On our farm we are using permanent raised beds. The beds are based on a german/austrian raised bed technique which they call Hügelbeet some folks in America are calling it hugelkulture. Which roughly translates to mound bed and that describes them pretty well. Our beds are free standing but they can be placed in traditional garden containers such as 2x4s or logs. Before we get into our beds lets talk about what we see as the pros and cons of the run of the mill raised beds.
They get warmer earlier in the spring.
In wet area they raise the roots out of the super wet soil.
The aerated soil stimulates root growth and allows for greater root penetration.
The overall yield in raised bed tend to be significantly higher that non raised beds.
The added heat and drainage leads to potential drying out which means the need for an increase in irrigation.
If you use a containment system the cost of beds is much higher.
The building of beds is labor intensive for double digging and must be redone when compaction occurs.
Large scale tractor built beds must be rebuilt every year after fields are tilled.
A significant amount of inputs are needed to maintain fertility and organic matter in a tractor based tilling system.
Our beds are an attempt to take advantage of the pros of raised beds while limiting the number of negatives. Our beds are designed to last anywhere from 5 to 20+ years depending on the original composition and size with little to no major work done to the internal structure of the bed. They will be able to absorb and release large quantities of water, which will avoid the irrigation problems. They will have a slow release of nutrients that will aid in the long term fertility of the bed. They will using in a no till situation so the need for tractor use is reserved for the construction only. The construction of the bed will lead to answering the question of how can we achieve these results compared to a normal raised bed system.
The bed starts in a similar fashion to a double dug bed. After marking out where the bed will be the sod should be removed and piled somewhere close by. Then a hole should be dug out out about 12 inches deep a standard shovel blades depth is about right. Depending on your scale the soil can be put on a tarp or just piled up to be used towards the end of the building process. Once the digging is done you are set to start piling up your material.
The first layer is made up of your largest woody materials. For use that meant logs that ranged from 5 to 20 inches. We had to cut down several trees to improve light to our garden and to a few fruit trees that were in the area. We used these logs as our core layer. The smaller logs we piled in the center around 1 to 2 feet tall and ran the length of a bed. The larger diameter logs had a few smaller logs placed around them to fill in gaps around the bottom. We also made a few notches around an inch deep along the length of the larger logs to help increase the initial breakdown of the wood. You can use fresh or rotted wood for this core. Some folks say rotted wood is better because the partial decay will help avoid the potential problem of nitrogen robbing and result in better water absorption sooner. We went with fresh because it is what we had as well as we believe there will be no problem with nitrogen robbing and that the fresh logs will result in a longer life span of the bed. If you are making a small bed you do not need full size logs. Branches and twigs can be your first layer.
The second layer is made up of your smaller woody material and can be the first layer in smaller beds. We used the branches and tree tips from our cuttings hugging the material around the core layer so it starts to resemble a woody onion. We made this layer around 1 to 2 feet thick but it will be compressed by the next layers reducing its total width. This layer will start breaking down much sooner that the core and will be the initial area of water storage and fungal activity.
Those two layers represent the largest change to a standard raised bed and deserve some discussion. There are four major reasons we add these two layers.
The first is water retention. As the wood begins to break down it becomes very sponge like, gaining the ability to hold large quantities of water. Water is taken in when there is a surplus of water and is released when water becomes more scarce. In many cases beds need no irrigation once established.
The second function of the woody layers is long term nutrient release. As the wood is broken down by fungi and little soil critters the nutrients that were built up while the tree grew. This slow release helps regulate nutrient flow within the beds that with a good rotation or crop mixture leads to long term fertility with little outside inputs.
The third function is the main process that leads to the water retention and nutrient release. The presence of the woody material leads to a fungal dominated environment. The wood is broken down primarily by fungi. Leading to the spongy woody texture. The fungi can also move nutrients around the soil where it is needed by different plants. It has the ability to retain collect nutrients that would normally be leeched out by water trickling down into the subsoil.
The last function I’ll talk about is the physical expansion, contraction, and shifting of the woody materials in your pile. In our Vermont climate the wood goes through many temperature swings as well as moisture fluctuations. As the inner core moves it shifts the above soil which helps to break up compaction that has occurred. The shifting is a property that double dug beds lack and is one of the reasons that hugelbeets last longer while not compacting.
The final layers are similar to what you would find in a lasagna style bed. The sod should be laid down next grass side down covering the branches and twigs. The next layer should be made up of leaves or old hay/straw depending on your bed size the thickness of this layer will be different. The next layer is made up of your greens like fresh grass clippings and food waste. You can make a few of these layers with smaller amounts in each layer to avoid creating anaerobic zones. In our case we had a large amount of horse bedding that we used as our precomposited material. An optional layer which helps get the piles started breaking down would be a mix of unfinished compost. Finally you should cover the pile with the soil that you dug from out of the original hole. Once we had the final shape of the bed finished we took an old round bale that had been sitting out in a field for who knows how long and mulched the beds for the winter.
If you are planning on growing non-nitrogen fixing vegetables the first year you should make sure that there is a distinct separation from your woody and soil layers. You want to limit the interaction of because there is a potential of nitrogen robbing occurring when the wood begins to break down leaving little for your crops. But there are countless German sites that have planted into these beds right away and have had very good success. We made our first set in the fall so they could breakdown a bit before there first year of production.
The beds will shrink with time and should have a top dressing of compost added to them from time to time. You can also use mulch to cover the beds which will help keep the beds moist, slightly cooler in the summer and have a much lower weed pressure problem. If snails and slugs are a major pest pressure in your location the beds can perform when not mulched and will help reduce their numbers if no other solution works for you. I have a video of some of our beds being built in the process of being edited and will add it to the post when its ready.