Just about everybody agrees that raised beds are the way to grow vegetables in a home garden, especially when your alleged “soil” is lousy clay or construction fill.
Raised beds elevate plants up and out of that compacted menace, they contain root-friendly and nutrient-rich organic matter, they warm up and dry out faster in spring, they allow for closer planting and more production in less space, and they never need to be tilled since you walk in the paths and not on the soil.
Where things get contentious is in exactly how to build the beds and what to fill them with.
Even the experts disagree, leaving gardeners scratching their heads over advice that sometimes contradicts.
If you’re starting a new vegetable garden this year (or discouraged with an existing one), here’s a raised-bed rundown.
To dig or not to dig?
Gardening “systems” such as lasagna gardening and Mel Bartholomew’s Square Foot Gardening method advocate saving work by simply laying a sheet of cardboard or four to six sheets of newspaper over the area to be planted. The raised-bed soil then goes on top while the vegetation underneath dies and decomposes.
Penn State Extension gives the nod to that, and University of Nebraska Extension educator John Porter says this no-dig approach also avoids stirring up weed seeds.
“Simply adding a thick layer of compost and topsoil on top of the soil in the general dimensions of the bed is a good way to start a bed,” Porter writes in a post on the Garden Professors website. “No need to till or disturb. Over time, the organic matter will work its way down into the soil.”
In clayish soil, Porter recommends at least a six- to eight-inch layer of compost or topsoil over the existing ground.
On the other hand, the French Intensive or “double-digging” method not only advocates removing existing vegetation but digging down two shovel depths, then mixing that soil with organic amendments to create the raised beds.
Cornell Extension suggests a similar approach, except digging down four inches after removing turfgrass. It advises setting aside the dug-up four inches, then flipping the sod pieces upside down and covering them with a mix of the excavated soil and at least two inches of compost.
Washington State University horticulture professor and author Dr. Linda Chalker-Scott advises against any drainage-impeding underlayment, with the exception of a membrane in case of contaminated soil.
“Underneath the beds is… nothing,” she writes in a Garden Professors post. “If you have no (contamination) issues, it’s soil next to soil, meaning we have great drainage.”
Chalker-Scott says this layering no-no goes for adding gravel or other coarse material to the bottom of a raised bed.
“The change in soil texture creates a perched water table, which makes for a soggy planting bed and optimal conditions for soil-borne diseases,” she writes.
To box or not to box?
The main disagreement here is over what framing materials are OK (or not) to contain raised-bed soil.
Everyone agrees it’s a bad idea to use railroad ties in a food garden – or any other materials that contain creosote, chemical wood preservers, or arsenic (a metalloid used to pressure-treat wood prior to 2003).
The split starts when it comes to current types of pressure-treated boards, which use alkaline copper quaternary instead of arsenic. Some say food contamination from copper is “negligible” if any, while others say any leaching is too much.
Some advocate recycled plastic boards instead, again pointing out that leaching is negligible and that these are a good way to recycle plastics. Others don’t want any plastic in the garden, including sheets of plastic mulch or hoop covering.
For those with concerns, rot-resistant cedar, black locust, redwood, and oak boards are wood options as well as interwoven tree branches. The main downside is that they’ll decay sooner than pressure-treated wood.
Although it’ll be more expensive (unless you score someone else’s giveaways), other long-lasting soil-containing options are brick, concrete blocks, wall stone, and manufactured blocks.
For that matter, framing isn’t necessary at all if you’re willing to rake fallen edges back into mounds each year.
How big?
Most agree on the width of raised beds. If you’re reaching in from one direction, about two feet is big enough for most people’s arm length.
If you’re accessing a bed from both sides, four to five feet of width is the recommended max – a reach of half from either side. Go wider, and you’ll end up having to step in, which will compact the soil.
Those four- to five-foot beds can be any length, although most people stick with commercial board sizes that run eight feet, 10 feet, and 12 feet long. To prevent bowing, hammer in pegs at least every four feet along the outside perimeter.
Bed depths are where disagreements come into play.
Clemson Extension says that four to six inches of depth are enough – if you’ve loosened the soil underneath.
The University of Minnesota Extension and the Old Farmer’s Almanac recommend six to eight inches. So does University of Maryland Extension educator John Traunfeld… in cases where you’re sticking with shallower-rooted crops like leafy greens, beans, and cucumbers and growing on a hard or untilled surface underneath.
The University of Missouri and the Square Foot Gardening Foundation suggest raised-bed depths of six to 12 inches, while Penn State Extension goes a tad farther at eight to 12 inches.
Maryland’s Traunfeld says you’ll want beds 12 to 24 inches deep if you’re growing bigger crops like peppers, tomatoes, and squash on those untilled or hard surfaces.
And Washington State University Extension leans to the deeper end in general with a recommendation of 15- to 18-inch-deep beds.
Deeper beds essentially mean you’re growing in a giant container. While deep, loose soil is good for root growth, it also means more watering and more amendments that you’ll have to buy and move when building the beds in the first place.
Most everyone suggests two-foot-wide pathways between the beds. To prevent erosion and hold down weeds, these can be topped with straw, wood chips, wood mulch, crushed gravel (“stone dust”), crushed sea shells, river pebbles on landscape fabric, flagstone, bricks, or manufactured pavers.
How to fill the boxes or beds
This one’s the king of raised-bed discord.
Most extension services and horticulturists say it’s best to use all of your existing soil – including raking soil from the paths into the beds – and then improve it with compost or similar organic matter.
On the other hand, the Square Foot Gardening system and former Organic Gardening magazine editor Mike McGrath are among those who say existing soil is one of the worst things you can add to your raised beds.
Garden soil is “loaded with weed seeds, insect eggs, and potential pathogens,” says McGrath, who hosts PBS’s “You Bet Your Garden” program. He adds that many urban soils are also contaminated with lead.
McGrath recommends filling raised beds with half compost and half purchased and screened topsoil along with a “generous amount” of perlite – a super-heated mineral that puffs up into tiny, white, light-weight little balls.
The Square Foot Gardening system recommends filling with a “Mel’s Mix” of equal parts of three ingredients: 1.) compost, ideally homemade from at least three to five ingredients (purchased is OK if also from multiple ingredients); 2.) peat moss (or coconut coir if you’re trying to avoid peat moss), and 3.) coarse vermiculite or perlite.
The lasagna gardening system sidesteps soil as well and recommends alternating two-inch layers of “brown” and “green” materials – up to a level of 18 inches to three feet. The layers break down and shrink significantly in six to 12 months, after which the bed is ready to plant.
Brown materials include chopped leaves, straw, sawdust, wood ash, wood chips, pine needles, and shredded paper. Green materials include grass clippings, kitchen scraps, rotted manure, coffee grounds, and plant trimmings from the yard.
Among the soil-is-good advocates, Ohio State Extension recommends a raised-bed blend made out of one-third native soil, one-third compost or peat moss, and one-third coarse sand.
Penn State Extension recommends beds that are 70 percent native soil and 30 percent compost, while Washington State Extension suggests working one to three inches of compost into the top six to eight inches of loosened native soil.
Chalker-Scott goes so far as to say that you might not need anything other than mulch-topped, loosened native soil – raked from paths and/or bought or brought in to raise the beds.
“The best material for your raised bed is actual native topsoil,” she writes.
Contrary to the no-soil advocates, Chalker-Scott says the worst raised-bed additive is bags of soilless media (i.e. “potting mix”), which are mostly comprised of wood waste, bark fines, and peat moss.
“Read the contents,” she says. “If it’s all organic material, you are going to have to fill your beds every year. This is both expensive and time-consuming. Plus, you could very well have excessive levels of some nutrients that will build every year as you add more.”
These are the same reasons why it’s a bad idea to fill entire raised beds with compost or an all-no-soil mix, says Robert Pavlis in his 2023 book, “Compost Science for Gardeners” (New Society Publishers, $22.99 paperback).
That medium might work for some, Pavlis writes, “but we are also seeing poor growth and stunted plants in such systems. When the soil is tested, it invariably shows toxic levels of phosphate.”
Excess phosphate (phosphorus) can block other nutrients and stunt growth, plus it’s a leading waterway pollutant when the excess runs off.
Compost also can become compacted as it breaks down, while manure-based organics such as mushroom soil can increase soil salt to plant-harming levels.
Maryland’s Traunfeld says it’s difficult to make a blanket soil-building recommendation at all. He says soil should be tested before adding anything to determine what amendments should be added to a particular garden and in what amounts.
Chalker-Scott advises regular soil testing – not just at the beginning of a new garden.
“Soil testing is really crucial for any garden but perhaps most important in vegetable gardens where harvesting may decrease key nutrients over time,” she says. “It will also guide you in identifying potential heavy metal problems. The money you will save in not buying unnecessary fertilizers and other amendments will pay for many soil tests.”
Watering, topping and ongoing care
Most everyone agrees that the most important downside of a raised bed is that it’s going to need more water more often than an in-ground garden. Those mounds drain quicker and dry out faster where the wind has more impact.
To counteract that, one option is fitting a raised-bed garden with drip irrigation. These plastic lines can be snaked through the beds with holes poked at desired intervals. Not only is drip irrigation efficient because it delivers water directly into the soil, the systems can be hooked up to automatic timers to eliminate the tedious job of hand-watering.
Covering the beds with organic mulch is another way to slow evaporation and hold down water demand.
Mulch layers also cool the soil in summer, help keep a lid on weeds, prevent soil erosion, and add nutrients and organic matter to the soil as the mulch breaks down.
Chalker-Scott recommends a coarse organic mulch – in particular, a four-inch layer of wood chips. Alternately, she suggests pine needles or straw.
“Fine-textured organic materials like compost should never be used as a mulch,” she says. “Thick layers of compost are more restrictive to gas and water movement and also facilitate weed growth. Save compost for a thin top-dressing when your soil is in need of organic matter, and be sure to cover it with woody mulch to keep those weeds out.”
Nebraska’s Porter is also OK with wood chips – “so long as you don’t let them mix in with the soil. That’s something I never can do in a vegetable garden where I’m planting and removing things on a regular basis.”
Porter’s veggie garden mulches of choice are straw and shredded newspaper.
Everyone is on board with the idea of scrapping tilling and annual digging. Once the beds are built and not walked on, compaction shouldn’t be an issue, and weeds will be less since you’re not continually stirring buried weed seeds to the surface.
