How much shrinking to plan for?

I'm building a hugel that will be home to a couple elderberry bushes. It's in the front of the house, so I'm digging down to accommodate a deep hugel without having a huge mound. Since the bushes are long-lived, they will sink as the hugel sinks. I don't want the bushes to sink below the surrounding soil grade level, as that could put them in a muddy pit. So, I need some idea of the amount of shrinking to expect over the lifetime of the hugel/bush pairing to know how much of the hugel can be initially below the soil grade level. I realize the shrinking will be influenced by what I put in the hugel and how I pack it, but what would be a good rule of thumb?

By shrink, do you mean the initial settling phase or the total amount of size reduction over the life of the mound?

Growing mounds, depending on how they are built initially, can settle as little as 5% of the height and as much as 30% of the height in the first few years. This happens as the topping (soil, mulch, etc.) sinks to fill in the gaps between the pieces of wood. When the wood rots away, almost all the volume previously occupied by the wood will settle, leaving a small mound where a tall one once stood.

What I'm trying to allude to is the fact that the amount of settling is relevant to the amount of time post build.

Laid wood construction, (that which is placed horizontally) will tend to settle faster than standing log construction by quite a lot.

In the end run though all mounds will end up just about the height they would have been if only the soil was piled up, this could take, depending on the type of construction and the size of the largest logs used, up to 100 years (given logs of 1.5 to 3 ft. diameter, just beginning to decompose when placed, being used as the bottom layer and building up to a height of 5 feet).
There are mounds that were built long before the European invasion took place that are still here today, when sectioned for analysis these mounds are now mostly soil, some artifacts, lost to the mound during construction are in these, but a lot of the "Indian Mounds" were refuse piles (the local dump) and when excavated lots of the trash of old is still there to be found. These types of mounds do not settle as much, due to the contents and the many layers of soil (think modern land fill).

I know of many old growing mounds that I've been told were around 6 feet tall when constructed and today they are more bumps than mounds. These are on lands the US Government took and can no longer be used or disturbed in any way. If you decide to hike many of the trails found in Hot Springs National Park (Ouachita territory) and were to wander off the beaten paths, you might stumble across some of the many lost growing mounds where villages used to be. All of these mounds are not recorded, since the real purpose of taking the lands of Manataka and making the first National Park out of them was to keep the Indians out and take away their place of peace. I'm sure it also saved many lives, both white eyes and Indian alike, but this place was a "no weapon" zone for the people, the hot springs have always been a popular gathering place for anyone who found them.

By shrink, I'm mostly concerned with how far down the top of the mound will drop over the full lifetime of the elderberry bush that's planted there.

My current plan is to dig down 3 feet and build a 6 foot mound, so that initially there's 3 feet of hugel above the soil grade. I think an elderberry bush has a productive lifespan of about 20 years. So the question becomes, will a 6 foot hugel shrink to less than 3 feet in 20 years, including the initial settling?

To keep the mound above the soil grade, I'm wondering if I should build the lower half of the hugel with upright logs closely packed with soil. Maybe roots would then fill in the gaps left by the rotting wood and stitch the soil columns together.

I think the idea of using vertical stacking would perform as you desire. Usually trees are not planted on growing mounds but at the base of them, however it is very possible to plant something such as elderberry on a vertical stack mound that has already had the gaps filled with soil and mulch. I would recommend watering the mound as you add the soil so that it will go ahead and settle into the spaces better than if just spread on and only nature providing the water. For the root system to be able to anchor the tree(s) I would perhaps give a little space between the logs where I planned on placing the tree(s) so the roots can dive as deep as they want to and so hold the tree against any strong winds.

I like the idea of spacing the logs further apart under the bushes. The extra soil might also prop up the hugel more in that area, so that even if the surrounding parts fall below the soil grade, the bush's root crown might stay above the grade. Thanks.

As Bryant said, it's not really recommended to plant trees and large shrubs on the hugul bed. The elder is a reasonably light and relatively short shrub that may be able to handle the settling process. They do tend to like damp semi swampy areas which are not super stable ground. That said, I've seen many large wild red elders that are split due to snow loads, heavy branches falling on them, or unstable situations (like growing on a very rotten log). With careful observation, you may be able to remedy this somewhat preemptively with propping "crutches" and ropes to stakes or any number of similar orcharding tricks, but this is not just an issue of settling in terms of sinking (as you describe), it's an issue of the entire structure giving way underneath the weight (and wind torque) of the shrub with a large amount of it's foundation "soils" disappeared. It might be best/ easier (than propping up a problem tree) to stick to smaller shrubs, like blueberry (which can be re-stabilized easily with some coarse mulch), on the mound and plant the elder on level ground at the base on one side or the other. The Elder will naturally, search out the water/nutrients/community in the hugulkultur. Is there a particular reason why you feel the need to put the shrub on the hugulkultur?

Different elderberry varieties have greatly different sizes, some of them clearly trees rather than bushes. The varieties I've been considering all claim to top out at about 6 feet. That seems in about the same range as a blueberry bush, which I've read do well on a hugel. As you've noted, the elderberry also prefers the moist soil that a hugel provides.

I'm planning to use the edges of the hugel for actual tree size fruit trees, so also don't want to give up that space for the elderberry.

Since we're talking about it, another question I've had is how close to the hugel to plant the fruit trees. I haven't found anything more specific than "next to". Is two or three feet away from the edge of the hugel the right spacing?

Tyler Close wrote:Different elderberry varieties have greatly different sizes, some of them clearly trees rather than bushes. The varieties I've been considering all claim to top out at about 6 feet. That seems in about the same range as a blueberry bush, which I've read do well on a hugel. As you've noted, the elderberry also prefers the moist soil that a hugel provides.

I'm planning to use the edges of the hugel for actual tree size fruit trees, so also don't want to give up that space for the elderberry.

Since we're talking about it, another question I've had is how close to the hugel to plant the fruit trees. I haven't found anything more specific than "next to". Is two or three feet away from the edge of the hugel the right spacing?

Hau Tyler,
The Elderberry that you are talking about would indeed grow on a vertical hugel.

I plant trees about a foot from the bottom edge of the wood core, this means that they appear to be "in the mound" but are actually at the edge of the innards of the mound. The thing to remember is the subject trees root system, I would feel fine planting at the bottom edge of the cover (the dirt mound over the wood core), and three feet away from this would also be just fine. IF you plant your elderberry trees on the mound, do remember to take into account the shading these will provide and plant your fruit trees so they get full sun. Keep in mind that elderberry likes full sun too and so when the fruit trees grow up, will they shade out the elderberry trees?

You can build any growing mound as wide as you desire, as tall as you desire. I have seen some that are 50 feet wide and around 20 feet tall. I don't make mine that huge but I do have one I am building that will end up about 10 feet tall and 50 feet in width.

The trench my wife and I have dug is a rectangle 5 feet wide and 22 feet long, with the long side on the north-south axis. The plan is to put avocado trees on either side of the north end of the hugel. The avocado varieties are supposed to grow tall, but narrow. The first elderberry will then be planted on the hugel, about 6 feet in front of the avocadoes. Then 5 feet down from the elderberry comes a couple of dwarf citrus, again on either side of the hugel, then finally the second elderberry. I hope this creates a sun trap effect, with the elderberry in the understory. The elderberry should get plenty of sun, especially as the sun swings around and hits the hugel from the side in the late afternoon. I also want them to get some shade from the citrus, since the summers here routinely spend a few weeks in the 100s. When grown, the whole arrangement should also provide a nice evergreen privacy screen between the street and the house.

We've started laying the wood in and are using vertical logs for the lower half of the hugel, as you suggested. It's astonishing how much wood fits into a trench of this size. I've also now got a lot of black clay that I'm trying to figure out what to do with.

sounds like a good plan. What varieties of Avocado trees? I am thinking of growing a couple of these myself, I know there are varieties that can get over 50 feet tall (used to live in LA and had a friend who's family grew Avocados for markets). In my area I think I could grow some of the Haas variety with some care and winter green housing.

A lot of information online suggests that my zone gets too cool in winter for avocado, but there are a few mexicola grande trees that are doing well in my neighborhood. However, they can grow to 25 feet and are spreading rather than columnar. I've got enough space to put a 20 foot separation between them, but am going to check the nurseries for a columnar variety that's also cold hardy.

Yes, most of the Avocado trees want zone 10 or 11 as their minimum winter temps. That's why I will be putting up winter green houses over mine, it will be very helpful to their survival and productivity.

You can use pruning to tell the trees what size you will allow them to be. The end result would be a tree with many branches, all of which would produce fruits. The thing to think of here is how best to make the tree do what you need it to do and still be able to protect it when needed. I plan on using the Bonsai type of pruning, I will let my trees trunk grow to a desirable height then start pruning back to two bud sites on each branch. This will let the tree grow but it will have very dense branching as it grows older. It will also allow me to put up winter green houses around them, since I know I am two zones from their ideal habitat.

In the 6'-7' hugels I put in 13 months ago I seem to have lost about a quarter to a third of the height. In places where the logs were very tightly stacked, less.

Trees are such moisture seekers that burying wood on one side or two sides of the tree will do the trick. The tree roots will send out towards the buried wood. I base this on seeing a papaya tree sending a thick primary root into a raised garden bed that was next to it. The papaya was in a 3 gallon pot, busted out of it and sent roots first into the soil, but then rocketed roots out into the wetter raised bed. Which also had more nutrients.

Not that I am making real and proper hugelkultur mounds...but I have buried wood before planting a few guavas and grapes. Even planted them in HugelK raised beds that had lots of buried wood which would allow for sinking to soil level as the wood decayed.

Some of my trees have graft lines 3 inches above the soil level. You definitely don't want that graft sinking below soil level and getting perpetually wet and moist. It can rot it and kill the tree. In this case dig Hugel holes on two sides of the tree and bury wood in those holes. This way the tree is planted in firm soil