Hugelkultur using Palm trees?

Hey all,
I have a small yard that's only been growing 2 things since I got here, Bermuda grass and palm trees, both of which I consider useless and both want my expensive California water. As my water bill is $135 every other month BEFORE they add the water cost that I actually use, and we average 11 inches of rain each year here... I'm hoping to improve my soil by adding organic matter. Perhaps I could solve several issues at the same time, by cutting down the palm trees  and burying them under what little soil I have, and continue adding the free compost I can pick up from the city?

I've heard from a landscaper that the palms don't decompose well, so they tend to not put their cuttings in the green recycling. But if it takes longer to break down in the soil I don't think that would be such an issue for Hugelkultur beds, so long as they eventually do and also help retain moisture along the way. So far the first baby steps have included scraping back the few inches of topsoil next to the patio, tossing some palm trimmings that I had in, and then tossing the soil back over along with a load of compost. That was followed by putting together a small raised bed made from cedar and corner supports held in place with rebar. That bed was filled with chopped up dead bushes I had out front, and then covered with another load of compost.

So phase 3 would be more significant by removing the palm trees if they could be used effectively. Being massive they would certainly add a lot of material to the ground, and removing them would increase the water in the soil for other plants. Haas avocado trees do well here, and getting fruit for 10 months of the year would be great. They like a well-drained soil, so if I can build up the yard a bit and then plant the tree where it can get to the water that would be great. My only current source of organic matter is picking up compost that's made by the city, and perhaps I divert some liquid fertilizer that I currently flush away with my drinking water.  

The starting soil condition: dry, full of dormant Bermuda, patrolled by the faithful hound

A few cuttings tossed in (I know it's not Hugelkulture), which later had compost added

Chopped up bushes from the front were piled into a raised bed

The bed made from cedar boards, I dug into the existing soil a bit them added carbon and compost

So if anyone has any tips regarding the use of palm trees in Hugel beds I'd appreciate that feedback. The trunks are about 12-14" in diameter and 20-25' in height plus the canopy of big leaves. I have 3 in the back and 2 in the front strip between sidewalk and street. I'm thinking the tough outer husk on the trunk would really impair the process, so perhaps I saw the trunks down the middle to expose the insides? Or I could buck the trunks real short to expose more cut ends?

I should also add that I can also get rough mulch for free from the city, so another option would be to remove the top soil in a given spot, toss in a pile of the woody mulch, then cover it with the top soil and compost mixed in to reach a proper ratio of wood/soil, plus a bit more height.

Since I have several years (5-6) before I would retire, I could just remove the palms and try them out to see what happens. That amount of time should let them break down if there's enough moisture, I just want to be sure I'm not selling the house with large mounds in the yard confusing buyers.

Hi;
I generally favor palms. I think they fill a significant niche in Mediterranean garden. They can grow tall with umbrella shape crating a huge shadow. Shade is really important for water-wise garden. They do that without lower branches, which enables you to grow various crops in pots beneath (or palm roots find a way to get in). They have a different root system, like a weed I would say. It is hard to dig around full grown palms because of many finger-thick roots in all directions and all levels. Its canopy concentrate rain over their trucks. Above all, you can hang bird nests and similar items enabling a 3-d garden layout. Sparrows and many other birds make nests and find protection. They are reliable organic matter producers. Even though the fonds are hard to pass through a wood chipper (the fibers wrap around the blade-plate and choke the engine), the resultant material worth the effort. It does not last long as wood chips but they are soft, with good water holding capacity and without seeds. I use them around dwarf fruit trees. By palms I mean palms like date palm, Phoenix Canariensis or similar. Not something like bananas (musa) or similar, that I saw in the pictures.
But..
If you want to use them in hugels, they are slow to decompose. You can use them at the base level to increase water holding capacity. I don't think they don't help much with nutrients but they soften considerably. Roots can easily penetrate they in 2 years. Put a lot of manure (one wheelbarrow-full for each 3-4 Ft) over them to speed up the process and increase nutrients. It is better to chop fonds to speed up the process. If you want to pass them through a woodchipper, either use them really fresh (like a day old, tops) or when they are very very dry. Also, exposing insides will speed up the process considerably for sure, but it is quite a work!

About getting the garden ready in 5-6 years: The perfect way to dig small basins (10-12 Ft diameter, with 6ft deep) and fill them with logs and etc to create buried wood beds. You can search permies for that method. OR cover the whole garden with compost first and them with woodchips. You don't have to do any work other than that. To speed up this process you can pass with a broad fork, flood the garden (you will do this only once) cover it with compost (this step is to bring in organisms, fungi etc) and then cover with coarse chips/ rough mulch. I would not add 1 Ft of mulch in one go. It will gradually build up.
Hope it helps!