One of Friar Laurence's passions or hobbies is his herb garden. He tends to it gently and attentively. These herbs will come into importance later in the play when he uses them to make a potion to drink that will help Juliet fake her death to avoid having to marry Paris.
The observations Friar Laurence makes about his herbs and flowers have to do with the healing and harming powers they possess. They have the power to heal, yet they have the power to harm, as well. Think of it as the venom from a snake. If a snake bites you, the venom can kill you. However, a tiny amount of that venom is used to make the anti-venom that is given to snake-bite victims to heal them. The Friar then makes a connection to humans. They also have the power within them to either heal or harm. We all have both sides of our personality within us and it's a matter of what we choose to utilize. This is his commentary on the two families who have been fighting for generations.