In order to maximize botanicals' magic potential, magic rituals and spells are incorporated into their harvesting.
Because plants are alive, removing them from Earth is a risky operation. One has the option of increasing and enhancing their inherent power, or of offending Earth and the presiding Plant Spirits.
Once upon a time, all harvesting, for magical or other purposes, was accompanied by spells, rituals and propitiation of various Earthly and Spirit forces. Unless you purchase your botanicals from magically oriented vendors, one can safely assume that modern harvesting is accompanied by no such rituals or spells.
If you practice extensive botanical spell-casting you may wish to incorporate similar gestures in other ways, to enhance your spells and to provide spiritual protection for oneself. If however, one grows and harvests one's own botanicals, ancient spells, and rituals may be borrowed or adapted.
Because they're alive, have power, and must be treated with respect, it's not appropriate to just go out and grab a handful of plant. Botanical materials are safely harvested through magic ritual. Essentially you cast a spell in order to gain materials to cast more spells.
The plant (or its presiding spirit or Earth herself, however you best understand this) must be addressed. The purpose for gathering should be explained. Because of the principle of reciprocity, gifts are exchanged. Libations of water are always appropriate, howver different traditions favor different gifts. native Americans offered pinches of tobaccop; Anglo-Saxons once offered oatmeal. The ancient Romans offered bread and wine. Honey, wine, and menstrual blood are popular offerings. Fragrant incense may be burned in the vicinity as a gift.
Use your intuition and your judgment to discern what is proper when collecting in the wild. If botanicals are purchased, consider a blessing spell and/or some kind of offering and acknowledgement of the disrepect the plant may have suffered before it came to you.
source: Element Encyclopedia of 5000 Spells