The appeal of building a raised garden bed with cedar fence boards stems from a desire to blend the material’s durability with a lower price point than traditional dimensional lumber. Raised beds offer gardeners control over soil quality and drainage. However, using thin fence pickets for this application is not straightforward, as the material’s structural limitations must be addressed. This article explores the properties of cedar, the challenges posed by thin boards, and the specific construction techniques required for this cost-effective approach to succeed.
Why Cedar is a Preferred Material for Raised Beds
Cedar is widely regarded as an excellent choice for outdoor construction due to its natural resistance to decay and insect damage. This durability comes from a high concentration of natural oils and tannins within the wood fibers. Compounds known as thujaplicins act as fungicides, inhibiting the growth of decay organisms. This inherent protection means untreated cedar can last upwards of 10 to 15 years, even when in constant contact with moist soil.
The safety profile of untreated cedar is another significant advantage for growing edible crops. Unlike pressure-treated lumber, which relied on chemical preservatives that could leach into the soil, natural cedar is non-toxic. Its natural defenses eliminate the need for chemical treatments, ensuring a safe environment for vegetables and herbs.
Structural Limitations of Thin Fence Boards
The primary hurdle when using cedar fence boards is their lack of thickness compared to standard lumber. Fence pickets are commonly milled to a thickness between 5/8 inch and 3/4 inch, a dimension chosen for lightweight vertical fencing, not for bearing lateral load. When filled with soil, a raised bed subjects the walls to significant outward pressure, which increases dramatically when the soil becomes saturated with water.
This hydrostatic force causes the thin boards to warp and bow outward over time, compromising the bed’s structural integrity. Unlike thick dimensional lumber, a thin fence board lacks the mass and rigidity to remain straight. The failure point is not decay, which the cedar resists, but the mechanical collapse of the side walls due to insufficient thickness.
Essential Construction Techniques for Longevity
To successfully use thin cedar fence boards, their structural weakness must be counteracted through specific reinforcement techniques. The first step involves installing sturdy internal corner posts, typically made from 4×4 or 2×4 lumber, which are significantly thicker than the fence boards. These posts anchor the sides together and bear the outward soil pressure at the corners.
For beds longer than four feet, vertical internal bracing must be added along the long sides to prevent bowing. This is achieved by installing additional 2×2 or 4×4 posts mid-span, secured to the inside of the thin walls. These intermediate supports act as buttresses, distributing the lateral force across the length of the board.
Shorter beds, generally 10 inches or less, will experience less internal pressure than taller constructions. Lining the interior of the box with a thick plastic sheeting or pond liner can extend the wood’s life. This barrier prevents direct contact between the moist soil and the cedar, slowing the decay process at the soil line.