Use a clear plastic sheet and peg the corners down make sure that there is a slight gap at the bottom so it can breath.
you should mabey cover it with tree covers I think
winter
the best answer is part.
In the winter when the tree is dormant.
avoid it. trees are best trimmed in spring or fall not winter, its a common mistake.
When tree is dorment, in the winter is the best time to get the clippers out. The tree is asleep and will not suffer loss of water from the cutting.
The best time would be in winter or when the tree gets too woody and stops producing proper leaves, the tree is not deciduous so should not be pruned too much.
Squirrels can jump from tree to tree and collect acorns for the winter.
In the winter a tree enters a dormancy stage, hibernation, and won't grow at all throughout the winter, it is paralysed for the winter. :)
The only part of a healthy tree that dies in winter are the leaves and this is because the tree does not need to take in solar energy as it does not grow in winter.
in the dead of winter when growth is most impeded.
Deciduous Tree.