In a perfect world, you would not trim a tree when it's wet outside. Moisture increases the risk of disease spread, being that most tree diseases are caused by moisture-loving bacteria and fungi. There is also more risk of the trimmers slipping and making jagged cuts, which can be hard for the tree to heal. But sometimes situations are not ideal. Maybe your schedule is tight, or perhaps you're leaving town in a few days—and your only option is to trim the tree in wet weather.