The answer to this question can be found in Act II scene 3 of this excellent tragedy, which is of course the first time we are introduced to Friar Lawrence in the play. His introductory speech, before Romeo enters, makes it clear his expert knowledge of the world of plants, herbs and flowers and their usage. Note what he says in these lines:
O, mickle is the powerful grace that lies
In plants, herbs, stones, and their true qualities;
For naught so vile that on the earth doth live
But to the earth some special good doth give;
Nor aught so good but,strained from that fair use,
Revolts from true birth, stumbling on abuse.
In these lines Friar Lawrence thus indicates the power that lies in knowing the "true qualities" of plants and herbs. He says that everything on earth has some importance or special quality. Friar Lawrence's skill and knowledge lies in knowing the "true quality" of such herbs and plants and how to use them to help man. This knowledge of course becomes critical for the plot of the play when he gives Juliet a potion that will put her in a death-like state but without killing her.