Thanks Dennis. That trench digger is great its small enough to be wheeled into the yard.
Overall, what I understand is the clay soil makes it harder for the roots to go deeper due to its hardness with results in less root mass to support good vigor. Is the poor drainage an issue in winter/spring when we get a ton of rain but the trees are dormant or in summer when there is no rain and they are actively watered.
Same principle but I didn’t use the perforated ag pipe or fabric, just gravel. The rainwater will naturally find it’s way to the bottom of the hole, and instead of potentially sitting there will now drain away from the root zone. It just drains a couple of feet away to a lower area and into the surrounding ground. I was really just making sure the roots won’t sit in water and making sure the hole doesn’t trap water. It was probably overkill but better safe than sorry. To test just pour a bucket of water in the planting hole and make sure it eventually drains. If you have standing water or a retaining wall etc you’d need a proper french drain.
Like almost anything else, the law of diminishing returns is at work.
That said, bigger (wider, mainly,) is better. A lot more people under dig than over dig.
As a doughy office worker, this is the way I handle it: I pick a likely spot and dig pretty hard with a pick and shovel for exactly fifteen minutes. I use the alarm on my phone. I repeat the next day and every day. I think of it as part my daily exercise.
Somewhere between day 3 and day 7 it starts to get a little ridiculous. I stop. I try to screen the dirt a little. Rabbit wire is ideal in my area.
The tree can stay in its pot for a few more days. A bare roof plant’s hole can be dug a week before scheduled delivery.
Just a note about excavators to Oregon_fruit_grow, you might be surprised where a 1.7-2 ton excavator can fit. They have retractable tracks and get down to 39" wide. There are even smaller 1 ton machines that go down to around 32". Of course if you have stairs or a gate with an overhead constraint that may rule it out.
I brought one home from work last night and did some clean up of random dirt piles and dug four wide, shallow tree holes in not enough time to drink two beers. Though, to be fair, with a little care I could also probably get a Cat 320 in my backyard 
My hunch is that your builder imported the clay and compacted it to a uniform CBR (California Bearing Ratio) in order to assure the house foundation does not settle. Typically most building codes require a certain standard and builders do want to achieve a certain standard to protect their reputation. So that explains why your clay could be acting like a hard pan. If you do a perk test it will answer your question. Dig a 2’ deep hole, fill it with water and see how long it takes to drain. How many inches per day did it drain? Then compare that to your most intensive winter rainfall events. Most trees require drainage year round even when dormant to avoid root rot
Dennis
Kent wa
That is probably the case, our house/yard is on top of a small hill roughly 675ft elevation. There is approximately 8ft difference between the front door to backyard. I did a perc test at couple of places, at one place a 12" hole took 3 hrs to drain, and other 18" hole took hour and a half. But we have also got inch and half rain this week. I probably should have tested in summer.
Your drainage is fine. Plant your trees in a normal hole and quit fretting. You’ll spend a lot of time and money and not improve a thing.
Those are good results! You have good drainage, only difficult holes to dig due to compaction, but tree roots can easily penetrate clay, still I would excavate a large hole and use some of that clay to enrich your garden while mixing in at least a 10% organic material which could be any one of these: compost, composted wood chips (at least 3 years old from middle of a large wood chip pile), spagnum peat moss, or bagged top soil with vermiculite). I would not be concerned at all about drainage with those results. Good luck
Dennis
I did a small soil drainage test. Posthole to 20 inches.
Filled with water to 6 inches below grade. Drained 8 inches in about 5 hours. Dry in the morning after 20 hours.
My soil is loam with no smearable clay. Breaks up when pressure is applied.
There may be a different layer further down. This winter, for information, I’ll dog a much deeper hole and test it.
Regardless my method of planting seems fine.
Those planters look awesome. Are those just 16x8 inch concrete pavers from HD/Lowes? Are they also cemented in?
Thanks Bombers nice of you to say. We just found the pavers underneith the house and have been using them for garden beds. The size is around 15x7.5". Not cemented in, just pavelocked. Cementing would probably be better.