Building Raised Beds: Materials, Dimensions, and Soil Mixes

Why Raised Beds Transform Pacific Northwest Gardens

Raised beds have become the go-to solution for Pacific Northwest gardeners, and for good reason. Our region’s heavy clay soil, wet winters, and short growing season all favor elevated planting areas. Whether you’re dealing with compacted urban soil, poor drainage, or simply want a more organized garden, raised beds offer advantages that in-ground planting can’t match.

Benefits of Raised Beds in Our Climate

Better Drainage

The most compelling reason for raised beds in the Pacific Northwest is drainage. Our clay soils hold water like a sponge, creating soggy conditions that rot roots and prevent early-season planting. Raising your planting area even 6-8 inches above grade dramatically improves drainage, allowing excess water to escape and roots to breathe.

Earlier Soil Warming

Raised beds warm faster in spring than in-ground soil. The sun heats the smaller soil mass from the top and sides, sometimes gaining 2-3 weeks of growing season. For heat-loving crops like tomatoes and peppers that struggle in our cool climate, this head start makes a real difference.

Extended Growing Season

The same thermal properties that warm soil faster in spring keep it warmer longer in fall. Combined with easy-to-add season extension like row covers or cold frames, raised beds can stretch the Pacific Northwest growing season from March through November.

Complete Soil Control

When you build raised beds, you control exactly what goes in them. Instead of fighting native clay or contaminated urban soil, you start with the ideal mix of topsoil, compost, and amendments. This investment pays dividends for years.

Reduced Compaction

Well-designed raised beds eliminate the need to walk on planting areas. Uncompacted soil drains better, allows deeper root penetration, and requires less work to maintain. In our wet climate where walking on soil creates compaction, this benefit is substantial.

Choosing the Right Materials

Untreated Cedar

The premium choice for Pacific Northwest raised beds. Western red cedar, abundant locally, naturally resists rot and insects without chemical treatment. Expect 10-15 years of service from 2-inch thick cedar boards. Cedar weathers to an attractive silver-gray that blends into Northwest landscapes.

Pros: Long-lasting, naturally rot-resistant, locally available, beautiful appearance
Cons: Higher cost, heavy to work with
Cost: $150-300+ per 4×8 bed depending on lumber prices

Douglas Fir

Another locally available option, Douglas fir costs less than cedar but doesn’t last as long—expect 5-8 years without treatment. Some gardeners prefer fir for its lower cost, planning to rebuild beds as they age. Using thicker boards (2-inch) and lining with landscape fabric extends life.

Pros: Lower cost, locally available, easy to work with
Cons: Shorter lifespan, may need replacement sooner
Cost: $75-150 per 4×8 bed

Galvanized Metal

Galvanized steel or corrugated metal raised beds have become popular for their modern aesthetic and long lifespan. Metal won’t rot and can last 20+ years. However, metal conducts heat, which can be both advantage (faster warming) and disadvantage (overheating in summer sun).

Pros: Very long-lasting, modern appearance, no wood decay
Cons: Can overheat, sharp edges if not properly finished, more expensive
Cost: $200-400+ per 4×8 bed

Concrete Blocks

Standard concrete blocks create sturdy, permanent beds inexpensively. The hollow cores can be filled with soil for planting herbs or trailing plants. Blocks do raise soil pH over time as lime leaches, which may require monitoring in our already-acidic soils.

Pros: Inexpensive, permanent, no rotting, easy to configure
Cons: Can be unattractive, may raise pH, heavy to move
Cost: $50-100 per 4×8 bed

Materials to Avoid

• Pressure-treated lumber: Modern treatment (ACQ) is copper-based and considered safer than older CCA, but many organic gardeners still avoid it. If using, line the interior with plastic.
• Railroad ties: Treated with creosote, a known carcinogen. Never use for food gardens.
• Pallet wood: Often treated with unknown chemicals for shipping. Only use pallets marked “HT” (heat treated) and verify origin.
• Composite decking: Contains plastics that may leach chemicals. Not recommended for food growing.

Optimal Dimensions for Pacific Northwest Gardens

Width: 3-4 Feet Maximum

You should be able to reach the center of the bed from either side without stepping on the soil. For most people, this means 4 feet maximum—narrower if the bed is against a fence or wall.

Length: 6-12 Feet

Length is limited only by materials and budget. Eight feet is popular because it minimizes lumber cutting (boards come in 8-foot lengths). Longer beds may need internal supports to prevent bowing.

Height: The Pacific Northwest Sweet Spot

Bed height depends on your goals:

• 6-8 inches: Minimum for improved drainage over clay soil. Adequate for most vegetables but roots will grow into native soil below.
• 10-12 inches: The sweet spot for most gardens. Deep enough for all vegetables, keeps soil warmer, requires less bending.
• 18-24 inches: Ideal for accessibility, root vegetables, and gardening over poor or contaminated soil. Higher cost but eliminates almost all bending.
• 30+ inches: Table-height beds for wheelchair access or those who cannot bend. Require substantial soil volume.

For Pacific Northwest conditions with heavy clay underneath, we recommend 12 inches as the ideal height—deep enough for excellent drainage while keeping material costs reasonable.

Site Preparation and Installation

Choosing Your Location

• Sun exposure: Most vegetables need 6-8 hours of direct sunlight. South-facing locations maximize sun in our northern latitude.
• Level ground: Beds on slopes will have uneven soil depth and drainage issues. Level the area first or build stepped beds.
• Access: Leave 2-3 foot paths between beds for walking, wheelbarrows, and kneeling. Wider paths allow for easier harvesting and maintenance.
• Water access: Locate beds within easy reach of a hose or irrigation system.

Ground Preparation

You don’t need to remove sod or dig up the existing soil, but some preparation helps:

1. Mark the bed outline: Use stakes and string to ensure square corners.
2. Level the area: Remove high spots and fill low areas so the frame sits evenly.
3. Kill existing vegetation: For best results, lay cardboard over grass and weeds. This smothers vegetation and eventually decomposes into organic matter.
4. Consider drainage: If your yard has severe drainage problems, dig a shallow trench and add gravel beneath the bed.

Assembly Tips

• Use exterior-grade screws, not nails, for corners and joints. Screws hold better and allow disassembly if needed.
• Pre-drill screw holes in hardwoods like cedar to prevent splitting.
• For beds longer than 8 feet, add vertical supports every 4 feet to prevent bowing.
• Stake tall beds into the ground with rebar or wood stakes for stability.
• Line the bottom with hardware cloth (1/2-inch mesh) if gophers or moles are problems.

The Ideal Soil Mix for Pacific Northwest Raised Beds

Don’t fill raised beds with native soil—you’ll just move your drainage problems up higher. Instead, create a custom mix optimized for growing vegetables:

The Classic Mix: 60-30-10

• 60% quality topsoil: Provides structure and minerals. Avoid “fill dirt” or soil with heavy clay content.
• 30% compost: Adds organic matter, nutrients, and beneficial microorganisms. Use aged compost that’s dark and crumbly.
• 10% additional amendments: Perlite or pumice for drainage, aged bark for structure, or sand for very heavy mixes.

Buying Soil in Bulk

A 4×8 foot bed that’s 12 inches deep holds approximately 32 cubic feet or just over 1 cubic yard of soil. Bulk delivery from landscape supply companies is far more economical than bagged soil for multiple beds:

• Look for “raised bed mix” or “garden blend” specifically formulated for vegetable growing
• Ask about composition—quality mixes include compost already blended
• Expect to pay $40-80 per cubic yard for quality bulk soil plus delivery
• Order slightly more than calculated—soil compacts as it settles

Local Soil Suppliers

In the Seattle area, reputable bulk soil suppliers include Pacific Topsoils, Tagro (Tacoma), Cedar Grove, and local landscape supply yards. Ask specifically for raised bed or vegetable garden mixes rather than general topsoil.

Maintaining Raised Bed Soil

Annual Replenishment

Raised bed soil levels drop 1-2 inches per year as organic matter decomposes and soil compacts. Replenish each spring with:

• 1-2 inches of compost worked into the top few inches
• Organic fertilizer based on what you’re planting
• Additional topsoil if beds have sunk significantly

Crop Rotation

Even in raised beds, rotating crop families prevents disease buildup and nutrient depletion. With limited space, this can be challenging. At minimum:

• Don’t plant tomatoes and potatoes in the same bed two years running
• Follow heavy feeders (tomatoes, squash) with legumes (beans, peas) that add nitrogen
• Alternate root crops with leafy greens and fruiting vegetables

Cover Crops in Raised Beds

Pacific Northwest winters provide perfect conditions for cover crops even in raised beds. After fall harvest:

• Plant crimson clover for nitrogen fixing and spring beauty
• Use winter rye to prevent nutrient leaching and add organic matter
• Austrian winter peas fix nitrogen and can be chopped for compost

Turn under cover crops 2-3 weeks before spring planting.

Season Extension with Raised Beds

Raised beds adapt easily to season extension structures that can add months to your growing season:

Low Tunnels

PVC hoops covered with row cover fabric or plastic extend shoulder seasons by 4-6 weeks. The fabric allows water through while trapping heat. Install permanent anchor points on bed frames for easy cover installation.

Cold Frames

A clear-topped box that fits over the bed creates a miniature greenhouse. Traditional cold frames use old windows; modern versions use polycarbonate panels. In the Pacific Northwest, cold frames allow winter salad production and early spring starts.

Cloches and Row Covers

Individual plant covers (cloches) or floating row covers draped directly on crops provide a few degrees of frost protection. Essential for protecting tender transplants during our unpredictable springs.

Common Raised Bed Mistakes to Avoid

Building Too Wide

A 6-foot-wide bed seems efficient until you try to reach the middle without stepping on the soil. Stick to 4 feet maximum.

Skimping on Depth

Six-inch beds don’t provide enough root zone for tomatoes, peppers, or root vegetables. Invest in 10-12 inch minimum depth.

Using Poor Quality Soil

The advantage of raised beds disappears if you fill them with clay or low-quality fill dirt. Invest in good soil from the start.

Forgetting Paths

Beds placed too close together make working awkward and tempt stepping on soil. Plan 2-3 foot paths minimum.

Ignoring Irrigation

Raised beds dry faster than in-ground gardens. Install drip irrigation or soaker hoses before planting—it’s much harder after plants are established.

Building Your First Raised Bed: A Quick Guide

Materials for One 4×8 Foot Cedar Bed (12 inches high)

• Four 2×12 cedar boards, 8 feet long
• Four 4×4 cedar posts, 12 inches long (corner supports)
• 24 exterior wood screws, 3-inch
• Approximately 1 cubic yard of raised bed soil mix
• Cardboard for bottom layer (optional but recommended)

Assembly

1. Cut two 8-foot boards to create four 4-foot lengths for the ends
2. Attach 4×4 corner posts to inside corners of end boards with screws
3. Attach long sides to corner posts
4. Level the frame in your chosen location
5. Lay cardboard on the ground inside the frame
6. Fill with soil mix, leaving 1-2 inches below the rim
7. Water thoroughly to settle soil; add more if needed

Your raised bed is now ready for planting. With proper soil and regular compost additions, it will produce abundant harvests for many years to come.