How does friar describe herbs and their nature

Friar Lawrence implies that like people, plants can be manipulated to produce good things as well as combustible things.

The earth that's nature's mother is her tomb. What is her burying grave, that is her womb; And from her womb children of divers kind We sucking on her natural bosom find, Many for many virtues excellent, None but for some, and yet all different.

Friar Lawrence believes one must be careful about matching both plants and people because the the reaction can be dangerous. He uses the metaphor of plant chemistry with human social chemistry to make his point. The union of Romeo and Juliet certainly would produce a combustible situation!