Thanks for the feedback!18" worked great for me.
I planted 50 fruit trees last Spring; it's fun and very gratifying. What varieties are you planting?
Also, planting that many trees is plenty of work, but please don't skimp on the cage/screen. It's such a bummer to hear the stories every year from guys that forget or otherwise skip it and report the damage to the trees from deer/rabbits.
I used 12 inch auger to me the key is getting deep enough not so big in diameter I am sure 18 is even better but if all you have is 12 I think it will work
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I agree with Aero. The variable would be the type of soil you have. Heavy clay you'd want to make bigger. Sandy you'd want bigger so you can add back lots of stuff like Crazy Ed does. Nice loam you could go smaller. Luckily I have a lot of that.
I cut my holes about 18-20" with a shovel. If you have a 12" I'd use it. If you're renting I'd go bigger.
I use a 12" and just rock it back and forth a little before bringing it back up to soften the edges. It's big enough for about 99% of them. I have to plant several hundred at a time, and it works fine. If you have something bigger, it nice to have a bigger hole. The bigger you go though, the harder they work going down.