Planting now would definitely be my choice. In your zone there is no advantage to waiting. Your tree has a much better chance of survival and will be able to start becoming established if planted now. Follow proper planting procedures (see link below) and get it in the ground soon.

The answer to your conifer question (in the Conifer Forum) was slightly more debatable, but I think this one's pretty clear.

Here is a link that might be useful: Planting a Tree or Shrub

Tree will be more vulnerable to cold in a pot than in the ground. However, root growth during winter is minimal - and it is desirable to wash the soil off at planting time. If it can be kept from freezing up hard and from being too damp (if the soil it is in now is on the soggy side) it might be better to wait until winter is over, so you can free it of the soil on the roots at planting time without worrying about it being damaged by cold later.

Trees planted with intact root-balls may actually fail due to textural differences between the soil inside and the soil outside the root-ball. I have myself lost a couple planted with intact field soil root-balls in recent years. If I had washed the roots instead of fearing the particular kinds would not be able to take bare-rooting I might now instead have established, happy specimens - instead of wasted time and money.

Here is a link that might be useful: Horticultural techniques for successful plant establishment PDF

If you have heavy clay soil I would garage and wait. Just make sure the soil in the pot doesn't dry out completely.
If your soil is good and can be worked easily now I would go ahead and plant.

If your soil is heavy and the plant has been grown in a potting mix, at planting time I would plant the maple high, plant half the depth of the pot in the ground and then mound good soil around it. Also make sure the hole is twice as wide as the pot. I have good success using aged pine fines to ammend heavy soil. If you can find the product in 2 cubic ft. Bags from Mulch Manufactures it is a great product. I add about 20 percent to my soil and till in.

Also make sure the roots are not pot bound.

Not a good idea to add any amendments to a planting hole, heavy clay soil or not. If you are working wih a clay soil, dig a very wide (3x the rootball diameter) but shallow planting hole and place the plant high. Backfill with indigenous soil only and use whatever amendments you may deem necessary as a mulch to cover any exposed portion of the rootball.

And I would consider washing or bare rooting the root mass before planting. Any root problems (circling, girdling, etc.) can be easily detected and corrected by doing so.

Gardengal48,

In this part of the country most all of us amend to some extent and all landscapers locally will recommend it. The trick is to not add to much amendments to the soil to avoid the bath tub situation. Aged Pine Fines are excellent for breaking up clay and keeping it that way for sometime. When this is done and then the plant is planted high like you recommend plants seem to establish faster.

David

all landscapers locally will recommend it.

That doesn't mean it is an accepted or currently advised method :-) Professional landscapers often practice ill-advised planting methods on a routine basis, simply because they don't know any better or are not members of the professional associations that keep them abreast of current practices.

The rule of thumb regarding planting hole backfill for any type of planting is what comes out goes back in. No amendments of any kind. If you need to amend or improve the soil - and there is often good justification for this - amend the entire planting area, bed or anticipated mature root zone, not individual holes. That prevents soil interface issues or the combining in close proximity of various soils or potting media of different textures. These disparate soils prevent percolation of rainwater or irrigation and impede good drainage. Enriched planting holes can also retard the spread of the roots properly into the indigenous soils and impede establishment.

This is also the reason bboy suggested washing or bare rooting the rootball before planting. It removes yet another planting medium from the mix, reducing the potential for interface issues, as well as allowing the planter to visually examine the rootball and make any necessary corrections, which are often required with any container grown stock.

Here is a link that might be useful: Myth of Soil Amendments

This is one of those issues that requires site specific knowledge and knowledge of how much amendment to ad to the soil. In my subdivision all the natural top soil was removed. As such I amend the clay and plant high. This practice has had a positive impact on what will grow there. At my parents house 10 miles away that is 100 years old there is 1 ft of dark top soil. At their location I don't amend as it is not needed.

If money was not an issue I would have tilled in 4-6 inches of pine fines over my entire yard but that is not practical for my 1/2 acre with some mature trees. Plus what works in the Pacific North West may or may not work well with us her in the midwest. All I am doing is giving my best recommendations based on my actual experience in this area with our soil.

Again it is important to remember thatI don't add a large amount of material, 10-20 percent max, and I don't dig deep holes to avoid some of the issue you outlines above.

David

"Research findings and new technology are slow to be incorporated into nursery and landscape practices. Tradition is important and has a distinct place in our society, however, blind allegiance to tradition stymies progress. In 1968, a study was begun to determine the "optimum" amount of soil amendments to use in the planting hole since recommendations varied from 5% to 50% by volume. The optimum amount turned out to be none"

--Carl E. Whitcomb, Establishment and Maintenance of Landscape Plants (1987 (1991), Lacebark Inc., Stillwater)

Here is a link that might be useful: Establishment and Maintenance of Landscape Plants II