Raised beds are the most requested thing we build. They are also the job where the visible part gets all the attention and the part that decides whether anything grows gets almost none. We have lost count of the gardens where someone has built genuinely tidy beds, then filled them with whatever came out of the patio dig. Subsoil, bits of brick, a fair amount of clay. The beds look sharp on handover day. Two summers later the planting is half the size it should be and nobody can work out why. This guide is about permanent raised beds for shrubs, perennials and ornamental planting, the sort we build into landscaped gardens, rather than veg boxes on an allotment. > The short version: fill the rooting zone with good topsoil blended with well rotted organic matter, open enough to drain freely. Allow around 450mm of depth for shrubs. Leave the base open when you are building over soil, loosen whatever is underneath first, and expect to water more than you would a normal border. :::takeaways - The soil in the bed matters more than the bed. Good topsoil with organic matter through it, not spoil from the dig - Around 300mm of depth suits shallow planting, 450mm or more for shrubs and anything permanent - Leave the base open over soil and loosen the ground first. Over paving, build drainage openings in - Finish with 50mm or more of mulch, kept back off woody stems - Free draining cuts both ways, so a raised bed needs more water in summer than the garden around it ::: ## What soil should go in a raised bed Good topsoil, with well rotted organic matter blended through it, and enough mineral structure that it stays open instead of packing down. The bit worth being fussy about is where the topsoil comes from. Topsoil sold to the recognised British Standard, BS 3882:2015, has been tested and graded, and any supplier worth using will send the analysis over without being chased. Look at it when it arrives too. It should smell earthy and crumble in your hand. If it turns up full of roots, plastic, brick or wet clods, send it back, because you are about to bury it under everything else you are paying for. As a rough starting point, something in the region of seven parts topsoil to three parts organic matter and two parts sharp sand suits most ornamental planting. We shift that about depending on the site and what is going in. A shaded bed of moisture lovers wants different treatment to a hot sunny bed of Mediterranean shrubs. What not to use matters just as much. Subsoil, the pale dense stuff from lower down, and the mixed spoil off a patio dig have very little structure or life in them. On a deep bed there is a fair case for clean subsoil or clean rubble low down as bulk fill, purely to save on soil. It just has to sit well below the rooting zone, with proper growing soil above it. Fill higher than you think, because fresh soil with air in it settles. A bed filled dead level in September looks sunken by spring, so we overfill and let it drop. !Long raised sleeper beds filled with fresh topsoil, with a stepping stone path between them and a pergola at the far end. ## How deep does it need to be Depth is set by what you are planting, not by how the bed looks. Around 300mm of decent soil is enough for bedding, herbs and shallow rooting perennials. For shrubs, fruit bushes and anything meant to be permanent, work to 450mm or more. That is soil depth, measured after settling, not the height of the timber. Where the bed sits on open ground you get some latitude, because roots will push down into the ground below once they are established. That is the whole reason we leave the base open. Over a hard surface there is no such luxury, and whatever is inside the bed is all the plants will ever have. !Raised sleeper beds filled with topsoil framing a large porcelain patio at a house in Nottingham. ## What is the bed sitting on This is the question that changes the build, and it gets skipped constantly. On normal garden soil, leave the base open. Nothing across the bottom, no membrane, no gravel. Loosen the top of the existing ground with a fork or a mattock before filling so there is no hard pan between the new soil and the old. On heavy or compacted clay, the same but with more effort. Dense clay drains very slowly, so water arriving at the bottom of the bed has nowhere to go and the lower half stays sodden through winter. Breaking the base up properly before filling makes a real difference. If a site still holds water afterwards, the cause is usually wider drainage or a high water table, and that is a separate job rather than something to bury and hope about. On paving, concrete or a patio, you cannot leave the base open, so drainage has to be built in. That means openings at the base of the bed so water can escape, and the full rooting depth held inside the bed, because there is nothing underneath to grow into. !Cross section diagram of a raised sleeper bed showing sleeper courses fixed together, membrane on the inside face only, soil filled proud with mulch on top, an open base and loosened ground beneath. One thing that sounds sensible and is not: a layer of gravel under the soil. It does not help water away. It creates an abrupt change in texture that water is reluctant to cross, so the soil above sits wetter for longer. Gravel is a surface mulch, not a hidden layer. ## Working out how much soil you need Simple sum, and worth doing before anyone orders. Length times width times fill depth, all in metres, gives you cubic metres. A bed 4m by 1m filled to 0.5m is 2 cubic metres. Add roughly ten percent for settlement, because it will drop. Order it in bulk rather than in bags if there is any access for a tipper or a dumpy bag. The price difference on anything more than a cubic metre is not close. ## Sleepers, and the reclaimed trap New treated softwood sleepers are what we build most beds from. Straight, clean, easy to cut, and they take a stain if you want them darker. Reclaimed railway sleepers are the ones to avoid. The genuine ones are creosote treated, and creosote is not for garden use where people are in regular contact with it, let alone anywhere food is grown. On a warm day they sweat oil, and they are heavy, inconsistent and filthy to handle. We do not use reclaimed creosote treated railway sleepers in domestic raised beds. If you want the aged look, a dark stain on new timber gets you most of the way there. ## Building them so they do not spread Wet soil pushes outwards, and it pushes harder than people expect. A bed is a small retaining structure, so it needs building like one. - Fix the courses to each other with long structural screws or bar driven down through them. Stacked sleepers with nothing holding them together will move once the soil is wet and loaded. - Line the inside face with a membrane so wet soil is not sitting against the timber. It buys you years. The base stays open. - The taller and longer the bed, the more sideways load it carries, and the more the fixings and the ground it sits on have to be thought about rather than assumed. That goes double where a bed is holding back ground on one side, because at that point it is doing a retaining wall's job. ## Planting it so it still works in February The usual mistake is planting entirely for July. Everything peaks at once, looks tremendous for six weeks, and from November to March you are looking at bare soil out of the kitchen window. Start with structure. Evergreens give the bed a shape that exists all year, and ornamental grasses hold their form well into winter. Put those in first and space them for the size they will actually reach rather than the size they are in the pot. Then match the rest to the conditions, because that is where most planting goes wrong. Aspect, how much moisture the bed holds, the soil pH and how exposed the spot is all matter more than what looked good at the garden centre. - Sunny and free draining: hebe, lavender, rosemary, cistus, with a grass like stipa through it. These want the drainage a raised bed gives and resent sitting wet. - Part shade, moisture retentive: hydrangea, viburnum, hardy geranium, ferns. Hydrangeas in particular want moisture at the root, so they are a poor choice for a hot bed that drains hard, and a very good one for a bed that holds a bit of damp. - Exposed or low upkeep: euonymus, photinia, small ornamental grasses and a few tough perennials that can be cut back once a year and left alone. Resist filling every gap in year one. New planting looks sparse and the temptation is to cram it. Two seasons on, a well spaced bed closes up by itself, while an overplanted one turns into a fight. ## Free draining cuts both ways This is the part most raised bed advice leaves out. The drainage that makes a raised bed good in a wet winter is exactly what dries it out in summer. A bed standing above the surrounding ground loses moisture from the sides as well as the top, so it can be dry while the lawn beside it is still damp. New planting is the vulnerable bit. Through the first growing season it needs proper soakings rather than a daily splash, because light watering only wets the surface and encourages roots to stay up there. Warm or windy spells are when beds get caught out, and wind dries a bed faster than heat does. Mulch does a lot of this work for you, which is the real argument for it. ## Mulch, and what happens after year one A layer of bark, compost or gravel over the surface once planting is in holds moisture, keeps light off weed seed, and makes the bed look finished rather than like a building site with plants in it. Go 50mm or more, and keep it pulled back off woody stems rather than heaped against them. Piling mulch against the base of a shrub keeps the bark permanently damp, which is the opposite of helpful. After that the soil looks after itself, more or less. It settles, the organic matter breaks down into it, and the level drops. Top the surface up with compost or bark every year or two and you are feeding the soil at the same time. Nobody needs to dig a bed out and start again. ## Ground conditions vary across Nottingham Raised beds are particularly useful on the heavier and more compacted ground we come across regularly around Nottingham, though conditions vary a lot across the city and the county, and two streets apart can be genuinely different. That is why we would rather look at the ground before saying what a bed needs. A bed on free draining sand is a different build to one on compacted clay, and the difference is all underneath. ## When to build and when to plant Autumn is usually our preferred planting window. Soil still holds warmth from summer while the air has cooled and the rain is back, so plants put roots down instead of struggling to keep leaves going. For perennials that generally means late September into October, and trees and shrubs go in well from autumn through winter as long as the ground is not frozen or waterlogged. Spring planting works perfectly well too. It just means considerably more watering through the first summer. So if you are thinking about beds now, late summer is a good time to get them built, so the soil has settled and everything is ready for autumn planting. :::cta Thinking about beds or borders? Send us a photo of the area and rough dimensions, and we'll tell you what would work there, what depth it needs, and what should go underneath it. ::: Beds are one of the cheaper things we build and one of the biggest changes to how a garden feels. Just spend the money on what goes inside them rather than only on what holds them up. We build raised beds and plant borders across Nottingham, and you can see more of our work nearby.
