Undergrowth regards plants over all life. To him, they are his "children". He sees humans as fragile, temporary lives that won't last long; "Humans are merely a temporary weed in the garden of life... a weed that can be removed." He prefers using humans to do his dirty work. However does show some respect to humans who want to help nature, such as rebirthing Sam as his "daughter"
If he has any weaknesses, the main one would be extreme cold (and assumably fire). Danny's freeze powers weakened Undergrowth completely. His roots & brain are also his main weak spot. Also, when destroyed he will revert into a weed-like state that might take a long time to return to normal. When last seen at the end of "Urban Jungle", he had managed to regenerate into a small, rudimentary form, only to be reduced to mulch by Tucker's lawnmower.