You can use earthwork to remedy many different situations in gardens. In drylands, sunken beds, as opposed to raised beds, help retain moisture by keeping soils cooler, protecting soil and plants from wind and capturing water during rains and condensation.
Though we get 50” of rain per year, our farm is located on a sand dune that is 70 feet of almost pure sand. The drainage is extreme, and in the dry season, which is 8 months per year, the sand wicks moisture away from tree roots when the wind isn’t evaporating it.
We thus find drylands water retention techniques useful. We have implemented a modified, much more shallow version of Mollison’s “donut” (Images A and B) in Florida, as long as drainage is sufficient. In wetter areas of Florida, we tend to build up on mounds rather than go deep to keep the roots out of perpetual soggy soil unless they like that.
At Pine Ridge reservation, we planted trees in pits created by a post hole digger. This worked very well to protect them, keep the soil from drying out in the heavy winds, and capture more moisture and nutrition. We also dug post holes on either side of the tree and filled them with wood chips and organic material. That was the equipment we had, so we improvised. Those holes captured water, shaded and protected it, allowed organic material to build, and offered roots a place to access it all.
We also discovered that tipi poles inadvertently stored around a young tree protected the soil around it enough to nurture the tree. They caught some of the organic material and soil that blew around and allowed it to build around the tree. They shaded the soil, protected it from wind, and therefore allowed moisture to condense around the tree as well. In other words, they acted as nurse logs do, in a forest for young seedlings and trees.
The tree that was thus protected grew about twice as big the next year as other trees planted at the same time and is now more than twice as big, though the poles have long ago been moved. Nurse logs placed around trees are very useful in a system like this, where the soil is so exposed to elements.
This tree is now big enough to provide a bit of shade.
We added logs in a fish scale pattern, and rocks near other trees to help capture water and nutrients (this earthwork is in progress, the line of rocks and logs were moved closer to the tree and in a V shape). The piles of rocks are for condensation.
This earthwork also helps hold down weeds, though the weeds also make great chop and drop. We used wood chips to hold down weeds as well, which worked well as they didnt’ break down for years at this drylands site.
In drylands, rocks can gather moisture from condensation collected from underneath them at night. When placed around trees, they can collect enough moisture to make a difference. This is a technique used by a number of indigenous tribes.
For the garden at Pine Ridge, we used earth berms similar to drawing (F) and tire berms to protect veggies from harsh and drying winds. We built a large swale for the food forest that also protected plants from winds as well as capturing water, aiding their establishment. This swale was another place where trees grew bigger than ones planted in an open pasture.
The lesson was, it was well worth it to create a microclimate similar to a forest climate, focusing on protection for the soil, around trees in a dry prairie system. We’ll go more into this in the section on food forests.
Feel free to combine different water capture techniques.
We used a similar approach to the one in the above drawing for our polycrop orchard. We put the logs in the swale, filled it with wood chips and planted trees and crops on the berm. That’s because we have extreme drainage on our sandhill. The swale will spread the water just enough to soak the logs - on our sand dune, water tends to move straight down through the sand as opposed to moving downslope under the surface in a lens. But we know we can hold water in logs and wood chips that roots can reach.
Again, solutions should be site specific. What works in a northern climate and clay soils isn’t necessarily going to work in Florida. Thus far, the modified system has worked great and our young mulberry trees have gotten sufficient water except for our recent severe drought - they were watered 2X in six months of drought, and fruited copiously. We will continue to observe and test this system and any lensing effects.