I followed this procedure as a guideline for fall collecting:
Art Of Bonsai In Wales Show
bonsai4me.com
It seems it can be done and with higher success rate than in spring for certain tree types. I'm not sure,
@rockm , why you would advise against it. I'm new to bonsai, and I figure the more experience I can develop in collecting the better.
You know that article is about collecting about 3,000 miles away in a completely different climate with completely different species, right?
Collecting in autumn is not ideal for a number of reasons. First, you're facing winter only weeks after you dig up the tree and remove most of its roots. Roots are the engine that stores a lot of the energy that will power spring growth. Roots won't begin growing until the soil warms up and the tree has completed its chilling cycle.
Second, unless you have a frost-free location to store the collected tree, you're putting yourself and the tree in a very difficult situation. Freezes will kill roots, as well as the top of the tree, or severely damage it. Keeping the collected tree inside where it won't experience dormancy will force weak growth in really bad conditions (extremely low light and desert-like humidity conditions produced by your home heating system. An unheated garage or shed may work, but that can be a crap shoot and you have to understand conditions inside and what the tree needs.
Third, and this is related to both of the above, cut roots sitting in the cold will rot and/or dry out, depending on your ability to understand when the tree needs water (and it will need water if its freezing out--water insulates to some extent and protects roots in very cold conditions. Dry soil and freezing leads to dead trees.
All of this is in addition to your problem with identifying a dead tree from a trees preparing for winter.
Springtime collection gets past ALL of those problems, as you collect trees when they are already on their way to their most active period of growth. They have time to recover more quickly and thoroughly from the drastic treatment you're giving them.
Sure you can collect in Autumn, but unless you know how to care for what you've collected, your wasting your time and killing trees for no real reason. There will be folks who will tell you that process will teach you how to collect. I don't think that's true. Collecting in spring teaches you more, since the tree is actually growing after you collect it, has more resources and is more responsive.
And FWIW, asking people at these organizations is going to get you more accurate info than from someone working in the U.K
www.midwestbonsai.org