I have a stand of 80 foot tall pine trees growing out over the grass at the bottom. I would like to prune the bottom branches so I can maintain the grass. When would it be safe to do so? I think they are white Balsam Fir.
Best time to do so is late winter early spring, before new growth but late enough in winter to avoid exposing newly cut tissue to zero or worse weather. Understand it takes away natural growth and any new growth below will have to be taken off too. Also, turfgrass growth underneath evergreens does not do well to non existent, due in part to acidic soils caused by leaf drop.
Read more here: http://www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/pruning-evergreens
Then there are those who talk down the practice of limbing up evergreens and yes, besides sometimes looking odd, it could affect the health of the tree long term: