Probably not. The holes would soon be filled by soil as the holes collapse and you'd be exactly where you started.
Unless you have some extreme situation (low lying area where water collects) or soil type with an underlying strata of perhaps cliché or rock, most clays are usually a good growing medium without additional drainage.
Your best avenue would be to prepare the planting holes as suggested in most literature, that is... dig the planting hole three times as wide and perhaps one to two times the depth as the root ball. Build a small "cone" in the bottom of the planting hole and set your plants at the same exact soil depth as they were grown. Start refilling the planting hole about one third at a time, packing slightly as you go and adding water before adding more fill. Typically, you may mix a little compost with the fill if you like but be aware that adding too much may end up producing a "saucer" effect where water will collect, effectively depriving the root system of air in times of heavy rain.
On the whole, clay does drain on its own even though it may not seem like it. On the other hand, most clays also hold water well in times of drought.
Hope this helps.
Steve

Hello, Steve

Thanks very much for your response! I am going to do everything you suggest. I think you're completely right. The holes would probably fill with soil as their sides collapse, leaving me just where I started.

I do have an extreme situation. The area planned for Carmine Jewel and paw paws is at the bottom of my yard, which used to be separated from the top of my yard by a slope so steep that last summer I converted it into two large terraces, now filled with topsoil. This spring, plants that die in clay will go into the terraces.

So my slope is resolved, but I still have two levels. Carmine Jewel and paw paws are supposed to tolerate clay, but not standing water. As the soil can stay squishy in the low area reserved for them for long periods, I think I've got to at least try to enhance the drainage.

In my original question I mentioned that I plan to fill the holes with large pebbles. I'm thinking that they will prevent the collapse of the holes you warn against. At least, my system of holes 2.5" wide x 3' deep, filled with large pebbles, won't hurt. (I hope.)

Thanks again for reading and responding to my post!

Bernard

i was gardening in clay soil area in NC for a while and i remember reading about drainage ditches as you describe to hold the water in times of heavy rain. i think it's an 'old world' practice. basically they used to dig a large pit and fill it will rocks - so it would fill with water drained off the neighboring plot.
what you suggest is not far fetched. but i think the hole needs to be wider like 6"-12" at least. also remember if it fills up with water - it is like a reservoir, it'll slowly release it into soil. effectively you might surround the plant that does not like to be damp with a wall of water.
so perhaps the ditch needs to be in an area away from the plants that dislikes to be wet.
one of the solutions could be a ditch below the area that you need to drain - at the lowest level so to speak - and a little away.
i'll try to look up the drainage ditch technique in my books.

Hello, Petrushka --

Thanks very much for responding to my question. 7b! Too hot for me! But I bet your garden is beautiful.

A system of narrow ditches lined with fine pebbles to drain surface water away from the paw paws and cherries sounds like a fine idea, especially given the topography of my yard. I'll try it.

For the main drainage, I'm thinking about not just one hole, but about ten per plant. I'll dig a planting hole for each paw paw and cherry. At the bottom of each planting hole I'll use my augur and cordless power drill to drill ten holes 2.5" wide x 3' deep. Instead of filling each hole with pebbles to prevent collapse as I originally planned, I'll make cylinders of hardware cloth and slide them down the holes, topping off each cylinder with a piece of hardware cloth.

(I made a mistake before. My augur is 2.5" in diameter, not 1.5".)

Snow is 8" deep today... It's all just a dream.

Bernard

Oops, forgot to say that the planting holes will be extra large in diameter, will extend from ultimate drip line to ultimate drip line (or as far as my aging body will allow).

As all those drilled holes will be at the bottom of the planting holes and re-enforced by cylinders of hardware cloth, I'm really talking about an underground drainage system.

Bernard

no garden for me now :( = just a balcony.
do a google search on 'french drain system' and also 'soakaway' - yep, that's the technical term :). and look at images - there are a lot of schematics that you can learn from and get ideas just at a glance.
in clay soils it is best to plant trees elevated - not sunk in.
so that the crown of the tree is 6"-12" above the ground - that is to prevent flooding of the crown.
also it is actually not recommended to dig large pits - what happens is all water runs into them from the surface and it creates a pond/well! so your tree will be sitting in a wet pit.
elevation is more effective. raised beds/terraces/slopes.
they do put down gravel /rocks in the bottom of the dug pit to help drainage though. and also when planting smaller plants - it's good to create a mound with rock/gravel high in the center, low on sides and then fill up with soil - this is better then just a pit with rocks on the bottom.
shallow french drain sloping away from the tree is prolly even better if you flood a lot.
it could be shallow 12" V shaped laid in a grid.
they make 'aquacells' now - sort of hard plastic crates that link up that you can sink into trenches for hard-surface stability. a la shipping plastic milk-crates, turned upside down.
the deep holes on the drip line is a very good idea for clay actually when it is hard-dry - filled with gravel they are stable and you can water in water/fertilizer directly into holes for deep watering that is very beneficial and there's little water waste.
however if you have heavy floods - the wells directly under the tree will hold the water for a very long time, especially if your water table is high.
i guess it also depends on how thirsty the trees are and how much water they can consume.
it would be good to know where your storm drain is too and whether you can drain the water thru a drain pipe someplace - either storm sewer or pond/ditch etc.

as a side thought , may be you could drill narrow holes evenly thru the yard and fill with gravel/rocks. so you'll have small deep draining wells that will then slowly dissipate water.
i saw you mentioned the creek at the bottom of your yard - can you run your drains into it?