If the tree is growing in grass I would suggest Liquid iron, it is suited to perk up generally poorly trees you can use as a spray on the leaves or water directly into the soil if you water it in over the "drip" area of the tree this is where the tree would naturally take in nutrients.

I wouldn't suggest iron tonic unless your trees are acid lovers. First, Mlt, what are the trees? And can you explain more clearly what you mean by "tired", please.