People often ask me: When is the best time to install my tree tubes?
My answer: Today! (The second best time: Tomorrow.)
Seriously, there's no bad time or wrong time to install your tree tubes. They can be installed when seedlings are still dormant, or they can be installed when your seedling are in full leaf.
If you install them later in the summer after the seedlings have already pushed new growth you might not see a dramatic increase in growth from using the tree tubes until next year... but that's OK - at least your trees will be safe from the deer!