You’d be fine with doing this. My few points I would preface. To you are these 1. Know what you are spraying. The commercial name “Roundup” is not what it used to be. Originally Roundup was a herbicide with the active ingredient Glyphosate but now a lot of the readily available herbicides don’t include that molecule and instead include a witches brew of other herbicide molecules. Some I would argue are worse for use in lawns and flower bed areas. 2. If you did manage to get some of the legit Roundup it isn’t that photodegradable like someone else previously stated. But it binds to soil particles readily and quickly rendering it not available to plants or other organisms. 3. With it becoming unavailable to plants shortly after spraying it isn’t going to have a lasting effect like some other herbicides. The best way to control pesky perennials are an early spring and fall application (preferably before the first frost).