As the Shrub Knights emerged during the interval in which Adler Gawainsson was in the Lowfolk Country, we are not certain precisely why, or how, they were recruited to serve. There is evidence to suggest that the population of the Capitals was declining owing to the economic disruptions caused by the emergence of Gaps (which impeded commerce and traffic), which in turn caused troop shortages. In any event, by the time Prince Adler returned to Faerie, the Shrub Knights were very much visible members of the armed forces.
Usually they are seen as static guards, with a weapon propped up against one of their branches. They are able to communicate, apparently through a variation of Voice of the Forest. How effective they are is open to question.