Raised beds have become a staple of urban gardening and for good reason. Artificial beds have plenty of advantages:
- Excellent drainage
- Easy to keep tidy
- Able to be installed over poor soil or potentially contaminated soils
- Good foundation for garden infrastructure (irrigation, row covers), see photo.
- Prevents soil compaction
- Can keep away some pests
- Can be easier to work (depending on height)
They do have drawbacks, but they are few:
- In hot climates, plants might dry out
- Requires bringing in soil and amendments (usually)
- Requires work and materials to build
Most of the downsides can be mitigated to some extent, depending on your circumstances. It is possible to install slow-release watering containers in hot climates. I’ve installed an in-ground irrigation system in my raised beds. Depending on your local soil, you might have to bring in soil from elsewhere whether or not you are building a raised bed, so this might be a false drawback. If you do have good soil, it might be possible to dig down to the bottom of the rich, organic layer, install the bed, and then put the soil in the bed, leaving the walkways dug down to subsoil. Finally, while they do take work, they are no more labor intensive than any other gardening activity, and if you are not handy, this might be a good way to get to know a handy acquaintance, as the work is pretty straightforward.
Deciding on Materials
You could build raised beds out of anything (really!), but you should avoid certain things: materials that leach chemicals and/or don’t hold up well to the elements. Most people build out of three types of materials: wood, blocks, plastic wood-like composites. Let’s look at each of these in turn.
Wood
Wood is the classic choice and my preference. The internet is rife with debate over whether or not it is better to use modern treated wood or naturally weather-resistant species. Modern treated wood has less nasty chemicals than the old style green lumber, however, it still does have chemicals it will leach into the 2-3″ of soil next to the walls. Some folks line the walls with plastic, but that has chemicals that leach as well. Natural woods, like cedar or cypress, are expensive and often harvested with less-than-ideal lumber practices (look for the FSC seal). My Take: I have read that after 15-20 years, treated wood has leached everything it is going to leach, so I have used 25-year-old treated wood in my beds. I also was able to scavenge cedar wood, which I use when I can. Perhaps reclaimed wood is the ultimate answer because it doesn’t cause new materials to be harvested and processed.
Blocks
At first glance, cement blocks can be ugly. Bricks can be beautiful, but are harder to come by for a reasonable price. One big advantage of using blocks, though, is that they will last longer than you will be gardening. The other advantage is that they are more or less inert when it comes to leaching chemicals. On the other hand, they take work to install and might require mortar or cement to hold them in place (although cement blocks can be secured with stakes through the hollows). If they are just sent in place (which is perfectly fine), they will dislodge and shift over time and require a bit of maintenance every few years. They do contribute carbon dioxide to the environment when they are made, but since they last so long, that might be discounted. If you’re worried about the carbon footprint, find used blocks or bricks. My Take: Blocks are great if you know you’ll be there a long time. Paint ’em fun colors if the drabness bothers you (on the outside with safe paint).
Composite Plastic Wood-like Boards
To be honest, I’ve never worked with these boards. I have looked at them in the big box stores and from what I saw and have read, they seem to be long lasting but flimsy. You might have to brace these things more than with real wood. They are made of plastic, but I haven’t seen studies looking at what sorts of chemicals they leach out, so you’ll have to do some digging if you want to use them. My Take: I suppose it is good that recycled materials are being used, but I’d rather go for the more direct use of recycling as with blocks or wood, above.
Alternatives
Logs: Easy to use but can harbor pests.
Wine Bottles: Pretty, but will break and spread broken glass around.
Straw Bales: Nice but will degrade after a year or two.
Pallets: Okay but be sure to get the heat-treated pallets, not the chemically preserved ones.
Basic Process
First, you’ll need to find your site. Measure it out and stake the corners of where you want your beds to go. Don’t make them wider than 4′. This is because you can’t effectively reach to the middle of a bed that is any wider than that. I make mine 2-3′ wide when possible.
Second, you should flatten out the base of where the walls will go. If you’re on a flat lawn, you’re probably okay, but if it slopes, you’ll want to dig out the high sides and possibly build up the low ends. The most accurate way to do this in a low-tech way, is to buy a masonry line level* and masonry string. On the highest point next to the bed, put in a stake. Tie the string to the stake about 6″ above the ground. Attach the line level and pull the line taught. This string is now level and you can measure the distance to the ground with a measuring tape. Say the lowest point of the bed’s wall foundation is 18″ below the line. That means you’ll want to dig out the rest of the wall line to 18″ below the line (or dig the high spots and fill in the low spots until it is all even).
Third, if you have grass or other vegetation underneath where the bed will go, you have a few options. First, you can dig it all under, turning the veg. over. If you have clayey soil, you probably want to leave it at that to allow the bed to drain. If you have sandy soil, or don’t want to turn over all that area, you can cover the ground surface with cardboard. A layer of cardboard will smother the underlying plants, help retain water, and disintegrate after a while (oh, probably 5-15 years depending on conditions).
Fourth, you can start building. If you’re using wood, I recommend using 2″ dimensional lumber (2-x-4″, 2-x-6″, etc.). Anything thinner than that will probably degrade too quickly. Measure the sides and cut the boards to length. Instead of just attaching one side to the other with a basic butt joint, I use either 2-x-4″ or 4-x-4″ cut to the height of the raised bed wall, plus 3″ as posts to reinforce the corners. The extra 3″ lets you bury the excess beneath the beds, adding a bit of stability. The easiest way to attach these, in my experience, is to lay the corner posts down and place the short-side boards over it (posts flush with each end). Attach the boards to the posts (see below about fasteners) as in no. 1 in the figure. When both short ends are done, stand them up with the extra 3″ in the air (post-side facing each other) and then attach the long sides to the posts and short sides, as in no. 2 in the figure. Once everything is attached, flip it over, dig out pockets for the extra 3″ of the post, and install. A Note About Fasteners: You’ll need something beefier than deck screws or the like. I use these things*, which are essentially a 1/4″ lag bolts with a built-in washer. It is worth the extra cost in the long term. Essentially the same process can be used for composite wood-like boards. I highly recommend predrilling the holes. It takes an extra 5 minutes but probably adds a year to the life of the bed.
If you’re using blocks, you’ll need to be sure your foundation is level. Then it is simply a matter of laying blocks in an overlapping pattern. If you’re using bricks, you might want to do two parallel layers because the weight of the soil and water might push out a single layer of bricks, depending on the height. If you want to use mortar, well, that is beyond the scope of this little tutorial. Avail yourself to YouTube or better yet, find a friend with skills to teach you in person.
Fifth, you can fill your beds. The nice thing about a raised bed is that you can tailor it to what plants you’ll be growing. Root veggies do well in sandy soils that they can dig down into. Other plants like more water-retentive soils. Do your research. My basic fill starts with a base of woodchips mixed with compost and soil. After that comes more topsoil mixed with compost. Many municipalities give away free compost, topsoil, and woodchips. Check out your local government to find out if they do. It can be a big savings if you have a truck (or a friend with a truck), but the quality is much less uniform than buying from a supplier. You can build up your own soil through intensive composting and amending of your existing soil. It takes patience, but it can be done!
Maintenance
Raised beds, when built well, are fairly maintenance free. Each spring I check the corners to be sure they are sound. I tap some of the older wood if it looks rotten. If it sounds hollow, I’ll consider replacing that board. Each fall, I put a few buckets of compost on every square yard of raised beds and then gently turn it in with a spading fork (I do NOT turn my soil deeply each year, so as not to disturb the microbiome overmuch). If I forget and end up doing this in the spring, the squirrels will dig up all my freshly planted starts because the compost smells so interesting. Plus I think the winter lets the compost and existing soil structure get mixed together better.
One thing that I think looks particularly nice is putting mulch over the walk ways. Each spring I use a hoe and scrape up the old mulch, cleaning out any weeds that have started. I put in a fresh layer of mulch and toss the old stuff on top of the beds (to help retain moisture) or into the chicken run (the next year, the poo-laden mulch goes into the compost).
*I am not affiliated with Johnson Levels or FastenMaster.