I would personally backfill with a more sandy mixture and just get rid of the clay soil instead of adding it to the hole. I like the idea of planting it on a mound and since you are a zone 8 I also have clay soil and although certain plants appeciate the additional moistur, extreme cold should not be a problem for palms like Trachycarpus.
I also have clay soil. Some of my plants are okay with it (my windmill palms dont have a problem, the brugs are okay with it, and my crape myrtle also seems to like the clay soil), other palms and plants do like to have there soil amended with things that are better draining like potting soil (sometimes I completely replace the soil with potting soil). Clay soil isnt as much of a problem here during the summer months because it gets very hot and sometimes pretty dry too, its the spring and fall that cause some of the plants to suffer from too much moisture.
So basically, this is what I would do...

Make a hole that is a bit deeper and a bit wider than you need

Add course sand AND small rocks to the bottom

Put the palm in the hole in a way where the soil level of the palm is a bit higher than the soil level of the ground around it.

Mix sand, small rocks, and nutrient rich potting soil together and fill up the hole with that.

The result is a palm tree on a small mound. The small mound and sandy soil will keep the palm from being over watered. Some light mulch might help keep the soil in place when there is a lot of rain and also protect the roots of the palm from an occasional cold night during the winter.

Good luck!
-Alex

Not really an expert but I also have clay soils. Palms do just fine here so it could when your planting them. I agree with tropicalzone7. One thing I noticed that gives me good results is planting just before sun set and watering well right after. The reason I think I see good results is it gives it time to recover and water loss is small at night than in sun where the plant wants to grow and use its water supply. Also planting it a little higher is to insure that the plant doesnt drop below ground level. When the soil settles it will drop, so planting higher can level itself to ground level with out making a hole. Good luck!

- US_Marine

I'm curious - isn't a zone 8 in the northwest different from a zone 8 in, let's say Virginia or North Carolina?

I was just reading my post above and the first paragraph didn't make much sense so Im going to re-write that...

"I would personally backfill with a more sandy mixture and just get rid of the clay soil instead of adding it to the hole. I like the idea of planting it on a mound and since you are a zone 8 I also have clay soil and although certain plants appeciate the additional moistur, extreme cold should not be a problem for palms like Trachycarpus"

I would personally backfill with a more sandy mixture and just get rid of the clay soil instead of adding it to the hole. I like the idea of planting it on a mound and since you are a zone 8, extreme cold should not be a problem for palms like Trachycarpus

mastiffhoo the only difference is NC and VA have a longer growing season cuz they are warmer in spring/summer and warm up faster from winter. Other than that a zone 8 lows are zone 8 lows no matter where ya at.

I think that is what i meant - the growing seasons are different. Also, I was wondering if the earlier and faster warm up meant a quicker recovery for margainal palms.

Can't comment on your particular climate, but here in So Cal Dr Hodel did many experiments with several more common species of palms and found that planted exactly as you suggested (hole size of pot and backfill with original soil, not ammended in any way) set the palms back the least of any other method. Backfilling with different soil, or sandier, or looser etc. soil sometimes put the palms through a bit of a 'coma' and they often took longer to resume growth. In some situations there simply was no difference at all. But in no situations was ammending or changing the soil in any way an improvement (other than mulching the top soil well). Sometimes the simplest method is the best.

I agree with the Lizard doc.

It was thought that amending the soil was the way
to go but many palms(etc),roots did not grow out
of the "new"soil around the palm....
Best for the palm to at least have a mixture of the soil
it's in and where it's going.