Root re-enforced earthen structures.

Just like it sounds.
Jute bags of earth,with willow between each course.
Twin lines of willow stakes,backed by cardboard, filled with soil.
Willow grid atop a tarp roof ,mounded with soil.
OK,  could work.
But what other plants would you include aside from willow?
Grapevines?
Alfalfa?
Elderberry?
Alder?
Nannyberry?
What do y'all think?

It would help if I had some idea of what you want this structure to do/be?
Be a cold cellar - I can picture that, but I think critters might be *really* motivated to dig into it so if it was me, I'd want to line it with the heaviest 1/2 inch hardware cloth I could buy.
Be living accommodations - cool. I've seen something like that somewhere on the web, and I'd recommend you look for ways to include a thermal break and insulation unless you tolerate quite cool living arrangements. I'm picturing a variation on a WOFATI only the supporting structure is living.
I've got a vague memory of someone suggesting that with enough roots designed into the project, the roots would become a solid living roof with no cracks for water. What I'm less sure of is whether this was "theory" or actual proven results. For a sheep hut or cold cellar, it wouldn't matter and be a great experiment to try. I'd love to see pictures of it at different stages.
I'd also suggest you consider lifespan. Willow grows fast and tolerates being messed with, but there might be longer-lived trees that would still tolerate training but would take longer to create, but would also live much longer - a project for the future, rather than the present. I don't know if there's a tree that would fit that description, but it would be cool for someone to inherit 100 years from now!

I'm always very interested in this sort of topic/experiment.  One concern would be how much a willow would want to root through a tall earthen wall or berm as I think of willow liking moist soil.  If that's a big issue perhaps a plant like goumi might be helpful as it's doing very well in fairly dry soil at my place and fruits like crazy, but pretty sure it won't root easily like willow, so you might have to use a bunch of rooted plants.  Elderberry roots very easily, but I also associate that one with moist soils.  In a warm environment figs would root really well and they don't mind dry soil.  

What do i want this structure to do/be?
Good question!
I was just musing on the nature of earthen structures, and trying to think of easier-cheaper ways to do them.

Walls have so many uses,  with many functions within each of those uses.
Security, privacy, thermal moderation, wind protection,  water exclusion...

I'm planning a 200 square foot shed.
Pallets fit the budget, as do tarps, and I already know that that can work out quite well.
But tarps disintegrate in the sun and pallets die from water and micro organisms.
So, how about a triple thick pallet wall, filled with rocks, soil,  woodchips, and planted with willow,  grapevines, elderberry and alfalfa?
Plant both horizontally and vertically, and plan on watering in the beginning.
Hammer some willow all the way down into  the ground underneath the wall,  to give it a head start on rooting there.
Plastic on the inside of the structure , to deal with damp.
Eventually,the pallets embedded in the earth walls would fail, but stakes could be driven into the wall as new anchor points,   or roots might be available by then.
Three layers of pallets makes for a wall foot thick.
Thicker might be better,  but you could add layers as you go.
I really like that idea,  and it makes me think of stepping down each additional layer by half a skid, creating something of a slope.
As the pallet wood and steel disintegrates, the plants are there to hold the soil together.
The roof could start with heavy duty rafters, add a roof deck of pallet stringers and layers of carboard and plastic.
The slightly peaked roof, edged with stringers,  would be covered with soil and plants.
It might be worth it to mix in perlite, vermiculite, or biochar to lighten the load, but ultimately the idea is to have soil protect the plastic and the plants to hold the soil in place.
Unlike most earthen structures,  we would want water to hit the walls,  to encourage the growth of the trees.
In fact,  peeing on your wall might be a good idea.

I am inspired  by the post mentioned above:
https://permies.com/t/27725/Photos-growing-eco-buildings

Konstantin Kirsch, have some great wood working skills .
Me, not so much.
Thus, I would like to mimic him using  pallets


Posts like these by  Alder Burns  also inspire possibilities:

Alder Burns wrote:You might think along the lines of the carpet-sandwich idea for pond liners.  I've roofed a couple of cabins with a similar technique.  Basically you protect a couple of thicknesses of heavy plastic with carpet (or something else durable and puncture resistant....I've uses silt-fence fabric in a pinch) on both sides.  In a pond, algae and mud build up on the upper carpet.....and soil and rocks can be put on top of it where it is above water, mostly to protect it from slowly degrading in sunlight.  For the cabin roof, I laid the plastic in overlapping courses on a deck of cardboard placed on the frame, and then covered this with overlapping courses of carpets, which were then stuccoed with a soupy mixture of cement, which hardened in among the carpet fibers and formed a rigid surface.  Moss eventually grew on it, and I imagine some soil could have been added and a living roof done that way.  The roof, of course, had some slope to it......

The lack of used carpet availability near me, points me towards using roots to keep soil in place.

The idea seems sound enough to at least try it, so I'm going to do just that.
I've been focusing on plants that grow well from live stakes, this,  elderberry for instance.
Some dogwoods are also possibilities, but not the ones that  are known as food plants.
Willow has a well deserved reputation for being malleable, but it's actually too damned vigorous for me to want to plant near my house.
I will use it in my other property, but not yet.

William, I'm really glad you found that post I referred to so vaguely - I'm not at all good at finding things on permies!

Have you considered cutting some skids narrow and making the skids into boxes like planters for the walls? I've used skids for a number of garden jobs and many/most are treated to slow decomposition and filling them with soil is a struggle - it either falls out or leaves air gaps. If you built the walls more as a series of raised beds - think honey comb but square and vertically oriented- you'd get good width for lateral support I would think.

The thing I always wondered about with the original pictures from Konstantin Kirsch, was why the stems of the willow wouldn't rot where covered with soil, before turning into root? I'm not familiar with willow's characteristics all that much. I've been told that "burying the roots" of an established tree will kill it - but is that only true of some trees? Or an urban myth and what really killed the tree was damaged to the roots further out? Your idea of having some sort of a sloped wall by stepping down the skids would allow roots to get air at different levels.

If you're wanting a tree to grow a long way with less air exchange, would a tap-root type tree do a better job?

Please understand that I'm just trying to give you ideas to think about - I'd love to try something like this but feel like I have more questions than answers!

If anyone has ideas of a tree that would do this or even a living fence that would tolerate summer drought and low light levels, I've got spots I want to try this sort of thing. Using skids as temporary support  and deer protection would help here. Many plants that are supposed to root easily seem to struggle in my ecosystem, but maybe I need figure out the exact right time of year to try starting such a project.

Willows root more readily than nearly anything out there.  In my neck of the woods, they use them for river restoration, to keep river banks stable.  My wife worked on one of those projects, where a farmer had cleared down to the river.  He had been resistant to revegetating it until a large chunk of his land fell into the river due to a flood event.  Basically they just wove the willow sticks into a "mattress" and buried the whole thing.  Two years later it was a dense willow thicket.

Here's a company that does some of what Jonathan is talking about:  Salix

And here's a few pictures of one method they use....willow spiling:

As first made.


And after 3 years.

Wikipedia page on spiling gives some details on the construction method for these.

Now this has got me thinking about a duck house, walls and roof made of pallets with willow grown down through the pallet walls then woven through the roof and into each other to make a snug as a bug living duck house.

Hah! Or a wall to keep the world out.

Pallet-framed rammed earth core, tamped earth fill in and atop the outer pallet layers, a variety of trees chosen, some with taproots to pin corners together, some with dense root mats for roof or rampart structure and for holding on to earth, and probably some with thorns, flowers, perhaps fruit. Interdicting human and large animal access would prevent mechanical damage and ensure sediment from runoff remains trapped on the slope.



Realistically, this could be used to grow reinforcement into and over any appropriately designed structure. I think I see a problem with the idea that you want water to hit the soil covering directly, in that the first good rainstorm will wash you out. I think I would probably, at very least, explore what raw woven fibrous matting can be made from in situ resources that could be pinned down into the earth covering until such time as it is overgrown and rots; more likely, I would mix seed and water muddied with clay in an appropriate sprayer and I would hydroseed the surface, such that the clay mud mist creates a cohesive outer layer under which the seeds would germinate. I would then cover with mulch, or a made-to-decompose thatch roof.

-CK

I read an article exploring alternatives to willow as easy rooting plants for riparian restoration a while back, and I think the authors mentioned redtwig dogwood and twinberry as two good alternatives.  I know firsthand that twinberry roots nearly as well as willow from woody cuttings.  I bought two last fall for hummingbird/bird habitat plants.  The deer killed one right away, so I took 3 cuttings from the other one.  Then, in February I knocked one of its two branches off in a composting accident, so I threw that in a pot as well.  All of them rooted, so now I have 5 bushes.  The original plant was a single foot tall branch in March, and now is a 3 foot tall multistemmed bush.