How old can the tree be? Can greenwood be used? I have never attempted one and I have many young trees.

as long as you have a good branch that's hardened and can support the weight, you can air layer it. i never done it on green wood. the night temp out there should be around 50 or so. lower than that, there won't be much of root activity going on. 

I'm not an expert on air layering but I've done it on a growing green limb in mid summer. I don't think it matters on how old the tree is just as long as you can support the air layer. You can have a newly rooted cutting growing and do an air layer to double your stock. Just make sure your not cutting off the only growing branch and you should be fine.

Hi Brianm,
Basically you would wait for branching.
When the tree has two branches airlayer one and let the other as a "sap sucker" - at least this is how I do.
You airlayer to be sure to have two trees in the end.
If you airlayer on a one branch - you could loose the first pot as it would have no leaves left or bud left for growing - that is a risk I don't take.
I'm waiting myself for my "longue d'aout" to branch to airlayer her - This year should be the year for that around July not too soon, not too late.
I'll probably end buying another lda as mine is really a slow grower - so perhaps a shy tree - I need to figure out - If I end up buying a better lda, I'll burn the first one for revenge.