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.
Pros:
·
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.
Cons:
·
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.
Pros:
·
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.
Cons:
·
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.
Pros
·
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
Cons:
·
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.
