Late February through Early April
Before 60 degrees hits. Definitely do not prune in May, June, July or August.
Late February through Early May
There are some sap flowing trees you can trim during this time, but I prefer to wait so they don't look so bad with sap flowing down the trunks. I put asterisk* by those with excessive sap that you may want to do later in summer.
Coniferous trees that you want to be bushier can be tip pruned after the new shoots sprout forcing them to double sprout the following year on the tips. Do not cut all the new growth off.
May through June
Those that bloom in spring on last year’s growth should be pruned right after blooming.
My dad loves trees. He planted hundreds of them in neat little rows over the years. Really, hundreds - mostly Lombardy poplar, apple, Russian olive, ash, maple and spruce. We had windbreaks and shadow walls galore in our flat, frequently flooded piece of the prairie. For the most part, my job was to mow and trim around those dang trees. Remember hand clippers? They were like industrial horizontal scissors for trimming grass. I also had to prune and trim those trees by hand. My hands hurt. They still hurt. This week my dad called a few days ago and asked if could I trim his trees when I come to visit. He is nearly 80 now and has a hard time getting around. So of course I will. It is a bit late in the season for trimming, but who among us is going out in several feet of snow to trim our trees as we should in February and March? Fess us. We all trim a bit late. But I've found that works out just fine for some trees. So have no regret. Trim your trees a bit later and save the snow pants for skiing. Here is my guide to when to trim and prune trees in the (Minnesota - Dakotas - Wisconsin) north.