If you decide to amend a planting hole for a tree in heavy clay soil, you'll want to amend the entire future root zone of the mature tree. If you amend only the immediate planting hole with lighter soil, bagged soil, sand etc., you're making it too easy on the tree's roots. Because the roots have an easy time of it, they'll take off like wildfire until they hit the side of the hole. Since the roots have not been conditioned to work their way through heavy soil, they are often unable to overcome this formidable barrier, so they either stop growing or continue searching for more easy soil by circling around the planting hole (root girdling). The long-term result is a stunted tree. The roots of trees planted in native soil have to "fight" their way through this soil; as a result, by the time they hit the side of the hole they are conditioned to keep moving outward. Trees planted in unamended clay eventually outgrow (and outlive) their mollycoddled counterparts.

I agree with Ragweed. To realize long term benefits,you would be better creating good soil for turf surrounding the tree. That alow's moisture and air into topsoil. You shouldn't lay a water hose next to trunk,water turf well with a sprinkler.

As Babs said.........NO amending!! Dig a very wide and shallow planting hole and plant high. Backfill only with what you dug out and then use whatever amendments/soil you have on hand to mound soil up to the edge of the rootball (but not over the top).

This is THE recommended method for planting in a heavy clay!

Thanks for the advice, everyone! Had to delay planting to tomorrow anyway, now I know NOT to amend.

One more question - the soil was separated into layers - loamy clay topsoil and then heavy clay a foot or so beneath. They got mixed together while I was digging the hole, will that affect the roots at all? It's all native soil but I'm worried the different compositions mixed together will have an effect somehow.

If you plant high, any negative effect will be minimal.

You can amend a big hole now, perhaps 3 feet in diameter and then over the years keep expanding the hole diameter. Sounds like you have a foot of good topsoil. If you amend your hole and then over the years fork the lawn, or bed, around the tree and cover with an inch of mushroom compost or well composted manure the tree will appreciate it. I have spread an inch of mushroom compost and my 60 foot Norway Spruces looked noticeably better, as did the lawn.

Amend on the surface as John commented with the mushroom compost but to not amend the hole as he suggested. That has been disproven by research to be helpful, especially in clay soils.

Yes, amending a 3' diameter hole will only lead to a 3' diameter bucket of moisture retentive amended soil still surrounded by poorly draining clay....all you have accomplished is more work for yourself but nothing of benefit to the plant. Do not amend individual plantng holes no matter the size. Save any amendments to be used as a mulch or topdressing.

Roots don't like amended holes better or get deflected by walls of amended holes - failed plants dug up later and seen to have circling roots have these because of having been left in containers too long before planting. The main problem plants have with amended holes is how the amending affects movement of water into and out of the amended hole. With liberally amended holes dug out of damp clay soils specifically a collection point for water may result, this has been called the "bathtub effect". (And also likened to a sump).

^^ Not precisely accurate. Roots do tend to stay in heavily amended planting holes (whether they have been congested by container culture or not) as native soils tend to have fewer available nutrients, are more finely textured and less aerated and offer no incentive for the roots to spread. They behave in an amended planting hole the same way they do in containers.

False. Roots are geotropic and always grow away from the center of a plant unless blocked by an uncrossable barrier like a pot wall - look at what happens when roots escape pots via drain holes. And root right into the nursery fabric, crushed rock or uncomposted, low nutrient sawdust pots are sitting on or in.

Also it is characteristic for top growth of plants in amended holes to pick up markedly after the outer roots exit the limiting conditions of the amended hole and start penetrating the unmodified soil beyond the hole. Something that happens rapidly after planting unless plants are in too poor of a condition before planting - or too adversely affected by amending - to produce a normal level of growth.

This often quickly manifested post-planting response to escaping of the amended hole is one of the reasons why amending of holes is not readily noticed to be detrimental. And even believed to be beneficial - because the plant really took off a year or two after planting.

In response to the "unimproved" soil around the hole.

Here's the key in OP's 2nd post":

"loamy clay topsoil and then heavy clay a foot or so beneath"

I find it very easy to believe I could plant the bareroot seedlings I buy into a foot of loamy clay topsoil. The only discussion needs to be how much mushroom compost or well composted manure to mix into that fine sounding topsoil.

If OP digs up a 3ft diameter hole a foot deep and amends it somewhat and plants his seedling, The seedling will grow very well the first year. If during the first year he expands his hole 2 feet in each direction it will be 7 ft in diameter at the end of the first season. A ft in diameter annually in the next couple years will make this a field and then it's politically correct to amend it. If you never dig below the topsoil you haven't created a "hole".

I guess if you purport to speak with more authority on the subject than those who spend all their time researching and studying these issues, Embothrium, then feel free to call that false. OTOH, I tend to give more weight to opinions of actual horticultural scientists :

"Let's say you have incorporated the recommended 25-50% organic matter to your backfill.
(Remember that an ideal soil contains 5% organic matter by volume.) The initial results are positive;
roots grow vigorously in this ideal environment as long as irrigation is provided. But what happens when
these roots encounter the interface between the planting hole and the native soil? Native soil contains
fewer available nutrients, is more finely textured and is less aerated. Roots react much in the same way as
they do in containers: they circle the edge of the interface and grow back into that more hospitable
environment of the planting hole. The roots do not establish in the native soil, eventually resulting in
reduced growth rates and hazard status as crown growth exceeds root ball diameter"

The Myth of Soil Amendments, Linda Chalker-Scott, PhD

"But what happens when
these roots encounter the interface between the planting hole and the native soil?" Linda Chalker-Scott, PhD in The Myth of Soil Amendments

in·ter·face
/ˈin(t)ərˌfās/
noun: interface; plural noun: interfaces
1.
a point where two systems, subjects, organizations, etc. meet and interact.
"the interface between accountancy and the law"

off google, uncredited

If there's a continuous 1 foot depth of topsoil there is no interface, that's relevant.

while I agree about shallow wide holes (there is a University of Colorado pub about it), I have had 100% survival rate when planting in clay with this method: medium depth holes, rotten wood at the bottom. It is a variation of hugelkultur and the wood turns into well drained loam, while also getting mixed over several years. I have done it about 40 times, for my 25 tree orchard (winter girdling accounted for the excess).