While you don’t necessarily need to level your raised garden beds, it’s better if you do. If you build a raised garden bed on a hill, the plants at the top will dry out much faster than those planted at the bottom. Gardening on a slope is also challenging, so it’s better to level your raised garden beds while building them if you can.
No matter how you build a raised garden bed on a hill, you’ll have to level it. You can reinforce the hill or build the beds in a terraced garden style. You can also build a box, level it, add vertical supports, and then nail in the top boards for a level, sloped bed.
Building a raised garden bed on a slope may seem like an incredible undertaking that requires advanced carpentry skills, but it can be a simple DIY project. Let’s look at how you can build a raised bed on a slope, which material you’ll need, and what type of lumber will be best for the job, so you can easily plant and care for your garden without breaking your back.
How to Build a Raised Bed on a Slope
Regardless of how you build a raised garden bed on a slope, keep in mind that you’ll have to level it. The biggest reason for building a level raised bed is easier watering.
If you water on a slope, the plants at the top are much more likely to dry out before those planted at the bottom. However, with a level bed, the soil moisture will be much more evenly distributed, ensuring healthy plants across the slope.
Similarly, nutrients in the water will also flow down to the lowest point of the slope. But with a level bed, fertilizers and all other amendments you add to the soil will also be evenly distributed. Let’s look at a few ways to level raised beds:
Reinforce a Steep Hill
Reinforcing steep hillsides will help protect your raised beds from damage and prevent the bottom of the slope from flooding. The easiest way is by supporting fencing or building retaining walls to hold back the soil.
However, make sure you do this at a time when heavy rainfall isn’t likely to interrupt the construction, or you’ll run the risk of soil erosion while you’re halfway through the project.
A solid concrete retaining wall is around 24” to 28” thick, allowing it to handle the total weight of the hill behind it. These walls typically have two layers: an inward one that leans towards the slope and a second wall that’s the front surface you’ll see.
You’ll also have to add a bit of gravel between the hillside and the inner wall to provide drainage for excess water and stabilize the soil. Finally, ensure you plan out your retaining wall months in advance and leave enough space between that and any future gardening space.
Build Terraced Gardens
Building your raised garden beds in a terraced garden style will ensure even water and nutrient distribution and enhance your environment.
If you’re building a terraced garden, you’ll also have to build a sturdy retaining wall to prevent erosion. If you build anything weaker, most of the soil will spill out from the edges, reducing the amount of nutrient-rich soil you have in the upper levels. Unfortunately, this will also overflow the lower levels, so make sure you start out right.
By building individual raised beds that hold most of the soil, you won’t have to worry about erosion issues usually associated with terraced gardens. In addition, combining raised beds with terracing will create a great, sleek-looking gardening space.
How to Measure the Slope of Your Land
Before you start building a raised garden bed on a hill, you’ll first need to measure the slope of your land. This is simply the ‘rise over run’ or a measure of how far your land drops away across a certain distance.
The higher the rise, the steeper the hill will be. For accurate measurement, here are the steps you need to follow:
- Place a carpenter’s level down on the sloping ground.
- Lift the level’s lower end until it’s level or the bubble is in the middle.
- Measure the distance from the middle of the level to the ground. This will be your “rise.” For a better understanding, suppose this measurement is four inches.
- Measure the distance from one end of the level to the middle. This will be your “run.” Most carpenter levels measure two feet (24 inches) long, so this measurement will probably be 12 inches.
- The rise/run or slope of your land will be 4”/12”, which means that for every foot you move, the hill will drop four inches.
Regardless of the slope of your land, it’s better to position your raised beds lengthwise along the hill instead of running them down it. This is because the more you go down your hill, the higher you’ll have to raise your beds to make them level and the more wood or lumber you’ll need.
What Type of Lumber Should You Use for a Raised Bed?
There are several different lumber types you can use for your raised beds. However, each type has its advantages, so make sure you choose the one that best meets your project’s needs.
Pine is the cheapest and most readily available lumber type today. However, it rots out much faster than other wood types, so you’ll have to replace it every eight to 10 years. One silver lining to rotting wood is that you can use it to make compost and add healthy bacteria to your garden’s soil.
While cedar is costlier than pine, it has natural oils that help preserve the wood and ensure it lasts longer. You can easily find cedar lumber at most hardware stores.
Other Types of Durable Limber
Depending on your locality, hardware stores may have other durable lumber types, such as maple or oak. While these materials are costly for larger-scale projects, they may be just what you need for raised garden beds.
Pressure Treated Lumber
If you’re planning a flower bed, you can use treated lumber, particularly one dipped in chemicals, to prevent it from rotting.
Ensure you don’t use treated lumber when growing a vegetable garden, as the chemicals will leach into the ground and contaminate the food.
Natural Wood Treatment
You can also use natural and non-toxic wood treatments on your lumber to prevent it from rotting. These treatments are safe for cedar, pine, and other wood lumber and help extend their life.
What Materials Will You Need?
Once you’ve planned everything out, you’ll need a few materials to build the raised garden bed on your hill:
- 2×6 pieces of lumber measuring six to eight feet. Cut a few of them in half for the ends.
- A few extra 2x6s for filling gaps and vertical framing supports.
- Three-inch Ardox nails
- A circular or hand saw, like this one from Rexbeti (on Amazon).
- Hammer, like this one from Irwin (on Amazon)
- Carpenter’s level, like this one from Craftsman (on Amazon)
- Safety gear, such as eye protectors and gloves, like this pair from Dowellife (on Amazon)
Steps to Building a Sloped Bed
Raised garden beds should be around four feet wide for easy weeding. While you can build them half a foot deep, building them one foot deep is much better for most plants and vegetables. The length of your raised bed depends entirely on your preference.
You can easily build a sloped bed alone. You don’t need anyone to hold the other end of the boards or secure the finished bed. Here are the steps you need to follow:
Step 1: Make a Box
The first step to building a sloped bed is constructing its basic frame. First, nail the ends of your eight feet 2x6s to your four feet 2x6s to build a basic box.
Don’t worry if the box feels flimsy—this will be fixed later. Next, place the frame where you want the raised bed to be.
Step 2: Level the Box
Place the carpenter’s level on any one of the four-foot 2x6s and lift the box’s lower or downhill end until the bubble is centered between the two lines. Place a block under the box to keep it level. You can use scraps of wood, a rock, or anything sturdy as the block.
Next, place the level on any one of the eight-foot 2x6s and lift one of the box’s ends until the bubble is exactly in the center. Place a block under this side too. Check all the sides to ensure the box is perfectly level from every direction, and make adjustments as necessary.
Step 3: Add the Vertical Supports
In every corner of the frame, vertically fix a 2×6 so it touches the ground and goes at least six inches above the box’s top. Nail your box onto these vertical supports.
It’s also advisable to repeat this step in the middle of the eight-foot 2x6s for additional support. If you’re building a bed that’s longer than eight feet, add vertical support every four feet.
Step 4: Fill in All the Gaps
After adding vertical supports, you’ll notice a gap between the ground and the bottom of your box. Nail additional 2x6s to your vertical supports until you completely cover the space.
Depending on the shape and slope of your land, the gaps will usually be a bit funky, so you may need to trim the boards for a better fit. You can dig away some dirt so the boards are snug against the ground.
Step 5: Nail in the Top Boards
Nail the last layer of 2x6s to the vertical supports extending above the original box. You’ll now have a level raised bed that measures one foot high on one side and a bit higher on the other.
Step 6: Add Stakes to Prevent Your Bed from Sagging
Eventually, the weight of all the soil in the raised beds will put pressure on their sides, especially the sloping ones. To ensure your beds remain level and stable, you add stakes on the outside by nailing them against the sides of the beds.
You can use pieces of rebar, pieces of lumber sharpened to a point, or any other strong stake of your choice for this step.
Step 7: Fill the Bed and Start Planting!
Your sloped bed is now level and complete, so you can start filling it with whichever flowers or vegetables you want.