure.
Can I build a raised bed on a slope?
Yes, but you will need to dig out the high side and build up the low side to level the frame. This takes more work than building on flat ground. Alternatively, you can build a stepped bed where one corner is higher than the other, but this makes it harder to level the soil inside.
How much does it cost to build a raised bed?
A four-by-eight-foot bed made from cedar boards costs forty to eighty dollars in materials, depending on board thickness and local prices. Composite boards cost eighty to one hundred fifty dollars for the same size. Soil and mulch add another thirty to fifty dollars. Budget one hundred to two hundred dollars total for a standard-size bed.