Go ahead and plant them in the spot you think is the best. Actually, you are tresspassing in either case anyway. Just cause its posted or not posted does not make it more or less tresspassing. No matter where you plant, if it's not your land you are tresspassing. I look at it this way - If all your doing is digging a few small holes and planting a few weeds you are not causing any harm or doing anything morally wrong. I own land that is also posted with no tresspassing signs. The reason it's posted, is to deter tresspassers who would vandalize the property or leave garbage all over. I don't really care if someone walks over my land and does not destroy anything or leave any garbage. Just be carefull not to damage anything or leave trash around.

If tresspassing for the purpose of putting out a few weed plants is the worst thing you ever do in life, you will stilll be an honerable, decent, upstanding human being. No reason to doubt yourself.