I have just grafted a couple of Apples onto Bud 9 root stock. I understand they will need permanent stakes. Some say a 10 foot metal stake is best but some say an 8 foot stake will do. Which is correct? why?
I've seldom needed stakes more than 5ft above ground. This for trees reaching 8-9ft on M9. But it depends on your training system and desired height. Is it just for support or also as a training aid?
I think a 4-foot stake is just fine. I've got 4-footers for my young trees and I don't even use them, honestly. I think once the trees begin bearing heavily I will tie up the trunks. But I certainly fail to see how an 8 or 10-foot stake is necessary to hold up a 7-8 foot tall tree. 4-6 foot stake should be plenty fine.
Soil also has a big effect on how much staking you need. I have lots of M9/B9 and some parts of my orchard are heavily staked and other parts I never needed a stake. My lazy approach is I stake a tree when it first starts to lean, but not before then. I use 6' stakes. More than the size of the upper is getting the lower hammered in deep enough; you only need about 4' of upper to tie up the tree, any more is not going to make it more stable since the soil movement is the weak link in the chain.
I have two trees on B9: Empire and William's Pride. Each is staked with a 6.5' T-post, so that there is 5' of post above ground. They are in heavy soil, planted 8 years ago. I think they are benefiting from the posts, keeping them more upright than they would be otherwise. I don't think they would need anything taller. I wonder if variety would be a factor--vigorous or not--as well as the soil type that Scott mentions.