A plant of any variety that when you are walking or simply outside, is simply annoying, unnessecary or in the way. This also includes plants that upon being looked at, fill the person looking at them with unexplained irritability and annoyance. In other words, an obnoxious-looking or ridiculous-looking plant. Such as a shrub that is much too perfectly trimmed or a tree that has an uneven distribution of leaves. The word "plant" can be interchanged with any object, such as "tree" to specify what kind of plant, or even "streetlight" if a streetlight is being obnoxious.
"What an obnoxious plant! It's overlapping onto the sidewalk!"
"That tree is so obnoxious!"