If a plant likes something, such as another plant or a piece of furniture, how close does it have to be to have an effect?
Could I safely have a Laugh-o-dil and a Desparagus at opposite ends of my dorm room?
Usually I assume that the distance of a bug repellant spell is a good measure...bring it up, but don't cast...the circle appears, place one part of the circle on the plant in question, then plant the second plant at least as far away as they will not be in the same circle.