Trees on hugel beds

Kirby Fry from Texas recently said he would never put trees on a Hugel bed. Does anyone know why he said this?

Has anyone here planted trees on theirs? Did you notice any Ill effects from doing so?

Whoever Kirby Fry is, I agree with them.

The hugel bed settles and shifts as the wood portion rots; trees may tilt, fall over, die from disturbance of their root systems, or be left well above the final soil level where smaller plants would be less bothered.

Ideally hugels should be rebuilt on a regular basis, dependent on the wood and the climate; not easy to do with trees in them.

The rotting wood and voids can be very attractive to rodents and other critters who would be happy to destroy the root systems. Losing some annuals if this issue pops up is at least less painful.


Where I interned in 2014, many trees and bushes had been started(in 2011 IIRC) atop individual hugel-piles as part of an effort to get them above winter flooding. Issues with soil washing down and wood rotting/settling left the roots vulnerable around the edges; ducks went after the bugs in the mounds and and further damaged the roots. Many died, though it was a bad drought year and maybe this would have happened anyway since not enough labour was available to provide sufficient (hand-carried) water. If these mounds had been built with 6x as much soil, I think it would have worked better... but still not as well as skipping the hugel aspect.

I think a better way to combine hugelkultur and trees would be to implement a hugel bed/mound close enough to the trees that they will get roots into it later on, once the wood is well into decaying/settling... and since the tree won't be actually sitting on it, it will be less impacted by any shifts that do occur. In my climate wood inside a 3 year old hugelbed is close to fully rotted, and providing an excellent sponge.

Kirby Fry from Texas recently said he would never put trees on a Hugel bed. Does anyone know why he said this?

Because hugelbeds are meant for annual crops.
They are not suitable for massive, perennial root systems. Too much settling and shifting through the years.

I wonder if it would work with supplemental support, such as a few stabilizing rocks under the tree root ball? Or with yearly additions of soil to replace the settling stuff settling? Our property has numerous slash piles from the previous owner's logging, and we are turning them all into hugelkultur, and were originally planning them as the base/center for each tree in our various guilds. Most of the soil on our property is gravelly loam, with high drainage and low organic content, so not very hospitable without supplemental compost. We just got a delivery of 10 cub. yards of organic compost, which we were going to use on our "hugels" to establish our trees (fruiting cherries, mulberries, ornamental cherries, willows, fig, etc). Now I wonder how we should modify our plans to prevent the trees from losing stability over the years.

Maybe by planting the trees near the bottom on the downslope side? We are hoping to take advantage of the water storage to avoid irrigation as much as possible in the summer.

We are in the temperate rainforest of the NW, so we get tons of rain in winter and spring, but not as much in summer. Last year we had a crazy drought with NO rain in April and May (or June- midAugust), which was highly unusual and frustrating since we had built our hugelkultur in March, and didnt get a single good soaking after the compost was added. Both the cherry trees we had planted died as a result. (we are still hauling/storing all water, hoping to get a well before summer, but no guarantees).

Should I move the ones I planted about a week ago? I don't want to disrupt them, but would rather not lose them again. The hugel is about 2.5-3 feet off the solid ground at the most, with the bottom of the cherry roots about 1-1.5 feet from the ground. I sure hope they can manage that...really don't want to transplant if it can be avoided.

Seems like perennial shrubs and trees belong around the base of hugelkultur?

I think people get confused by images like this one that makes it look like the tree is planted on the mound:



I'm sure confused by it - I have no idea where that tree is supposed to be.

Hi Tyler:

I'm sure confused by it - I have no idea where that tree is supposed to be.

Judging by the branch structure it sure looks like the tree is actually behind the hugul instead of on top of it, as it shows very little trunk. I can see why this drawing might be confusing to some, but it isn't so much to me. I love these images, but can see it's potential for causing confusion.

Hi Denise and Eliza: It is not at all recommended to plant trees in a hugul for the reasons stated above by others. The hugulkultur is a dynamic structure that will shrink down over time, and it will do this in an uneven way. Trees and shrubs enjoy solid ground, so that their deep or spreading anchor roots are established only once. This stable foundation is extremely important to their long term happiness.

We just got a delivery of 10 cub. yards of organic compost, which we were going to use on our "hugels" to establish our trees (fruiting cherries, mulberries, ornamental cherries, willows, fig, etc). Now I wonder how we should modify our plans to prevent the trees from losing stability over the years.

Maybe by planting the trees near the bottom on the downslope side? We are hoping to take advantage of the water storage to avoid irrigation as much as possible in the summer.

Before I say anything else, Woohoo! 10 yards of organic compost! %$$$ &&&&& !! (me doing the happy dance for you) !! That's awesome for you.

I think that your natural conclusion to plant the trees on the downslope side is the best plan, particularly if you have water shortage issues, with one potential caveat based on this principal: There will be an increasing lens of water underneath and downslope of your hugul as time goes on; Some tree species might not appreciate that as much as others. Some might appreciate being upslope or to the side where they can seek water from the hugul, but will not have it continually delivered.

I think a better way to combine hugelkultur and trees would be to implement a hugel bed/mound close enough to the trees that they will get roots into it later on, once the wood is well into decaying/settling... and since the tree won't be actually sitting on it, it will be less impacted by any shifts that do occur. In my climate wood inside a 3 year old hugelbed is close to fully rotted, and providing an excellent sponge.

From my understanding, Dillon has it quite right here.

Now I wonder how we should modify our plans to prevent the trees from losing stability over the years.

What I would do is consider what the culture of each tree might be as an individual species and orient it around your huguls as best suits that. Some might prefer to be on the North side of a tall hugul, so that they do not come out of dormancy too early during a warm part of late winter, and then be frost killed or damaged or flower too early, for instance. There is a lot to consider with fruiting trees. After considering the species specific stuff, I would dig your hole in that specific orientation for the tree a shade deeper0 than you need (and place the excavated soil/sod down slope of the tree to retain more moisture at the tree). Sprinkle any supplements for root growth now (some folks add bonemeal, mineral dust, etc, though others find this un-necessary). Then fill the hole with water. (I know that can be a lot of hauling of water, but it's best practice in landscaping from my experience.) After the hole water disappears, some of it's sides will have crumbled in and settled to be approximately the right depth. Put the tree in, filling in any outer gaps with soil, then water these gaps a bit to blend and settle the sides of the hole and the transplant. Mulch this with your compost (gently water the compost and the tree transplant to settle and meld the sides of the transplant ball and the hole) and then add as much woody mulch as you have available on top of the compost (which will hold the majority of that moisture in the soil at the transplant (if you have a yarrow or oregano, strawberry, or thyme, plant some near the tree to start your guild system). Water the mulch too. The initial dump of water, will be below the transplant and will give it a place to head with it's roots for long term water sourcing in your deep water-table. When your hugul is established (rotting, full of fungi, giving off nutrients and water), it will be feeding (and possibly raising) the water-table. With trees it is much better to super charge with water on a very intermittent basis, and then allow the tree to almost be begging for it, then to provide steady water. Making the tree chase the water downwards, is very important for long term drought potentialities as the tree ages. Do not let the tree get dehydrated, but don't spoil it either.

Seems like perennial shrubs and trees belong around the base of hugelkultur?

Yes; that's what Sepp and many others say.

Should I move the ones I planted about a week ago? I don't want to disrupt them, but would rather not lose them again. The hugel is about 2.5-3 feet off the solid ground at the most, with the bottom of the cherry roots about 1-1.5 feet from the ground. I sure hope they can manage that...really don't want to transplant if it can be avoided.

If it's not too big a project, I would definitely consider transplanting them again. The one more time disturbance (before they have really established a long term structural relationship to the bed) is much better-in my opinion-then all the structural support you will have to supply as the hugul changes underneath it, perhaps repeatedly, over time, and the chance that the tree will not be nearly as successful if it has such an unstable foundation.

When I built a hugul at the Darfield Earthship's permablitz, we planted fruit trees and shrubs at the base of huguls, but not on them. It was explained that the hugulkultur provides a nutrient sink, and water storage, but the trees do not have to be planted on them to gain these benefits. The tree's feeder roots will be sent over to the mound, and they will seek the nutrients, or water that they need. We also built Holzer High Beds which were mounded gravels, sands, and soil but without woody cores; some of these had shrubbery plants planted on them, but they are unlikely to settle much at all. These high beds still create a lens of moisture beneath them and within them (much as large sand dunes do), though not the same as a hugul.

Hi Dillon; when you wrote this:

Ideally hugels should be rebuilt on a regular basis, dependent on the wood and the climate; not easy to do with trees in them.

I have to think that this is not necessarily the case, from what I understand. Although it is true that any alterations would be more difficult with trees growing on them, the primary concern is for the stability of the trees on the beds rather than the need to rebuild. The permablitz I went to was partly instructed by Javan Bernakevitch who studied directly with Sepp Holzer. From what he said, Sepp does rebuild his beds, but not so often that I would use the word regular (definitely too regular though for safety and stability of trees growing on them (which can grow to advanced ages), so the statement is basically correct. From what I have studied and understand the hugul beds are intended to be fairly long term structures (more than 10 years, and as long as you want). But also from what I understand though, it is not necessary to re-build them. The beds can be permanent (though shrinking) and be considered as a one time initial build like building a no-till raised bed (with a one time disturbance to the main soil structure), and simply be added to (with wood and other amendments), with no further disturbance to the established system.

Hi Eliza:

Our property has numerous slash piles from the previous owner's logging, and we are turning them all into hugelkultur, and were originally planning them as the base/center for each tree in our various guilds. Most of the soil on our property is gravelly loam, with high drainage and low organic content, so not very hospitable without supplemental compost.

Considering your lack of organic material, and that you live in an area where logging occurs, it might be in your interest to seek out a smaller mill, who often have an over-abundance of sawdust, and possibly piles of bark bits. These both will add moisture retention and heat to your composting/hugul process. If you have the truck to haul it, there is pretty much no other fee than your labor. Use this material to load the spaces in your slash piles and cover it as much as possible, and put pockets of compost in the sawdust, anywhere you want, or put lines of compost through it, and plant rows of annuals. Plant things like squash, nasturtiums, peas, and potatoes and such, in the compost pockets. Also mulch around the base of the piles with sawdust/bark mulch, so that there is a minimum of water loss. The more you can cover the wood with soil (even gravelly loam), the more the wood will be able to absorb water. Cover the slash piles packed with sawdust completely (sawing off odd up-angled bits first would probably help), with your loam, and then cover this with a dressing of compost, and then mulch it. If you don't have a truck or can't find any sawdust/bark mulch, then load the piles with your loam and put the compost on the finished piles and plant into it. If you have any mulch material at all, then seed the soil/compost heavily then mulch the surface lightly after seeding so that 50% of your soil surface is exposed. The excessive plant growth will block out most weeds, and the mulch will provide structural stability/microclimates on your soil surface.

Awesome stuff Roberto, especially the planting guide part!

Roberto pokachinni wrote:

Dillon Nichols wrote:Ideally hugels should be rebuilt on a regular basis, dependent on the wood and the climate; not easy to do with trees in them.

I have to think that this is not necessarily the case, from what I understand. Although it is true that any alterations would be more difficult with trees growing on them, the primary concern is for the stability of the trees on the beds rather than the need to rebuild. The permablitz I went to was partly instructed by Javan Bernakevitch who studied directly with sepp holzer. From what he said, Sepp does rebuild his beds, but not so often that I would use the word regular (definitely too regular though for safety and stability of trees growing on them (which can grow to advanced ages), so the statement is basically correct. From what I have studied and understand the hugul beds are intended to be fairly long term structures (more than 10 years, and as long as you want). But also from what I understand though, it is not necessary to re-build them. The beds can be permanent (though shrinking) and be considered as a one time initial build like building a no-till raised bed (with a one time disturbance to the main soil structure), and simply be added to (with wood and other amendments), with no further disturbance to the established system.

Let me clarify my initial statement a bit; I think we're pretty much on the same page though.

By 'ideally', I meant that if you want the full benefit of the spongey water-retaining goodness from the decaying/ed wood, you'll need to rebuild the hugel, because eventually that wood is no longer there; it has turned to soil. Once the wood is all converted to soil, 'all' you have is a mound, smaller than it was to begin with, of really nice healthy soil... how awful, right?

I have no intention of rebuilding my hugel beds any time soon; as long as there is space available I'd rather use the same resources to build fresh ones, and leave the improved soil for growing in, since it's much nicer than the rest of the soil I've got. Once everywhere I'd like to plant stuff has been hugeled once, I might loop back, but at this rate that will be someone else's problem!

Javan's a very nice and knowledgeable fellow; he facilitated Richard Walker's food forestry workshop here, back in 2014. My guess, though, is that hugel lifepan will vary quite a lot with location and methods. I am not familiar with how hugels age in Sepp's climate, or yours for that matter, but in fall 2014 I dug into a ~3 year old hugel bed that I had built here, in the process of converting it to a raised bed, and there was minimal wood remaining. When I added another layer of logs and dug in additional soil and compost last summer, I found no signs of wood at all. So, in my climate,using my methods, I consider the 'best' years of a hugel's life to be years 2 through 4, after which I don't expect it to really show much in the way of enhanced water retention.

My hugel was hand built, dug into the ground less than 2 feet, and around 3 feet tall; I used primarily deadfall/standing dead maple and alder logs around 5-6" diameter, with some branches and a bit of fir added, and it was pretty light on manure/soil. Sepp's massive equipement-built berms, or the ones Paul and the Ant's are throwing up all over the Lab, will surely behave differently... I have no trouble believing 10 years as a lifespan for Sepp's hugels.

I completely agree that the MAIN concern for trees/shrubs in hugels is the lack of stability. This should be enough of a deterrent regardless of other issues.

Roberto pokachinni wrote:There will be an increasing lens of water underneath and downslope of your hugul as time goes on; Some tree species might not appreciate that as much as others. Some might appreciate being upslope or to the side where they can seek water from the hugul, but will not have it continually delivered.

For exactly this reason my inclination *in my climate* is to plant trees upslope from hugels. On the scale that I'm building in, the trees are plenty close enough to this lens to access it as they see fit, with less risk of getting too much water in a crazy wet winter like this one.

Roberto pokachinni wrote:
Considering your lack of organic material, and that you live in an area where logging occurs, it might be in your interest to seek out a smaller mill, who often have an over-abundance of sawdust, and possibly piles of bark bits. These both will add moisture retention and heat to your composting/hugul process. If you have the truck to haul it, there is pretty much no other fee than your labor. Use this material to load the spaces in your slash piles and cover it as much as possible, and put pockets of compost in the sawdust, anywhere you want, or put lines of compost through it, and plant rows of annuals. Plant things like squash, nasturtiums, peas, and potatoes and such, in the compost pockets.

My biggest challenge on hugels in the first couple years is getting enough nitrogen into them. On my scale assiduous application of urine is adequate, barely. I had trouble at first wrapping my head around just how much nitrogen needed to be thrown at the hugel beds; that yellowing CAN'T be nitrogen deficiency, I've put so much urine into that thing! Must be something else! Nope, just nitrogen deficiency...

The massively increased surface area of sawdust will enable it to soak up a HUGE amount of nitrogen as it decays; I would expect it to hinder growth in the first year unless well balanced with a lot of nitrogen rich stuff. Nitrogen fixers are a good idea, but will need help at first. Mulching with sawdust on the surface shouldn't be as much of a concern, as long as it isn't mixed into the soil.

In the longer term the sawdust will all become nice soil regardless, just something to consider in the short term.

Roberto pokachinni wrote: Judging by the branch structure it sure looks like the tree is actually behind the hugul instead of on top of it, as it shows very little trunk. I can see why this drawing might be confusing to some, but it isn't so much to me. I love these images, but can see it's potential for causing confusion.

I think you're right about where the tree is supposed to be. I guess I'd like to see a view from the top to know how close the hugel is to the tree.

By 'ideally', I meant that if you want the full benefit of the spongey water-retaining goodness from the decaying/ed wood, you'll need to rebuild the hugel, because eventually that wood is no longer there; it has turned to soil. Once the wood is all converted to soil, 'all' you have is a mound, smaller than it was to begin with, of really nice healthy soil... how awful, right?

my way of thinking of it is that the really nice soil is now the spongy water retaining goodness, and that any disturbance to this to me and my own thoughts of developing no till systems, counterproductive. But that's if no till is the goal.

My hugel was hand built, dug into the ground less than 2 feet, and around 3 feet tall; I used primarily deadfall/standing dead maple and alder logs around 5-6" diameter, with some branches and a bit of fir added, and it was pretty light on manure/soil.

what was the depth of soil over your wood, Dillon? I would suggest that perhaps the soil depth was not enough, so that the plant roots were directly competing with the wood for nitrogen. I'm curious about your nitrogen draw-down to your wood that seems to be what you are referring with your yellowing leaves in the next quote:

My biggest challenge on hugels in the first couple years is getting enough nitrogen into them. On my scale assiduous application of urine is adequate, barely. I had trouble at first wrapping my head around just how much nitrogen needed to be thrown at the hugel beds; that yellowing CAN'T be nitrogen deficiency, I've put so much urine into that thing! Must be something else! Nope, just nitrogen deficiency..

my way of thinking of it is that the really nice soil is now the spongy water retaining goodness, and that any disturbance to this to me and my own thoughts of developing no till systems, counterproductive. But that's if no till is the goal.

Seems reasonable to me. It would be an interesting experiment to see how a no-till hugel would compare with a rebuilt one. My expectation is that the partially decayed wood will hold more water on a per-volume basis, but I have a hard time see that this would justify rebuilding the hugel.

what was the depth of soil over your wood, Dillon? I would suggest that perhaps the soil depth was not enough, so that the plant roots were directly competing with the wood for nitrogen. I'm curious about your nitrogen draw-down to your wood that seems to be what you are referring with your yellowing leaves in the next quote:

I would certainly expect that the roots were making it down to the level of the wood, but I would think that to be the case in all hugel beds, barring an abnormally shallow-rooted crop... Certainly the 2nd one that I built had more soil on top of the wood than most other hugels I've seen.

As mentioned, adequate HLF resolved the issue, and obv. I expect it to be a decreasing issue as the wood decays. Many references to hugelculture make the point that performance won't be so hot the first couple years, and this certainly matches the behavior of my first one...


In the first hugel I built, soil was as noted sparse, and little manure was added. Maybe 250L of manure, mixed types, for a ~14' hugel bed. Soil around 6" deep on top of the soil to begin with.

The second one, built last year, was about the same size, and I had a truck available, so I added a good 3 yards of well composted cow manure this time. I also brought in enough soil that between this and the manure I had about a foot above the wood. It produced well once I got it through my head that it needed about 0.6 people worth of urine on an ongoing basis.

Only other thing that comes to mind is there were woodchips mixed with the manure during composting; it's possible the ratio was such that little nitrogen was available for the wood in the hugel. Unfortunately this is the best source I've got at this point for manure, as I worry about persistent herbicides and other badness from most sources.

Many references to hugelculture make the point that performance won't be so hot the first couple years, and this certainly matches the behavior of my first one...

I think that there are two 'behaviors' that one might not expect to be adequate in the first few years, one is nitrogen issues, and the other is wood needing to break down more before it will hold water. You must really have had some rapid breakdown in your piles, Dillon, from what it sounds like. My hugulkultur experience is limited to that workshop of establishing one, so I'm certainly no expert in this regard, but I do have a pretty extensive experience with both composting and carbon/nitrogen soil building in the garden, as well as studying forest ecology.

My thinking is that the piles with shallower soil covers will tend to draw nitrogen from the plant root zone system, but those with much deeper covering soils will gain their nitrogen through fungal interfaces in the wood as it begins to absorb moisture. The carbon/wood core itself can only source nutrients from so far into the soil before it gains fungi and begins charging with moisture. If the soil is deep enough, the plant roots are only dealing with soil for the first couple or few years, but as the hugul develops it's full fungal interface and that networks into the soil, then it will be providing moisture and nutrients to the soil food web and thus the plant systems. That's how I understand it anyway.

The second one, built last year, was about the same size, and I had a truck available, so I added a good 3 yards of well composted cow manure this time.

If the manure was not composted, it would provide a lot more nitrogen to the wood than composted manure.

Only other thing that comes to mind is there were woodchips mixed with the manure during composting; it's possible the ratio was such that little nitrogen was available for the wood in the hugel.

Definitely the wood chips would draw a significant amount of nitrogen from the manure in the composting process. The composting process creates a lot of nutrient availability, but this is balanced by a lot of nitrogen being used to deal with the carbon in the chips.

It produced well once I got it through my head that it needed about 0.6 people worth of urine on an ongoing basis.

When I was building that hugulkultur with a large group of people we had urine stations which were all dumped on the wood and sawdust core (which was already moistened with water) before we began to add soil.

It would be an interesting experiment to see how a no-till hugel would compare with a rebuilt one. My expectation is that the partially decayed wood will hold more water on a per-volume basis, but I have a hard time see that this would justify rebuilding the hugel.

It would be an interesting experiment. My experience with rotting wood is that it has an incredible capacity to hold moisture, for sure. Instead of re-building, one can simply build another hugul on top of the previous one, thus not losing any of the good that has been created in the soil of the original, in fact, I'd suspect that the lower hugulkultur would rapidly inoculate/colonize the new upper one with fungi and other microbes.

This is a great point. I will move my seedling trees off of the hugel in the fall, when it is not so dry and crispy. I have let rambling thornless blackberries, thimbleberries, and salmonberries crawl up there, which they seem to like. They don't have extensive root structure and are mostly biennial in duration, until they start a new cane.  I'm thinking about adding some herbs, on top, which can tolerate Western summer dryness and generally don't like flooding winter wetness.   I think I might also put a rockpile up there, because it is higher and gets more direct sun early in the morning to attract cold blooded reptiles.  They might find homes there and eat my slugs. Plus, they're super cool.
John S
PDX OR

i planted fruit trees on the edges of my hugelpiles.
i lost 2 this winter to rodents. no good deed goes unpunished- they burrowed in behind logs that i put around their bases to keep me from hitting them w my bobcat.
most are doing well, tho. they got through a hot dry spring w almost no watering.

Hmm, so burying large pieces doesn't work well, but what about mixing in some wood chips in the planting hole and also thickly mulching with wood chips/twigs/leaf combination? I know everyone says mixing in wood chips is a no no because of nitrogen, but why simply not add more nitrogen (like urine) for the first few months. After that, it will give nutrients instead of robbing. Here is a link to the blogger who quite successfully experimented with this:
http://lowcostvegetablegarden.blogspot.com/2012/10/wood-chip-soil-pictures.html
He didn't do it with trees, but chips will not create much soil shifting, I think. I am experimenting with some chips for my potted trees, and so far they are doing very good. I also planted some strawberries a bit over a month ago -one pot has about a third of wood chips mixed with soil and on top, and other one just on top. They both got urine, and both are doing great. I have to remember to give them more soon. Actually they are greener than they were before. I also mixed chips in for alpine strawberries and they are doing great too.

Linda chalker-Scott has done a lot of research with Wood chips.  They aern't ready to be used by plants or microbes if put into the soil.  If you leave them on the surface, they will slowly decay and become ready to be used by all the microbiology when the worms have decided to take tiny chunks of them into the soil at night.  By the way, earthworms are my favorite invasive species.
John S
PDX OR

John Saltveit wrote:Linda chalker-Scott has done a lot of research with Wood chips.  They aern't ready to be used by plants or microbes if put into the soil.  If you leave them on the surface, they will slowly decay and become ready to be used by all the microbiology when the worms have decided to take tiny chunks of them into the soil at night.  By the way, earthworms are my favorite invasive species.
John S
PDX OR

Yes, that is the general opinion. However the guy in the link I gave you, had great results with them mixed into the dirt as long as he gave high nitrogen stuff for the first two months. He just used urine, but any high nitrogen stuff would work most likely. No research is ever perfect and works under all conditions.

We are utilizing large hugelbeds as "nurselogs"  with the primary goal to create a fertile microclimate for the trees longterm.  However, we NEVER plant large trees on the mounds themselves.  One should really, never, plant trees in unconsolidated earth. Whether it is a swale mound or hugel bed. They will most likely blow over when their tops get big enough to catch the wind.

We dig right where the mound reaches the ground, right in the "crotch" of the bed.  I'll try and find some pictures.

here is a picture of a hugelkulture garden with young trees planted at the foot of the beds.

Dillon Nichols wrote:

My biggest challenge on hugels in the first couple years is getting enough nitrogen into them. On my scale assiduous application of urine is adequate, barely. I had trouble at first wrapping my head around just how much nitrogen needed to be thrown at the hugel beds; that yellowing CAN'T be nitrogen deficiency, I've put so much urine into that thing! Must be something else! Nope, just nitrogen deficiency...

The massively increased surface area of sawdust will enable it to soak up a HUGE amount of nitrogen as it decays; I would expect it to hinder growth in the first year unless well balanced with a lot of nitrogen rich stuff. Nitrogen fixers are a good idea, but will need help at first. Mulching with sawdust on the surface shouldn't be as much of a concern, as long as it isn't mixed into the soil.

In the longer term the sawdust will all become nice soil regardless, just something to consider in the short term.

I second that one. Mixing sawdust into soil makes soil unusable for growing things for a very long time. I knew someone who spent years fruitlessly trying to convert old sawmill land into a garden. Rotting logs on forest floors apparently harbour huge quantities of nitrogen fixing bacteria, but perhaps grains of sawdust don't

We put in a series of fruit trees a couple years ago as part of a large hugel. I found a group of urban permaculturalists (I believe in Philly, trying to find their website right now) who had made low-lying hugels in an 'O', with the tree planted in the middle of the 'O'. This was entirely covered up with soil. We incorporated  this design for group of 5 dwarf trees. So far they are doing very well.

Edit: This looks like the site

I should note that all the wood used was in an intermediate state of rot from being in a woodpile for a few years. None of it was anywhere close to being fresh cut.

I've found that if you plant your trees FIRST, then you get them established in the native soil that they will live in for the rest of their lives.  THEN build a hugel a short distance away.  This does a couple of important things.

1.  It will not tie up the nitrogen in the immediate root zone of the tree.

2.  The shape of the hugel will shed water down toward your tree, which is essentially at the base of the "hill".

3.  As the hugel matures and the fungal network is established within the rotting core of wood, the tree roots will be drawn outward to the moisture and fungal colony below the hugel.  Thus, it's an incentive for the development of tree roots outward.

4.  As the hugel completes it's lifecycle and needs to be rebuilt, you don't have a tree growing right in the middle of it.

I've done this for years with large slow-rot/cold compost piles.  I build them away from the tree, but right on the edge of their drip line --- often halfway between two fruit trees.  After 2 or 3 years, that pile has mostly decomposed, and the trees have benefited from the water retention, nutrients and fungal network.

On my hillside, I build brush-piles/hugels BELOW the trees -- not above them.  I do this for a couple of reasons.  First, the down-hill side is south, so that's where the hottest sun bakes the hillside.  Having the hugel below the tree provides shade for the tree roots.  Second, it's nice to stand on the hugel and pick the fruit or prune branches.  It's a steep hill.  Third, if things crumble and move, they move away from the base of the tree, not toward it.  Finally, when you chop and drop, you don't want those branches immediately underfoot under the base of your tree -- so pushing them downhill gets them out of your way, while it replenishes your brushpile/hugel.

Here is our sea buckthorn (seaberry) plantation on hugel culture terraces for the first few rows.  This was to raise the land and keep the plants higher than the saturated soil below. Plus it was to minimize the slope as this corner was too low in comparison with the rest of the front of the property along the ditch.   The sea buckthorn will become a hedge sheltering the property from the prevailing winds.

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Go Permaculture Food Forest - our suburban permaculture journey