Hugelkultur, small branches, and the plants that thrive on it

Hello. Spring is getting closer and closer, and I still have a lot of tree branchs in my garden, from when we cut up some trees. The branchs are not big, decades old logs; some can be about an inch thick, but those are more the exception than anything; most are much smaller. So what I am doing is cutting those branches, and burying them in the hope to have some form of hugelkultur going on.

I have three questions.

Is there any size the beds need to have for the process to work properly ("heat" the bed, decompose properly, hold water...), any quantity of log required ?

Then, are there any plants that are not recommended to grow on those beds ? Root plants obviously, but are there any medicinals, aromatics... that will hate the place ? From what I know, there might be some nitrogen deficiency as the decomposition process will take some of it, but it'll be "given" back later, so maybe some plant that are obsessed with nitrogen just won't like it. Which one ? I've already put some strawberries on a few of those beds, but since it's winter, not much is happening. Can squash grow on those kind of beds ? Can cotton grow on it ?

And about the whole nitrogen stuff, should I add some nitrogen fixing plants in there ?

Thanks !

In general, in my experience, any woody debris will work to create some measure of hugel activity. With the size branches you are talking about the effect will be limited and relatively short lived because there isn't much volume to act as a sponge and it will also break down much more quickly than a large log.

I frequently bury wood/sticks similar to what you are describing in my kitchen and forest gardens.
I don't necessarily consider it to be a "hugel," per say; I just dig a shallow hole, bury them, and hope they help hold water in the soil.
A lot of the stuff I bury is the bigger pieces of sticks that were part of the wood chip mulch, and are still on the surface when I replenish my deep mulch in fall & spring since they get in the way when I'm planting. The rest is usually from the sticks/branches the trees shed during the year (that I usually end up tripping over).
Now that I'm finally getting a strong fungal network in the soil, the smaller sticks, and those that are dead/dry are usually totally broken down by the next growing season (fall & spring here). I've not yet noticed any issues with nitrogen being tied up, but I also usually work some rabbit manure, coffee grounds, etc. in the top inch or two of the soil, which may make up the nitrogen. I also don't put any totally fresh wood under the soil, and even the wood chip pieces that were totally green when chipped have been sitting on the surface as mulch for several months before burying.

Anyway, it does seem to help with moisture retention and (hopefully) it's helping me to build healthy soil.

I have made well over 10,000sq ft of hugel beds of varying heights from 2-7ft. While any height can benefit from the organic matter, nutrients, drainage, and water retention of the wood, I see benefits in going up as high as you can easily reach (5-7ft) in moisture retention, soil warmth, and getting above a high water table if thats a problem. Anything but treated, tainted, or well-preserved woods like locust, cedar or redwood will do in my observation. The harder the wood the longer it will take to really take off but the longer it will last and benefit.

Strawberries do great on hugelkulture, and I have been told by large scale producers Nitrogen, Calcium and Magnesium are its primary nutrients required, so maybe that is a counter-example to the nitrogen lockout hypothesis. I think the lag in taking off is related to the soil we put on it.  The first year seems similar to the productivity of the preexisting soil I put on the wood, with the added benefit of some of the greater moisture retention hugels are famous for. By year 3-5, it seems like it doesn't much matter what soil I put on the wood originally, its all good. If abundant woody debris is available, I would go as big as you can with hugelkulture, and renting an excavator can definitely pay off quickly.

I get a lot of coffee grounds, so I could also add that to my beds then. That's a good idea, as I am getting too much coffee grounds to feed my oyster mushrooms.