A raised bed is a frame filled with soil above ground level. It solves real problems: heavy clay or poor drainage, compacted yards, soil that warms slowly in spring, and gardeners who find kneeling or bending difficult. It also adds cost and some work that in-ground beds do not need, especially more frequent watering. This guide covers how to size a bed, how the common frame materials compare, what to fill it with and how to keep it watered. It draws mainly on University of Florida IFAS Extension’s raised bed publication, with supporting guidance from other extension services. It does not rank brands or kits; it explains what to check when you compare them.
Do you need a raised bed?
Raised beds are most useful when:
- Native soil is heavy clay, rocky, contaminated or waterlogged.
- You want a tidy, defined garden in a small yard.
- Bending to ground level is hard; a tall bed brings the soil up to you.
- You want soil that warms and drains earlier in spring.
They are less necessary where the native soil is good and drains well. In that case, preparing an in-ground bed with compost is cheaper; see soil and plant health. The trade-off is water: University of Florida notes that raised bed soil drains well and dries quickly, so irrigation becomes critical.
Choose the site
UF/IFAS recommends a level spot, or one that minor grading can level, with at least six to eight hours of direct sun and close to a water source. Vegetables in particular need full sun; University of Wisconsin–Madison Extension defines that as at least six hours of direct sunlight a day. If you are building over lawn, UF suggests removing the grass or smothering it with cardboard or several layers of newspaper before filling. Contact your local 811 service before driving stakes or digging posts.
Size it for reach
Width
Most people can comfortably reach about two feet in from the edge, so UF/IFAS advises keeping beds no wider than four feet when accessible from both sides, and narrower, around three feet, if your reach is shorter or the bed sits against a wall or fence. Four by eight feet is the most common size and uses standard lumber lengths with little waste.
Height
UF recommends a minimum of six inches and up to 24 inches for gardeners with mobility challenges. Taller beds hold more soil and dry more slowly, but cost more to fill and need stronger walls to hold back the soil’s weight. Longer beds need extra supports along the sides so boards do not bow outward.
Paths
Leave 18 to 24 inches between beds, and 30 inches if you plan to use a wheelbarrow, per UF/IFAS. Wider paths are easier with a wheelchair or a garden cart. Mulch or gravel on paths keeps weeds down and feet clean. Layout ideas for small spaces are in small garden layouts.
Compare frame materials
| Material | Lifespan and cost (relative) | Notes from UF/IFAS and EPA |
|---|---|---|
| Untreated softwood (pine, fir) | Cheapest; short-lived | UF, writing for Florida conditions, notes untreated lumber can rot within about a year. |
| Cedar or redwood | Moderate to high cost; naturally rot-resistant | Cost more than treated pine but avoid preservatives. |
| Modern pressure-treated wood (ACQ and similar copper-based treatments) | Moderate cost; long-lived | UF/IFAS recommends ACQ lumber for longevity and value and advises stainless steel screws, since the copper corrodes ordinary fasteners. |
| Older pressure-treated wood and railroad ties | Avoid | UF advises against lumber treated before January 2004 and railroad ties for edibles, because of possible arsenic and creosote. EPA describes arsenical (CCA) treated wood as the type used around homes before 2004 and advises precautions such as a dust mask when cutting it. |
| Composite or recycled plastic | Higher cost; long-lived | UF: lasts longer than ACQ but costs more. |
| Concrete block, brick or stone | Durable; heavy | UF: pre-cast concrete blocks are acceptable for edibles and last many years. |
| Corrugated or galvanised metal | Moderate to high cost; long-lived | Listed by UF as an option. Metal warms in sun; edges should be capped or rolled. |
If you want to avoid treated wood entirely for vegetables, cedar, block, metal and composite are the usual alternatives. Whatever you choose, check that kits include corner brackets or posts strong enough for the height you want.
What to check when buying a kit
- Inside dimensions rather than outside, so you know the real growing area and soil volume.
- Wall thickness and bracing for taller beds.
- Fasteners: stainless steel or coated screws for treated wood and outdoor use.
- Material treatment: what preservative, if any, and whether the seller states it is suitable for food gardens.
- Bottom: most beds are open to the ground so roots and water can move down. Hardware cloth across the bottom keeps out burrowing animals.
- Assembly: slide-in boards or corner brackets make a bed easy to move or rebuild later, as UF notes.
Fill it
A raised bed holds a lot of soil, and the fill decides how well it grows. UF/IFAS recommends a lightweight, organic-rich mix that holds water, drains and lets air reach roots, built from components such as compost, composted pine bark, composted manure, peat moss, perlite, vermiculite and topsoil, with lime if needed to reach the right pH. Many garden centres sell a ready “raised bed mix”; ask what is in it.
To estimate volume, multiply length × width × height in feet and divide by 27 for cubic yards. UF’s worked example, a 4 × 8 foot bed about 22 inches tall, needs a little over two cubic yards. Fill to the top: UF notes the soil settles by about two inches, and it continues to shrink, so top it up after each season.
At planting, UF suggests mixing a balanced or slow-release vegetable fertilizer into the top six inches, at the label rate for the bed’s area. A soil test of the fill after the first season shows what it needs next.
Water it
Because raised beds drain fast, plan watering when you plan the bed. UF/IFAS recommends low-volume methods, such as soaker hoses, micro-sprayers or drip tubing, ideally on a timer in the early morning. Seedlings need frequent, short watering; established plants do better with less frequent, deeper watering. Running a header line to each bed during construction is much easier than retrofitting. See watering equipment for drip layouts.
Plant it
Raised beds suit block planting, where crops are spaced evenly in a grid instead of rows with paths between them. UF recommends the square-foot method. Put tall crops and trellises on the north side so they do not shade shorter plants. See vegetable gardening for spacing, and the rest of our layout guides under garden design and raised garden beds.
