Many gardening articles expound the advantages of a gentle southern sloping garden, but sometimes the slope is a little too steep for effective gardening.
Or maybe the only space you have for your garden is a steep hill but terracing is too expensive and labour intensive. The solution: building raised beds on a slope.
Gardening on an incline can lead to soil erosion as the exposed soil and any accompanying nutrients are washed away down the hill.
The key is to build a raised bed that is level so that you stop erosion, while gaining the benefits of raised beds and terracing your garden at the same time.
Building raised garden beds on a slope might seem like a massive undertaking requiring advanced carpentry skills, but this can be a very simple DIY project. In this article, we’ll look at how it can be done with minimal tools, commonplace materials, and very basic building skills.
Can You Put A Raised Bed On A Slope?
Absolutely! In fact, there are many advantages to building raised beds on a slope, such as improved drainage, increased soil temperature for early planting and seasons extension, reducing erosion, and uniform water and nutrient distribution.
You just want to make sure that the finished box is level and does not follow the contours of the ground.
When building a raised bed on flat ground, you simply build a box and lay it down.
On a slope, however, simply laying the box on the ground would make the raised bed just as crooked as the ground. You want to lift one end up so that your raised bed is level when it is finished.
Do Raised Beds Have To Be Level?
No, they do not but it is better if they are. If the raised bed is sloped, plants at the top of the bed will dry out faster than those at the bottom.
Alternatively, the plants at the bottom run the risk of becoming waterlogged. The boards at the bottom will also rot out faster than those at the top as the water pools on the downward side.
As the water washes to the bottom of the raised bed, it also washes all the nutrients away, too. During a heavy rain, the soil itself can also wash right out of the raised bed taking newly planted seeds with it.
Making sure your raised bed is level will alleviate these issues.
The Slope Of The Land
The first thing to do is to determine how much your land slopes. This is often described as the “rise over run” or how far the ground drops away over a certain distance.
The greater the rise, the steeper your hill will be. Here’s how to measure the slope of your land:
When building a raised bed on a hill regardless of the slope, it is better to position the bed lengthwise along the hill rather than running it down the hill.
The longer you go down the hill, the higher you will have to raise your bed to make it level and the more lumber you will need.
Choosing the Right Lumber
There are many options of lumber available that you can use to build your raised bed. Each type of wood has its own benefits so choose the one that best suits your project.
Pine is the most readily available and also the cheapest. The downsides are that pine will rot out faster than most other wood so it will have to be replaced (our pine beds last around 8 to 10 years before they rot), but this has the silver lining that rotting wood can add compost and healthy bacteria to the soil.
Cedar is another option that is more expensive than pine but contains natural oils that preserve the wood making it last longer. Cedar lumber is available at most hardware stores.
Other Durable Lumber. Depending on where you live, hardware stores might have other types of durable lumber available, such as oak or maple. These are often prohibitively expensive for large scale projects, but might be what you need for your garden beds.
Natural Wood Treatment. You can also buy natural wood treatments that are non-toxic and will keep the lumber from rotting as quickly. (http://microfarmgardens.com/blog/2015/12/30/6-non-toxic-wood-preservatives.html). Most of these treatments can be used on pine, cedar, or other plain wood lumber to extend its life.
Pressure Treated Lumber. If you are planting a flower garden, you could elect to use treated lumber which is dipped in chemicals to stop it from rotting.
DO NOT use treated lumber if you are growing vegetables because the chemicals are carcinogens and will leach into the soil and contaminate your food.
What size of lumber? A standard 2×6 is generally a good size for building raised beds and they come in a variety of lengths.
Most lumber sold in hardware stores is construction grade lumber that is shaved to a uniform size. If possible, get rough cut lumber as it is thicker, more solid, and will last longer than construction grade lumber.
Nails or screws? You can use either nails or screws depending on your building preferences. Nailing your raised beds has the advantage that it holds the boards together tighter and creates smaller holes so less water gets in than screws.
I generally prefer Ardox nails because their spiral shape really holds the wood together and they are galvanized so they last a long time.
How Big Should My Raised Bed Be?
On average, raised beds should be a max of 4 feet (1.2meters) wide for easy weeding. They should be at least 6 inches (15cm) deep, but 1 foot (30cm) deep will accommodate most vegetables and plants. A raised bed can be as long as you want.
In this article we are building a raised be that is 4 feet wide, 1 foot deep (on the uphill side since it will be deeper on the downhill side), and 8 feet (2.4 meters) long.
How To Build A Raised Garden Bed On A Slope
Now that you have everything planned out, let’s look at how to easily build a raised bed on a hill that won’t break your bank or your back.
3: Make a box.
The first step it to build the basic frame of your raised bed. Nail the 4ft 2x6s onto the ends of your 8ft 2x6s so you have a basic box. The box will be quite flimsy but that is ok for now. Put the box where you want your raised bed to be.
4: Level the box.
Put your level on one of the 4ft 2x6s and raise the downhill side of the box until little bubble is between the two lines.
But a block underneath the box to hold it in position. You can use a rock, scarps of wood, or anything else sturdy as a block.
Next, put the level on one of the 8ft 2x6s and again raise one side of the box as needed until the bubble is centered.
Put blocks under this side as well. Check each side to make sure the box is level in every direction and adjust as necessary.
5: Put in the vertical supports.
In each corner of the box, stand up a 2×6 so that it touches the ground and extends at least 6 inches above the top of the box. Nail the box onto these uprights.
You also want to do this in the middle of the 8ft 2×6 as well for added support. If you build a bed that is longer than 8 ft, put in another vertical support every 4ft or so.
Tip: Instead of resting the supports on the ground, you can cut them into a point and hammer them into the ground. This will add support to your raised bed and you can skip Step #6.
5: Fill in the gaps.
On the downward side of your box, there will be a gap between the bottom of the box and the ground. Attach additional 2x6s to the vertical supports until the space is covered.
Depending on the contour of your land, these gaps will often be a funky shape, so you might need to trim these boards a bit so they fit properly or you can dig some dirt away so the boards fit nice and snug against the ground.
If you end up with a space that is too small for a 2×6 to fit you have a couple of options. First, see if you have a smaller board that will fit such as a 2×4 or a 2×2.
Or, if you are not worried about aesthetics (like most of my garden projects) simply hammer the 2×6 so that is overlaps the other boards. The final, and more finicky option is to cut a 2×6 lengthwise at an angle so that it fits perfectly into the space.
(Be very careful when cutting the boards lengthwise, as this can be quite dangerous unless you have the proper tools and training). This is mainly a personal choice based on practicality over aesthetics.
Don’t worry if you still have a few small gaps, as these will quickly plug over time. You can also plug them by placing a small rock on the inside of the bed to keep the soil from spilling out.
6: Add the top boards.
Now you can add the final layer of 2x6s by nailing them onto the vertical supports above your original box. This will give you a raised bed that has a finished height of 1ft on the uphill side, and even higher on the downward side.
7: Add stakes to keep the bed from sagging.
Over time, the weight of the soil in your raised bed will put pressure on the sides, especially the downhill side.
To keep your bed nice and square, you can drive stakes in on the outside up against the side of the bed. You can used pieces of lumber tipped to a point, pieces of rebar, or any other sturdy stake your choose.
8: Fill it and plant!
Your raised bed in your hill garden is now complete, and you can fill it with whatever growing medium you desire.
Building a raised bed on nice even ground is one thing, but we might get discouraged as we look out over our hilly garden plot and try to plan our raised garden.
Even with basic construction skills, you can easily build a raised garden bed on a slope, and I hope this article has given you the confidence to tackle your project and create a productive, beautiful, one-of-a-kind raised bed garden.
Amber Noyes was born and raised in a suburban California town, San Mateo. She holds a master’s degree in horticulture from the University of California as well as a BS in Biology from the University of San Francisco. With experience working on an organic farm, water conservation research, farmers’ markets, and plant nursery, she understands what makes plants thrive and how we can better understand the connection between microclimate and plant health. When she’s not on the land, Amber loves informing people of new ideas/things related to gardening, especially organic gardening, houseplants, and growing plants in a small space.