Earthworks are meant to create conditions conducive for growth and a beneficial interaction with water. Although raised beds seem to be intertwined somehow with Permaculture, sunken beds are often a better choice in some respects depending on your context and climatic factors. Although most often employed in drylands areas, they can be applied to any climate really, especially in terms of seasonal growing during either drier or hotter seasons. With the advent of climate extremes, these beds could prove to be a good precautionary choice for when drought strikes.
Why Sunken Beds?
Simply put, sunken beds act as if they were a valley instead of a mountain (raised bed). Valley landscapes tend to have more accumulated water, be richer in organic matter, and boast more biodiversity. While raised beds do have their advantages, there is the obvious pattern recognition that raised beds dry out more quickly than sunken beds. The sun is not able to hit the south facing part of the bed as easily while wind passes over the growing space instead of interacting with it in a drying fashion. Also with raised beds, organic
matter and mulch tends to slide down to the pathway over time and requires scooping it back up to the raised bed to truly harvest this carbon resource. However when working with sunken beds, the valley space accumulates organic material and does not allow it to wash away as it does with raised beds. This accumulation of organic matter reduces water consumption and boosts soil fertility, which cascades into more disease and pest resilient plants. It also brings the height of the bed quickly back up to almost a level ground surface after just a short period of intensive mulching. Furthermore, when irrigating the water doesnt shed quickly from the sunken bed making this precious resource even more efficient. This is very important in drylands areas and beyond as water becomes an increasingly scarce resource. In some cases I have even seen how sunken beds can easily be flood irrigated to create a large and even distribution of water where heavier clay soils are present. One main disadvantage of sunken beds is the ability to reach into the bed with a comfortable feeling. However by adding raised keyhole paths you can gain comfortable access without sacrificing growing space as the beds can be expanded from normal dimensions. Also sunken beds don’t warm as easily as raised beds but what I am finding in Portugal is that by April 15-20, you can still get your tomatoes in the ground early with no problem. That is a month ahead of my hometown in Cincinnati, Ohio, USA where we plant in mid May normally.
To create sunken beds, simply invert the process for raised beds. Their layout according to aspect, contour, and proximity to water resources is part of your design process. What zone they will be in, what sectors are they influenced by, and how they interact with other elements of the farm are all part of the process of design. For the part below I am referring more to the actual earthwork. However, size wise, they do often follow similar dimensions as a raised bed in terms of width three to four feet (1-1.2 m). To compensate for reaching down, you may want to reduce their width or better yet add a couple of stepping stones or keyhole paths to maintain access and space efficiency.
The first step once you have pegged out more or less where they will go is to remove the top soil. Usually you will be dealing with zero to four inches inches worth (0-10 cm). Any more and I would ask if it is even necessary for the earthwork as to not disturb richer soils. However even in deeper soils if water is scarce, if dry seasons are extended, this may indeed be necessary. Once you have removed the top soil layer store it aside where it will not be trampled on to resume later. From there dig down one to one and half feet down (30 cm-45 cm). With the subsoil at the bottom of the sunken beds bring that up to the pathway areas and begin to raise them. With raising the pathways your overall depth may go down to two feet (60 cm) in all. On slopes your bottom wall will be higher essentially making a terrace wall. From there continue the earthwork to loosen and level the sunken bed and if you have a broadfork loosen the bottom once more with depth. Once the earthwork feels more or less complete, put the top soil back in. This will mean there is some microbial life in tact and some organic matter already accumulated. While it does take more time to preserve the top soil, I do recommend this step as seeing it go into the raised pathway is a bit heartbreaking. From there put as much organic matter in as possible to fill the bed to a certain degree. I usually aim for about one foot (30 cm) in the first mulching implementation. I water this thoroughly and then apply a compost extract to reseed the microbes and speed up the healing process after this soil disturbance. The mulch should be a chop and drop of green material and be a diverse mix. The diverse approach is also true of the brown material that goes in layers with the green, which most often it is raked leaves or straw. If one has compost one can also insert in layers of this or just after the top soil. Often buying in a mix of horse manure and some bedding material is possible but do watch out for manures from animals that have been treated with chemicals. No matter the mix, in general, I leave this for six months to break down before planting. In Mediterranean climates one would want to do this earthwork in the fall when the autumn rains hit and organic matter is more plentiful. The humid winter aids in breakdown as the fungal strands have the right microclimate to really expand and start with this energy cycling. By the next summer planting season you will have built organic material and the bed in which to grow tomatoes or zucchini for example. In this waiting period I do water in dry periods and also continue to add compost extract. The more frequent the application of this microbial seeding, the quicker things will breakdown. After a growing season I do recommend either growing a cover crop or going again with a deep mulch for the winter period instead of growing a crop of say cabbage. After this second application you will be virtually at the top of the original soil level with your mulch and well on your way to growing a good crop the next spring or summer.
I have implemented this technique on several projects with differing contexts and scales. I first began to contemplate the idea shortly after arriving in Portugal in 2009. I was attending a Sepp Holzer conference on aquaculture but for whatever reason in that workshop we were tasked with reseeding a field at the host site Tamera. Oddly enough we were reseeding because the seeds were sown on the raised bed in the first seeding and largely failed to germinate. However those that did germinate were in the pathways from people dropping them or during irrigation the seeds had slumped down into the pathway. I asked Mr. Holzer about this and referred to the drawings I had seen in the Designers Manual questioning why we were seeding the mounds again when it was obvious that they had germinated better in the pathways. Tamera is in an extremely hot and dry part of Alentejo, which warrants sunken beds in my opinion. However Sepp told me to read the book of nature and throw away my Permaculture books. Well I read the book of nature there through this observation and began to implement them with passion after seeing my friends at Nangayla, Terra Alta neighbors, install their initial ones. They pumped food and continue to still with little maintenance. Admittingly the groundwater is high there, but to reach that valuable resource, the beds were sunken and have produced the largest vegetables at our site as they dont suffer from water scarcity stress.
The sunken beds we did install later at Terra Alta on our Camino das Fadas section of our food forest followed the above process. They are in their third year now (2016) and produced a nice crop last year for sure. After topping the beds up with mulch for this third straight winter with a few potato crops mixed in here and there, the soil level is already back up to the original level where we went only one foot (30 cm) deep and almost there on the ones where we went one and half feet (45 cm) deep. I have abandoned much of the raised beds at the site for any gardening besides perennials, trees, and winter veg. In this area we implemented four sunken beds squeezing them into our food forest to grow annual and perennial vegetables. One of them is on a steeper section and required us to dig deeper consequently raising the bottom wall quite dramatically. It has been the slowest to build soil but did produce an ok crop of Jerusalem Artichoke in its second year. After a third winter of intense mulching it is almost full of deep rich soil with a thick layer of mulch on top and ready for a crop of peppers (2016). We have switched many other beds in our Serpent Garden there at Terra Alta and were quite pleased with their performance in year one. The beds had already had years of soil improvement so they were easier to get back into production because of the accumulation of top soil.
Another example comes from a project that I worked on with my TreeYo running mate and good friend Karsten Hinrichs. We met many years ago at that Tamera conference and work together on designs and installs from time to time. We did one in early 2014 in Alentejo, Portugal mainly on a zone 1 garden. In much of the garden we combined techniques of slight terracing and raised pathways to create an expanse of keyhole beds yet sunken growing spaces. It turned out to be quite a beautiful garden and very productive in year one with nothing else than laying a compost/mulch mix. However we should have probably gone with a much thicker mulch to smother the rhizominous grass that we tried to weed out during the install. Because we plowed and tilled to open the soil before our hand earthworks, we propagated lots of it and should have been sheet mulched in these relatively shallow sunken beds. However due to client demand we went forward so always remember the context, climate and taking a small scale intensive approach.
With that in mind, only take on what can really be maintained and if water is scarce or drought seems to be hitting your more humid landscapes more often try them out. Even where I farmed in the humid tropics of lowlands Cota Rica (2005-2006) we had a very pronounced dry season of six months. In that case it would have been good to have both sunken and raised beds to switch back and forth from. This could be applied to the mediterranean regions where winter rains can be heavy and quite frequent in the winter. We have no problems with this at Terra Alta, however, as our soils are quite sandy with a coarse grade but mixed with clay. The winter vegetables seem to thrive just fine in them and for sure as our spring season dries out they already outperform the raised beds. Even back in the humid temperate landscape of my families land in Kentucky, with our sandy loam soils and low irrigation capacity unless we took on a pretty hefty investment, I would do sunken beds for sure for summer production. In the off season you can cover crop to help promote fertility on more of a farm scale and grow your own mulch right there in place. Finally if in extreme drylands conditions, well i feel its the best way to go. With our summer drought of five to seven months at Terra Alta, I am glad we have switched over to this technique. Have you? Let us know in the comments below.
Written by Doug Crouch
Header Art Bonita Edwards