Answer: If your willow is oozing sap, there's probably an injury at the site. I don't know whether it is an insect or disease problem or even the result of weed whacker damage. In any event, the future doesn't look too good for your tree. I would remove and replace it with a healthy specimen. When you install your new tree, water it deeply once each week (not daily). Too much water can suffocate the roots and kill the tree.
Better luck with your new willow!
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