I would absolutely not get a gift of any kind for your T on Valentine's Day. It's a romantic holiday for the majority of people. It's true that some people give something to their mother perhaps, and kids exchange valentines in school, but that's that's where the exceptions end. In your situation it's just not appropriate If you feel the urge to do so, like others have said, give her a small token of your affection (in the form of a plant for her office or a card) on a different day. Any other day than Valentine's day to be honest!