Juniper collecting

I just bought a new house and I FINALLY have a back yard so I can get back into bonsai's. It's been a few years but I found my first material on a river bank in the middle of the woods. When you be the best time to trunk chop this and transplant? Or should this be done in the spring at one time? I only have a few tropicals so I'm not as familiar with temperate species

Spring is always the best time to collect -- assuming that you own the land on that riverbank. Otherwise, you need permission from whoever DOES own the land -- person, company or government -- to do any collecting.

It really depends on the type of Juniper. Some seem to have a better chance of survival if collected during the natural rest period in mid summer. I would also do a test dig to see how far the roots spread out from the base, since if it is an old tree many of the important fine roots might be quite far from the trunk. I have had success digging the tree out over a number of years and not lifting it in one go. Good luck!