Best time to collect

Hey guys!
What's the best time to collect sour cherry and cherry plum(prunus cerasifera)?
Also what's the best time to collect oaks?

The standard answer is always going to be early spring. There may be other times these trees can be collected, but that will be the "best" time.

Jim is right early spring, after buds begin to swell, but before leaves have popped free of the buds.

There is a good "second best" season, especially if you can get enough roots, and that is middle to late summer. At least in the midwest of the USA, tree roots don't grow much in the heat of summer, when night temperatures are above 18 C (65 F) roots tend to slow or stop growing. When the nights start cooling off below 18 C (65 F) roots for most species begin their second flush of growth. Daytime temperatures are not that important, but when the night cools, and soil temperatures come down a little, then the second flush of root growth begins. In your region, you can start collection when this begins to happen. You will need at least 6 to 10 weeks for the tree to get new roots established and begin to harden off for winter, so stop collecting about 10 weeks before your average first frost. If you collect later than this you will need to protect your newly collected tree from frost through the winter. If you can keep trees frost free over winter (in an area kept between 1 C and 4 C (33F to 40F) you can collect all the way up to the time the ground freezes. This is not as good as collecting in early spring, but survival percentage is pretty good.

For my location, between Chicago and Milwaukee, this second best collecting time begins right around August 15. We freeze sometime near the end of October, so I stop collecting around Sept 1.

Can I get a tree to root faster if I keep it below 18C during the night?

To answer your question about oaks, read this: http://www.bonsai4me.com/AdvTech/ATquercus%20rootpruning%20and%20repotting.htm
I have no idea how well this works out for other climates but this is some serious learning stuff.

Thank you.