Somebody has told you to put a layer of gravel in the bottom of your raised bed for drainage. It is one of the most repeated pieces of gardening advice on the internet, and it is wrong. A coarse layer under finer soil does not drain your bed faster. It makes the soil above it hold water for longer.
The short answer
An open-bottomed raised bed sitting on unsealed ground needs no drainage layer at all. Gravel, rocks or broken pots at the base create a perched water table that keeps the soil above them wetter, not drier. If your bed genuinely drains badly, the cause is almost always the soil mix, a sealed base, or where the bed is sited.
This guide explains the physics in plain language, then works through the three situations where drainage really is a problem and what actually fixes each one. If you are still choosing a bed, our guide to picking a raised bed covers why an open bottom matters more than almost any other specification.
What you'll learn:
Water does not move freely from a fine material into a coarse one. It sounds backwards, and it is the single most misunderstood thing in container and raised bed gardening.
Soil holds water against gravity by capillary action, the same force that pulls water up a paper towel. For water to leave the soil and enter a layer of gravel below, the soil at that boundary has to become almost fully saturated first. Until it does, capillary attraction in the fine soil beats gravity and the water simply stays put.
The result is a saturated band sitting directly on top of the gravel, which soil scientists call a perched water table. Adding gravel does not lower the water level in your bed. It raises the bottom of the bed, which means the saturated zone now sits higher up, closer to your plant roots. Washington State University Extension has been correcting this advice for years, and their review of the container drainage literature is blunt about it.
Why This Works
Permaculture keeps returning to the same principle: connect systems rather than separating them. A bed with an open bottom is continuous with the soil beneath it, so water, roots, fungi and worms all move freely across the boundary. The moment you insert a gravel layer you have built a container with a false floor, and containers need managing forever. The fix is almost always to remove a barrier rather than add one.
Three situations, and only three. If none of these apply to your bed, you do not have a drainage problem and you should stop worrying about it.
The bed has a solid floor
A bed on a patio, balcony, driveway or decking has nowhere for water to go. This is the only case where drainage holes are genuinely required, and it is a container problem rather than a raised bed problem.
The ground beneath is compacted or waterlogged
Heavy clay, a former building plot, or a spot where water already pools after rain. The bed will drain only as fast as the ground it sits on, so the ground has to be dealt with first.
The mix itself holds too much water
Straight bagged garden soil, pure topsoil or heavy compost with no bulking material compacts and stays wet. This is by far the most common cause and it has nothing to do with what is underneath.
For an open-bottomed bed on soil, the honest answer is nothing. Put your soil mix straight onto the ground and let the two systems join up. There are three optional exceptions, none of which are about drainage.
| Layer | Purpose | When to use it |
| Nothing | Lets roots, water and soil life move freely between bed and ground | The default for any open-bottomed bed on unsealed soil |
| Cardboard | Smothers grass and weeds while breaking down within a season | Building directly over lawn or weedy ground |
| Hardware cloth | Blocks voles, gophers and moles from below | Only where burrowing rodents are a known problem |
| Logs and branches | Bulks out a deep bed cheaply and holds moisture as it rots | Beds deeper than about 18 inches, on cost grounds |
Note that none of these are drainage layers. Cardboard and hardware cloth are both freely permeable to water, and a woody base absorbs water rather than shedding it.
If you are filling a deep bed, the woody base is worth reading about properly. Our layered method for filling a raised bed covers the ratios and what it saves you.
Common Mistake to Avoid
Do not line the inside of a raised bed with plastic sheeting to "protect" the timber. It turns an open bed into a sealed container, traps water at the base, and cuts the soil off from the ground below. If you are worried about the material, change the material rather than wrapping it. Landscape fabric at the base is a milder version of the same mistake: it slows root and worm movement for no drainage benefit.
Work from the most likely cause down, and resist the urge to dig gravel in.
Start with the mix. If the surface crusts, cracks when dry and puddles when wet, the bed is short of coarse organic matter. Fork in several inches of compost plus a bulking material such as coarse bark fines, pumice or coir. This is the fix in the large majority of cases, and it is the one people skip because it is less satisfying than an engineering solution. Our guide to the best soil for raised garden beds covers the ratios.
Next, look under the bed. If the bed sits on compacted ground, loosen the subsoil with a fork before rebuilding, or accept the constraint and go taller so there is more freely draining volume above the problem. A broadfork does this without inverting the soil profile, which matters if you want the soil life to recover.
Finally, look at the site. A bed at the bottom of a slope receives everything that runs off above it. Moving the bed, or intercepting the flow uphill with a shallow swale or a planted strip, solves what no amount of gravel will.
Get Our Free Companion Planting Chart
Join 10,000+ gardeners getting weekly tips on what to plant together, soil health, and permaculture techniques.
Send Me the ChartThis is the one case where you are genuinely building a container, so container rules apply. Drill or leave drainage holes across the whole base rather than a few in the middle, because water leaves at the lowest point and a flat patio has several.
Raise the bed slightly off the surface on feet or battens so the holes are not sealed against the paving. Use a mix with more bulking material than you would in a ground-contact bed, since there is no soil below to buffer it. And go deeper if you can: a taller column of soil drains more freely at the top because the saturated zone sits at a fixed height above the base regardless of how deep the bed is. That is the same physics as the gravel problem, working in your favour.
Only if they have a solid floor. A raised bed with an open bottom sitting on soil drains through the ground beneath it and needs no holes at all. A bed on a patio, balcony, driveway or deck is effectively a large container and does need holes, spread across the whole base rather than concentrated in the middle, with the bed lifted slightly so the holes are not sealed against the paving.
No. A coarse layer beneath finer soil creates a perched water table: water will not move from the fine soil into the gravel until the soil immediately above is nearly saturated, so the wet zone ends up higher in the bed and closer to your roots. It also costs money and reduces the growing volume you paid for. If drainage is genuinely poor, the cause is the soil mix, a sealed base or the siting, and gravel addresses none of the three.
For most gardeners, nothing. Put your soil mix directly onto the loosened ground so roots, water and soil life move freely across the boundary. Cardboard is worth adding when you are building over lawn, because it smothers grass and rots away within a season. Quarter inch hardware cloth is worth adding only where voles or gophers are a known problem. In beds deeper than about 18 inches, a base of logs and branches saves a lot of purchased soil, but that is a cost decision rather than a drainage one.
Almost always the mix. Bagged garden soil, pure topsoil and heavy compost with no bulking material all compact over a season and hold water. Fork in compost plus a coarse component such as bark fines, pumice or coir. If the mix is fine and the bed still sits wet, check what is underneath: compacted subsoil, a plastic liner or landscape fabric will all hold water at the base. Beds at the foot of a slope also collect runoff from everything above them.
No, and it usually hurts. Landscape fabric is permeable enough that it adds no meaningful drainage benefit, while over time it clogs with fine particles and roots, slowing water movement and blocking worms and roots from reaching the soil below. If your goal is weed suppression when building over grass, plain cardboard does the job better and disappears on its own once the grass beneath is dead.
Depth matters more for roots than for drainage in an open-bottomed bed, because the ground below does the draining. Over a sealed base it matters a great deal: the saturated zone sits at a roughly fixed height above the bottom, so a deeper bed gives you more freely draining soil above it. Twelve inches is a sensible practical minimum on a patio, and more is better. Our guide to how deep a raised bed should be covers the crop-by-crop numbers.
Ready to Grow Smarter?
Get our free 20-page beginner's guide to backyard food forests, with two printable worksheets and a year-by-month planting calendar you can use this weekend.
Read the Free Guide