Is a hugelkultur raised bed for you? If done right, it absolutely should be. Using this method of filling your raised beds can be a major benefit, particularly if you want to focus your finances on the highest quality growing medium to top them with.
But what exactly is hugelkultur, and why is it a benefit to raised garden beds? Is this technique more valuable in its original hill-mound form, or does it perform better when enclosed in a raised garden? And does it actually steal nitrogen from your soil and start plant problems?
Let’s explore the hugelkultur way in much more detail. We’ll talk about what materials should go in your hugelkultur bed, what you absolutely shouldn’t include, and why these things matter. We’ll cover the mix you should top your bed with, too. And by the time we’re done, you’ll be set up with everything you require to ensure a raised bed success!
What Is Hugelkultur?
This funny-sounding word (pronounced hoo-gell-culture or hoo-gull-culture depending on who you ask) is actually German for “hill mound”. In its original form, a shallow pit would be dug and filled with rotting wood, gradually making a long and narrow mound. Atop the logs dirt would be mounded, creating a long, tall structure that has a lot more planting space than a flat garden bed would.
Initially, the logs add bulk to the bed. As they gradually decompose, the hill gradually sinks down, and the rotting logs inside hold extra moisture. As long as it stays planted out fully, there’s very little erosion of the soil layer, and it becomes its own little ecosystem.
In raised garden beds, this isn’t quite the process that’s most commonly in use. Certainly, you could create a long, narrow bed and fill the center with logs, then mound dirt over them for a more traditional hill mound structure. But for most people who grow in raised beds, they just want to have enough potting mix to make nice, level beds.
Hugelkultur raised beds provide the best of both worlds. When you build a hugelkultur bed, you’re using wood to make up a little less than half of the material to fill your raised beds. Other materials such as grass clippings, food scraps, coffee grounds, and the like can be used to fill in around your wood to speed its decomposition process. Atop all of this, you’ll add your preferred growing blend.
This method allows you to utilize permaculture techniques to reduce the amount of materials needed to fill your hugel beds. At the same time, you’re able to reduce the number of branches, logs, and other woody material you might have lying around. If done right, the hugelkultur portion of your bed should be free of charge and will save you some money.
Benefits and Drawbacks of Hugelkultur Beds
A lot of different benefits have been ascribed to hugelkultur beds online. Let’s list just a few of the most common:
- reduces watering frequency
- can use up waste wood or prevent it from needing to be burned in a burn pile
- increases the surface area of your garden beds
- may generate heat as the wood decomposes
- is an inexpensive way to create a tall garden bed
All of these things sound incredible, don’t they? But as with most things you find online, there’s both good and bad in all of those things.
Sure, a hugelkultur bed may be able to reduce watering somewhat. But most of its water retention benefits will be useless until the wood actually starts to rot to the point where it’s sponge-like and can share that water with the growing media around it. That can take years if you’re starting with gigantic logs.
Unless you’re growing many trees, live in a forest, or live near orchards, you probably don’t have access to a whole lot of waste wood. That means you’re likely to have to find sources of wood yourself. While there are options to find large logs, they may be bigger than you’ll want and you’ll have to get a log splitter to take them down to the size you want. Many people opt to buy a cord of firewood and utilize that to build their beds, but if you’re trying to build cheaply, that might not be ideal.
The mounds that are built in traditional hugelkultur methods will increase the surface space on your beds. But what if you don’t want to build mounds that rise above your raised bed walls? You won’t have the same increased surface space, and in fact, may have no increases in space at all.
Most raised beds already warm up earlier than an in-ground bed does in the spring, so the warming aspects aren’t necessarily needed. But wood is also carbon-dense, and usually doesn’t release much heat as it breaks down. It’s nitrogen-dense materials that provide warmth as they decompose.
So what -are- the benefits of building a hugelkultur garden bed?
In my experience, the real benefits of a hugelkultur raised bed are as follows:
- Great way to use up branches, small logs, wood chips, scrap lumber, and virtually any other woody material you have
- Will in time act as a water reservoir at the bottom of the bed
- Can be extremely cost-effective to create
- Adds an abundance of organic matter to your soil
- Definitely reduces the amount of soil you’ll need to start your beds
- Reduces compaction and shrinkage in your raised beds
This is a much more realistic depiction of hugelkultur as applied in a raised bed setting. It’s not going to work miracles, especially not when it’s brand new… but over time, the benefits far outweigh the negatives.
Does Rotting Wood Steal Nitrogen?
One of the most common issues claimed by those who have experimented with hugelkultur is that the decomposing wood robs the surrounding soil of nitrogen. Claims of failed plantings are widespread,
In those cases, I just have to ask how little soil they applied on top of their mounds.
You need to have more than half of the material in your raised beds as the soil you’ll be gardening in. Only the soil that’s directly against the wood will experience any sort of nitrogen loss, and your goal is to limit how much soil is pressed directly against that wood.
The best way to do this is to pack around the wood with nitrogen-dense materials. No matter if you’re using thin sticks or thick logs, you’ll need to provide a few inches of nitrogen-dense material right around it on all sides to produce the best effect and to help your bed break down evenly. This nitrogen-dense material will also fill in any gaps that exist, keeping the soil up where you want it to be.
If you incorporate those nitrogen-rich green wastes around your wood, you should experience no nitrogen theft by the wood as it decomposes. Assuming you’ll be watering as normal, some nitrogen will flush down through your soil over time anyway, replenishing the nitrogen supply around the wood as it breaks down. Fertilize as normal, and you’ll have no problem at all.
How To Layer Hugelkultur Raised Beds
Now that you have a better grasp on what hugelkultur is and some of the benefits and drawbacks, let’s talk about the right way to build a hugelkultur raised bed!
Framing It Properly
If you’re using one of the Birdies raised beds that are sold through the Epic Gardening store, you’ve got a galvanized steel bed that will last for literally decades. This makes the perfect framework in which to build your raised garden.
You can also use a wooden frame, but remember, the goal of a hugelkultur system is to decompose the wood in the bed. Your wooden walls may be at risk, too. If you’re going to use this method with a wooden frame, be prepared to replace your side walls as needed. Choose woods that are slow to decompose, like cedar. It may cost more, but it’ll be worth it.
You can also use brick, concrete blocks, or stone to create walls for your beds. These methods may cost you a bit to start, but they will last for years just like the Birdies beds will. The drawback here is that they’ll be much thicker, which may reduce the workable size of your bed itself.
Start At The Bottom
One of the coolest things about a hugelkultur bed is that you can repurpose some of the soil that already exists in your yard. If you have decent-quality soil, this is a great way to get the most out of your own top soil while still lifting the height up to a more manageable level!
If you want to do this method, dig out a couple inches of the surface-level soil in the area your bed will be in. Either leave some hard-packed dirt to support the sides of the bed, or bury some bricks under the walls of the bed to support them and prevent the bed from sinking down. Set this soil on a tarp and put it to the side, you’ll use it later.
Once you’ve prepared the shallow trench in your bed, pack that couple of inches with green waste. Trimmings from your garden, grass clippings, fresh manure, compost (whether finished or unfinished compost), kitchen scraps, and the like are excellent things to put in those first couple of inches of space. This creates a nitrogen layer that you can nestle your wood down into.
Work In Your Wood
Now it’s time to go for your wood. But it can’t be just any old wood. You need to plan a bit for this.
You want your wood to stay below the halfway point in your raised bed, but you’ll also need to have at least eight inches of pure soil to garden in. So the height of your bed matters here.
A 30″ bed can have up to 15″ of wood in it, because you’ll still have 15″ of soil above that level. But a 15″ bed should only have 6″ of wood at most. You have to maintain that 8 inches of growing medium as your top layer for gardening purposes.
For shallower beds, I actually recommend using branches and twigs as your wood source. Bundle them tightly together, tied with a twine-like jute that will also compost down in the beds. If you can, jam grass clippings or other organic material into each bundle to fill in any gaps. Push those down into your green waste layer.
Taller beds can use wood that’s taller, but still below the halfway point of the bed. Try to use dry logs or ones that are already starting to rot, as this will get you a jumpstart on their decomposition.
Include an inch or two of green waste packed down along the sides of the bed as you fill it with material. Ideally, you will be enclosing all of the wood in organic matter that will compost down over time.
If you don’t have logs, branches, or twigs, you can use wood chips. But if you do, I recommend arborist wood chips, ones that come with lots of green leafy material as well as the trunk material. These wood chips compost down faster because they come with a little of their own nitrogen source. Still, beds using these should also add extra organic matter, as the wood chips break down more quickly than larger branches do, and will need more nitrogen initially.
Once your wood layer is in place, add more nitrogen-dense organic material on top. Again, you’ll want at least 2-3 inches of this. Really pack it in there, pushing it down to fill any crevices that exist. This prevents your soil from falling down between the pieces of wood, and it also acts as a protective nitrogen barrier for the lowest level of your soil.
But Not That Wood
There are types of trees that are considered allelopathic. These trees have compounds in their wood or bark that work as natural herbicides, stopping the germination of plants.
While you’re going to be putting the wood down underneath your growing medium, you don’t want to risk those compounds somehow making their way up into the area the plant roots will be in. As a result, it’s good to avoid certain types of wood, leaves, or chips when you’re building your hugelkultur base, as it poses less risk to your plants.
Common allelopathic trees include walnut (particularly black walnut), eucalyptus, tree of heaven, manzanita, sugar maple, red oak, sycamore, goldenrod, American elm, pepper tree, and black locust. Including leaves or wood from these can potentially pose a risk to your plants. While they do eventually compost down and lose the dangerous compounds, it’s just better to avoid them from the outset.
Fill With Your Growing Medium
Now that the hugelkultur portion of the bed is in place, the next step is to add back in any soil you removed from underneath the bed. Spread it out evenly across the entire surface to enclose your fill material.
On top of this, you’ll want a soil medium that has lots of organic material in it and that can hold a considerable amount of water. I recommend either a blend like Mel’s Mix or something incorporating a lot of coconut coir and composted manure. You can add topsoil or other ingredients if you’d like.
Make sure that if you’re blending dirt into it, you know the type of dirt you have. Sandy soils may require a little extra moisture retention, so work in additional coconut coir, peat moss, worm castings, or vermiculite. Clay soils should be broken up by lots of leaf mold, plant compost, composted horse manure or cow manure, and the like. Silty soils should be extremely well blended with your other components to ensure they don’t sink straight through and form a thick, gunky layer on top of your logs or branches.
While you want this layer to retain moisture, you also want it to drain off excess water. It shouldn’t puddle up on the surface. Before filling, take a pot of your mix and soak it down with water. If any sits on the surface for more than a few minutes, work in some coarse sand or add some perlite to provide better drainage.
Plant It And Mulch It Well
Now that your raised garden beds are full, it’s time to move on to the final step: planting and mulching. You’ll want to prevent weeds from growing, and the best way to do that is to cover your soil entirely. Wherever a plant isn’t, you want fully covered with mulch to reduce the chance of weeds filling it in.
In addition, hugelkultur raised beds may help retain water once the wood starts to compost. But like all raised garden beds, you want to reduce moisture evaporation in the upper few inches of your growing medium. Since your wood is new, for the first growing season or two it’s best to ensure you’ve got something to slow evaporation from the surface. I often apply a thick layer of compost between the plants, then top that off with some wood chips. Taking that step greatly reduces both weeds and water waste. If you’d like to reduce water waste even more, use a soaker hose system to water underneath your mulch layer.
Planting out your raised beds is much simpler when you don’t have to bend over as far. You’ll find that it’s easier to weed and water them too. I love gardening in raised beds for this very reason, and you will too!