Your tree has stood in flood waters for weeks, or suffered through a long, brutal drought. Or major limbs were lost in a storm. Or maybe the poor thing was partially tipped over by high winds, and you've had to straighten it up and brace it. Whatever the insult, the tree now looks some the worse for wear, and you're anxious to make it healthy again.
So out comes the bag of fertilizer and...
DON'T! Just don't.
If there is one thing you should not do "for" a damaged tree, it's fertilize it. (The exception: if your soil is deficient in nutrients—and that has been confirmed by a soil test. Even then, only the deficient nutrients should be applied.)
I inadvertently broke that rule several years ago, after our beautiful pin oak was severely damaged by an ice storm. Come spring, I spread organic fertilizer on the grass, remembering too late that I needed to keep it well away from the tree's expansive root system. Sure enough, the injured tree put on a spectacular spurt of growth that year—which was not good.
When a tree is damaged—especially when it suffers root loss—it needs to pour everything it has into healing wounds and regenerating feeder and anchor roots. It cannot do that when its energy is being diverted into growing lush new leaves and branches.
Initially, fertilizer might give you what you want: a landscape specimen with a lush, green crown. From all appearances, your tree will have made a miraculous recovery.
But if the flush of new growth pushes the tree past its capacity to sustain, and the healing process stalls, your prized specimen could become even more susceptible to attack by diseases or pests. This year's miracle recovery can turn into next year's sudden decline.
In the case of our pin oak, we were lucky. The tree managed to heal its many wounds over the next few years despite the dose of fertilizer. But that healing would undoubtedly have progressed much faster if the oak hadn't been forced to support an unnecessarily luxuriant crown.
So what can you do for a damaged tree once any needed pruning repairs have been made? Of course, the tree will need a steady supply of soil moisture, especially inside its drip line. And if the specimen is particularly valuable to your landscape, you might consult a certified arborist as to whether it would be advisable to have it treated with growth retardant. (See Surviving Dead Roots, posted on 9/9/11.) This will keep crown growth minimal during the healing period, while enhancing root growth.
Other than that, one of the best things you can give your recuperating tree is time...time to catch its breath and heal.