Raised-Bed Basics

A raised bed is, structurally, nothing more than a frame that holds soil above the surrounding grade - but the specific choices inside that simple definition (what the frame is made of, how tall it stands, what fills it) determine whether the bed lasts five years or fifteen, and whether it actually solves the drainage or accessibility problem you built it for in the first place.

Material Trade-Offs, Honestly Compared

Untreated pine is the cheapest option and the shortest-lived, typically breaking down within 4-6 years of contact with wet soil. Cedar and redwood cost considerably more but naturally resist rot, often lasting 10-15 years without any sealant. Modern pressure-treated lumber (the ACQ formulation, not the older arsenic-based CCA treatment phased out for residential use decades ago) is now considered safe by the EPA for food-bed use, though some gardeners still avoid it on principle and line the interior with plastic sheeting as an extra precaution. Composite lumber and galvanized steel or aluminum panels cost the most upfront but can outlast wood by a decade or more, with no rot risk at all.

Height Is a Real Decision, Not an Aesthetic One

A standard 6-12 inch bed is enough depth for shallow-rooted crops like lettuce and radishes and is the cheapest to fill, but it does little to solve a bad-back or limited-mobility problem, since you're still bending close to ground level. A 24-32 inch bed, built tall enough to garden from a standing or seated position without bending, is a meaningfully different structure - it needs stronger corner bracing to resist the outward pressure of that much soil, and it takes substantially more fill material, which is the main reason most raised beds default to the shorter height instead.

Width and Length

Width matters more than length for a simple reason: you need to be able to reach the center of the bed without stepping into it, since stepping on growing soil compacts it and undoes much of the benefit of a raised, loosely structured bed. A bed accessible from only one side should max out around 3 feet wide; one accessible from both long sides can comfortably go to 4-5 feet. Length has no structural limit beyond your available space and the practical convenience of not needing to walk unreasonably far around a very long bed to reach the other end.

Filling a New Bed

A raised bed's whole advantage over ground-level planting depends on what's inside it - filling one with cheap fill dirt or unamended native clay defeats the purpose entirely. A workable blend is roughly equal parts topsoil, compost, and a coarse aeration material like perlite or coarse sand, giving you a mix that drains well but still holds enough moisture and nutrients to support the tight spacing raised beds are typically planted at. Budget for this mix as a real cost of building the bed, not an afterthought - a cheaply filled expensive frame performs worse than a cheap frame filled properly.

Settling in the First Season

Fresh soil mix settles substantially as organic matter breaks down and air pockets compress under watering - a bed filled level to the top in spring often sits an inch or two lower by midsummer. This is normal and not a sign anything went wrong; top off with additional compost as needed rather than assuming the bed was underfilled from the start. A bed that's been through one full season and refilled once tends to hold its level far more predictably in following years.

Drainage on Hard Surfaces

A raised bed built directly on soil drains naturally into the ground beneath it, but one placed on a patio, deck, or driveway has nowhere for excess water to go unless you plan for it - a bottom layer of coarse gravel before adding soil, or a bed built with a slight gap between boards at the base, prevents the bottom few inches from staying permanently waterlogged. This is easy to overlook when following instructions written for an in-ground bed, since the drainage problem simply doesn't exist in that context.

Corner Joinery and Long-Term Stability

Simple butt-jointed corners held with exterior screws work fine for a shorter bed, but a taller bed holding significantly more soil weight benefits from metal corner brackets or a full internal frame, since the outward pressure of wet, settling soil is considerably higher than it looks. A bed that bows outward at the corners over a season or two usually wasn't over-built for its height - reinforcing the corners before filling is far easier than fixing a bowed bed after the fact.

Where a Raised Bed Beats - and Loses To - Other Layouts

A raised bed's real advantages are soil control (you're not stuck with whatever native soil you have), better drainage, and a warmer root zone in spring, since the elevated soil mass heats up faster than ground-level dirt. It's a poor fit for very large-scale growing, where the frame material cost per square foot becomes prohibitive compared to simple row gardening, and it's not the right choice if you specifically want to practice multi-year crop rotation across a large footprint - Raised-Bed Crop Rotation covers that need with a multi-bed approach built around the same basic frame.