While researching raised garden beds for a client, I came across this helpful article comparing the materials used to construct them.
Cedar vs. Recycled Plastic vs. Composite Raised Garden Beds
by Greg Seaman
Raised garden beds have become very popular in home and commercial gardens as gardeners learn of their many advantages. But the wide selection of models can be confusing to an aspiring gardener.
We’ve used them all in our own gardens, and have years of track records to help you decide on the ideal bed for your garden.
Most raised beds available today are made of cedar, recycled plastic or a composite material using wood flour and polypropylene. Although you can fashion a raised bed out of other materials such as heavy timbers, landscape blocks or water-filled plastic, commercially available raised beds usually stick to tried and true materials, and designs which are easy for a gardener to assemble. Here below are comparisons of the three primary types of raised beds.
Cedar Raised Beds
Garden beds and planters have been traditionally made using one of several varieties of cedar. Cedar is a premium wood characterized by its natural resistance to rot and its ability to hold up well to the extremes of weather. Available in a variety of species, such as Western Red Cedar, Atlantic White Cedar, Yellow Cedar, Port Orford Cedar and Juniper, cedar is the wood of choice for patio decking, fencing, outdoor furniture and many styles of garden raised beds.
· Beautiful. Many gardeners consider the aesthetic appeal of their gardens to be as valuable as the harvested crops. Without doubt, an attractive garden feeds the soul. Wood is a natural material, and lends itself perfectly for garden beds which complement the natural beauty of the plantings.
· Weather and rot resistant. Cedar contains “extractive” chemicals, which make the wood resistant to decay. Second-growth cedar is just as rot resistant as old-growth cedar, but there should be no sapwood present in the boards, since the sapwood will rot readily. (Sapwood is the outer wood of the tree and appears milky white in contrast to the red-brown of the heartwood.)
· Easy to work. Woodworkers enjoy working with cedar because it is stable, once dry, and does not split readily at the ends. Pre-drilling is not required for simple raised bed construction.
· Relatively lightweight. Compared to other woods, cedar is relatively lightweight. This makes it easier to bring home, carry to the garden and assemble.
· Biodegradable. At the end of its lifespan, a cedar bed can be left in a low spot of your yard to slowly melt into the earth. Or the usable parts of the wood can be split into smaller pieces and used for garden stakes and trellises.
· Color turns silver-grey, unless finished. Cedar left untreated will fade in color to a silver-grey. Depending on local sunlight conditions, this usually takes 2 – 3 years. The outside of the beds can be treated with an exterior finish such as Tung Oil Finish, which will brighten and preserve the original cedar color. If you wish to apply such a finish, it is recommended to do this before the beds are assembled and crops are planted.
· Shorter lifespan vs. recycled plastic. It is difficult to predict how long a cedar raised bed will last since there are variables such as the type of cedar used, the soil conditions in your garden, and the weather patterns of different regions. In dry regions such as the Southwest, cedar will last a very long time. In the rainy Pacific Northwest, the wood may retain moisture for longer periods which eventually creates the conditions for rot and deterioration. However, there are inexpensive and non-toxic wood stabilizers such as Eco Wood Treatment which are effective at creating a moisture barrier and thus preserving the wood and increasing its lifespan. Bear in mind that this and similar treatments will also change the color of the board to a silver-grey. However, stains are available with these treatments which can impart different shades of color to the wood.
When treating cedar with a preservative such as Eco Wood Treatment, treat the wood on both sides and all edges with a liberal application. Once the bed is filled with soil it is too late to apply this treatment, since rot in cedar beds commonly begins on the inside of the boards, where the wood is in contact with the moist soil.
Recycled Plastic Raised Beds
HDPE (High-density polyethylene) plastic is the type used for most recycled plastic raised beds. This is an extremely durable and non-leaching plastic, commonly recycled from milk jugs, which is used not only for raised beds but for outdoor fixtures such as picnic tables, park benches, boardwalks, municipal waste bins and similar applications which must be durable, long-lasting and able to withstand the extremes of winter freezing and summer hot spells.
· Long lasting. Raised beds made of HDPE recycled plastic are commonly guaranteed for life. Manufacturers often cite a minimum life expectancy of 50 years.
· Durable. HDPE holds up well to use. If you ram into it with your wheelbarrow, it’s unlikely to result in any damage. Recycled HDPE plastic is resistant to cracking or chipping, even in extreme weather, hot or cold.
· Stable. Does not leach. Because HDPE is a stable material it does not leach any chemicals, toxic or otherwise, into the soil within the garden bed. Also, the ‘boards’ do not shrink, twist or warp over time. HDPE does not expand or contract during periods of freezing or extreme heat.
· Available in different colors. Dyes are added to the molten HDPE to provide several color choices for the recycled plastic boards.
· Smooth finish, retains color. The appearance of recycled plastic garden beds remains consistent even after years of exposure to weather. And because the color is added before the molten plastic is poured into its mold, the color runs through the boards, so if you should scratch the sides of the beds it hardly shows because the color is the same. Light scratches can even be repaired using a small propane torch to melt the scratch closed.
· Washable. Recycled plastic garden beds can be cleaned easily by washing the surface with a wet sponge or power washer. This may not be necessary for most gardeners, but since recycled beds are available in different color choices, some colors (especially white or grey) may lend themselves to cleaning at the end of each gardening season. Recycled plastics can also be considered an investment since they improve the perceived value of your property, so cleaning the beds can be of benefit especially if you plan to resell your home in the years ahead.
· Recyclable. Should the time come to dispose of your recycled plastic beds, after decades of use, the material is still 100% recyclable. HDPE plastic is so valuable that future recycling depots may even pay a premium for this material. HDPE plastic can be easily melted down and reused for new products.
· Heavy! When the package arrives at your door, have a few dollars on hand to tip the deliveryman and have a friend or two ready to help you move it to the garden. Recycled HDPE plastic is very heavy. However, once your bed is assembled and filled with soil the weight is no longer an issue.
· Not as much linear strength as wood. If you pick up one end of a recycled plastic board, the board will sag more than its wooden counterpart. So recycled plastic beds need some form of cross-bracing to stiffen the sides and prevent them from bowing outwards. A common solution is the use of aluminum “flat-stock”, which is just a straight bar of aluminum drilled on each end and secured to either side of the bed. Any recycled plastic raised bed 6’ or longer should have cross-bracing.
· Expensive. Because of its inherent qualities of durability and long lifespan, HDPE is considered the highest quality of recycled plastics. The raw material is costly to manufacturers of raised beds, and this cost is reflected in the price. Recycled plastic raised beds are more expensive than cedar raised beds. However, it is easy to calculate the long-term savings with recycled beds, since they do not need to be replaced.
Composite Wood Raised Beds
Composite ‘timbers’ are made of a blend of wood fiber and UV-protected recycled polypropylene. They have a wood grain texture and earth brown color. They are designed to be used with flanged corner joints which can be stacked to make the bed any height in increments of 5.5”. These anchor and stacking are made of durable high-impact recycled plastic resin.
· Lightweight. The composite timbers are very light. They are hollow boards with a central stiffener. These boards are very easy to lift and assemble.
· Easy to assemble/disassemble. Because the composite ‘timbers’ are lightweight, and since screws are used to attach the timbers to the anchor/stacking joints, these beds are easy to disassemble and reassemble without damaging the materials. If you move to a new home, you can bring your raised beds too.
· Uniform, natural look. Composite timbers have a wood grain imprint, and at a casual glance give the appearance of solid wood. But unlike natural wood, there is no variation is color or texture and there are no knots.
· Weather and rot resistant.
· Washable. The smooth finish lends itself to an easy clean with the hose. A light spray removes surface dust; any heavier buildup can be sponged off. Abrasive cleaners or scrubbies should not be used or they may scratch the finish.
· Can be reconfigured or expanded. The composite wood/corner joint system lends itself to expansion. It’s easy to add a layer of timbers to make a taller bed, and because the corner joint flanges are hinged, they can be swiveled. This enables you to design many different shapes of garden beds, using either straight or curved sections of composite timber
· The hollow boards can be damaged. You need to be careful with the wheelbarrow and shovel because these beds will mark, or even crack, if hit hard enough by a heavy tool. Take special care when using the weedeater, or the plastic whip may scuff the bottom edges of the composite bed.
· Taller, longer beds may bow outwards. As a raised bed is taller and longer, the increased weight puts pressure on the sides and can bow them outwards.
· Lightweight construction. Some reviewers feel the material is flimsy and the corner screws are not well anchored, but most reviews are favorable. Once the beds are assembled, however, they do hold together and look good.
· Some color fading occurs over time. These timbers do have a protective UV coating which provides stability to the finish for years of outdoor exposure. However, over time there will be some fading to the most sun exposed surfaces.
And the winner is …
Well, you knew there wouldn’t be a clear winner! All three of these style of raised beds are worthy of their place in the market. In our garden we use traditional cedar beds, 12’ to 16’ long and 12” to 24” tall. This looks great in our homestead setting. But in a commercial garden or garden center, the recycled beds make a lot of sense because they hold up to heavy use, occasional abuse and still look like new after a quick wash. And creative gardeners will appreciate the myriad design styles available with the composite bed system.