How to Build a Raised Garden Bed: A Step-by-Step Guide

Size it so you can reach the middle

Keep beds no wider than 1.2 m so you can reach the centre from either side without stepping on the soil, since stepping compacts it and squeezes the air out of the root zone. Length is entirely up to you; 2.4 m is popular because it divides neatly out of standard lumber. Make the bed 25-30 cm deep for lettuce, herbs, beans and most annuals, and 40 cm or more for carrots, parsnips and other long-rooted crops. If you garden from a wheelchair or want to avoid bending, build to 60-75 cm tall with deeper side boards.

Choose wood that is safe and durable

Untreated rot-resistant woods are the simplest choice: cedar, redwood, larch and cypress last 10-15 years, and 5 cm thick Douglas fir gives a solid decade at lower cost. Avoid creosote-treated railway ties and reclaimed timber of unknown origin. Modern pressure-treated wood (ACQ or copper azole) no longer contains arsenic and is widely considered safe for food crops, but you can line the inside with heavy plastic if you prefer. If you use pallet wood, take only boards stamped HT for heat treated, never MB, which means methyl bromide.

Assemble the frame square and solid

Cut four boards to length, then fasten the corners with butt joints and three or four exterior-grade deck screws (8-10 cm) per joint, pre-drilling to stop the wood splitting. Check the frame is square by measuring both diagonals, which should match to within a few millimetres, before you tighten everything down. For beds taller than 40 cm, add a corner post inside each corner so the sides cannot bow outward under the weight of wet soil. Set the bed on level ground; on a slope, either dig the high side in or step the frame so the top stays level.

Prep the ground underneath

You do not need to dig out the turf. Lay down a thick layer of overlapping cardboard or several sheets of newspaper, wet it thoroughly, and set the frame on top, which smothers grass and weeds while still letting worms and water through. Avoid landscape fabric under a soil-filled bed, because it clogs over time and blocks the worm activity you actually want. If the bed sits on concrete or a patio, make it at least 30 cm deep, add a drainage layer, and plan on watering more often since there is no subsoil reservoir beneath it.

Fill it with a balanced mix

The most expensive mistake is filling a bed entirely with bagged potting mix, which dries out fast and costs a fortune in volume. A reliable blend is roughly 60 percent topsoil, 30 percent compost and 10 percent perlite or coarse sand for drainage. To cut both cost and bulk, put a 10-15 cm layer of sticks, pruned twigs and partly rotted leaves in the bottom, where it slowly breaks down and feeds the bed. Fill to within 5 cm of the rim so you have room to mulch and water without soil spilling over the sides.

Water, mulch and top up yearly

New beds settle and drop by 5-10 cm in the first season, so keep a bag of compost on hand to top them up each spring. Run a soaker hose or drip line down the middle before planting, because a raised bed dries out faster than ground soil and hand watering rarely reaches the root zone evenly. Mulch the surface with 5 cm of straw or compost to hold moisture and suppress weeds. Refill with compost every year and rotate crops between plant families so pests and diseases do not build up.

The bottom line