If you are sure it is well-drained and not apt to holding water, the tree will do much better if the DG is broken up and amended.

The best tool for this is a pressure nozzle on a pipe, like the landscapers use for boring a tunnel under a sidewalk for running a pipe under this. You find a kit like this at Home Depot that glues on a 3/4" PVC pipe.

I'd add a boatload of compost to it.

Do you mean you would attach this to a hose and use water pressure to break it up? IME you need a lot of mechanical force to break it up. A pick axe or a crow bar work, but very slowly.

Matthew:

The other possibility is to make raised beds of the best topsoil you can find. This could be a mound for each tree or a berm for a row of trees. Bring in as much soil as you can afford, up to about 18-24 inches, and combine it with a good drip irrigation system to rewet that soil frequently. I like the drip tube with built in emitters on about a 12-24 inch spacing depending on soil.

This would also allow you to do some landscapping in the process if needed.

Your current soil sounds too hard to break up with anything short of a backhoe. If a backhoe will do the job you could combine some new topsoil and some loosening of what's there.

Review this info
http://www.ext.colostate.edu/mg/gardennotes/213.html

Here is a link that might be useful: planting in various soils, including DG

Update.

I was able to dig a few holes myself to about 6" and 24"+ wide and then I filled them with water and let them drain a few times. They drain within 3-10 hours. After a couple soaks, I was able to get one of these holes to 15", others to 10".

Where I want to put dwarf Reed and Fuerte avocados I could only get 3" down. An asphalt driveway was here before which probably compacted things. I had a landscaper use a breaker hammer to dig large holes (~18" deep, ~36" D).

Here's where my problem is. I filled these with 12" of water and two days later the water has only drained a couple inches. This surprised me because all other holes I've dug into the DG have drained well even though they seem like the same material.

I'm thinking I should build up the ground and plant them above grade. I know avos have most of their roots within the top 6" and the area has a few inches of existing soil, plus a thin layer of leaves and then 3" of fine bark mulch I put down. The hard DG also softens over time with water and organic matter, right?

Is this a good plan? I have broken concrete available for back fill. I think I should layer it with soil to eliminate air pockets instead of making a dry well which I don't think will drain.

Lastly, is this a good idea or am I fighting a losing battle? I chose avocados because avos are evergreen, this is an ornamental part of my property and we love and eat them every day. However, I don't want to kill two expensive trees.

One hole has the wheelbarrow over it.
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The other hole has some rubble in it (an attempt to raise the water level to possibly more porous soil.
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Proposed plan:
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That hole that doesn't drain is nothing more than a bathtub used to drown plants. Your decomposed granite isn't decomposed. It's just plain old impermeable stone.

Given that, you need to quit digging holes, or given the slope that you have, at least dig out the bottom side to allow water to drain down out of the "tub".

To provide root volume either make raised beds or holes that drain out the side, ie downhill.

In your picture above the rubble does nothing but fill in a hard won hole. You might as well fill the hole in with concrete if you don't drain it downhill.

I also think I'd amend the existing sandy clay heavily with better soil and compost. Enough that water drains downhill through that zone much better than now. Pour on soil and compost and till it in right down to the rock. You say 3" mulch now. Make that 6 inches topsoil and mulch and till it all in as deeply as possible. That should give you enough well drained media to grow good plants.

Yes, the rubble is just to undo the digging of the bathtub, so that instead of water filling up that area, it will drain downhill through the more porous layer.

It's depressing given the time and money it took to dig those holes. Who knew they wouldn't drain like the others?

So, how is this plan: Those holes are 36" wide. I pull back the mulch in the surrounding two feet so I have a 5' diameter area. Fill in the hole with existing soil/rubble, no air pockets (rubble is used because it's free and takes up space). Then make a mound of 6" topsoil/compost in that 5' diameter and put the 3" mulch back on.

That big hole with the rubble in it is on a steep slope. All you need to drain that downhill is a narrow slot filled with porous mix. At least provide drainage as far down as you can. Your drain would just need to be level or slightly downhill.

I understand.

The only issue is this stuff is incredibly hard to dig through. I'd need to get the landscaper back here to dig that 10' long trench which will surely cost a couple hundred dollars. At that price it'd probably be easier to bring in the new soil.

I think you're probably right. But still dig down as far as you can by hand. Save the upper area of the hole. You're trying to do it right. That spells future success, good luck!!

I live in San Diego and have the same type of soil. DG, with a little bit of clay, no organic material. It turns into concrete when it is completely dried out.

I originally dug holes in my yard in the middle of summer and nearly killed myself trying. I found out over the years that digging in the winter shortly after a heavy rain made it much easier. Almost like digging through butter in comparison.

The problem right now is that we have hardly had any rain this fall and winter. So the ground is still dry and hard.

Mulching every year has really helped. It has been slowly adding organic material to the DG/Clay mixture, breaking it up, keeping the soil somewhat moist and soft during dry weather.

I originally added new soil or amendment for the first few holes five years ago. Anything recent just gets the top 6-12 inches of soil loosened up and then mulch thrown on top. The results have been the same so I don't add new dirt anymore.

We live on a steep hillside covered in huge granite voulders. Some will be rocks forever, I suppose, but some are visibly decomposing. The ground is very sandy and covered in gopher tunnels.

We have one boulder that a stubborn root actually split in half! We had to take the tree out (Liquid Amber) and found that root trying to keep growing.

In the front yard a volunteer tree came up in pure granite. The little seedling found its way through and under. We are going to try planting white fir seedlings in that same spot and keep one survivor as an outdoor Christmas tree. I'm hoping it hides the dead truck across the street!

All over our property, we have found areas to plant next to the boulders. The soil here is well drained, so we must be missing the clay component and we don't need to water much because roots find it very damp under boulders. We have citrus, pomegranates, mulberry, wine grape vines, macadamia, persimmon, figs, Moringa Olefiera, Anna Apple, roses, avocado, and soon pistachio and pine nuts. Some of those are drought tolerant, and some need lots of water in well drained soil. We use a drip system that meters out exactly what they need, but with ground as hard as yours, that might not work at all.

Good luck to you!

econ, Yes! I had that experience soaking two of my 6-8" holes and letting them drain three times. Then I was able to get a shovel in there and really make progress. Very satisfying. I figured the whole property would behave similarly, but those areas weren't paved over for the last 60 years like the spot where I'm putting avocados, so I think they're different.

desertdance, I am recently very aware of trees growing through this type of ground (I won't even call it "soil"). There are definitely plenty of plants that fare well. There is a giant oak tree on the 40 degree slope behind me, plenty of jade, senecio, sumac, other natives I'm yet to identify.

I am sure that in time, my fruit trees will extend into the hard soil as they are softened by water and organic material. I'm concerned about the first few years, so I think bringing in a 6" mound of topsoil/compost will get everybody off to a healthy start.

I emailed the LA master gardener hotline with this question, so I'll report back what they say.

Thank you all,
Matthew