Only by vigilant weeding.
Nicotine: tobbacco Alcohol: sugar fermentation by yeast Heroin: chemical alteration of morphine morphine: opium poppies, papaver soniferum THC: canabis plant Cocain: coca plant Crack: basificaltion of cocain HCL salt Methamphetamine: chemical alteration od ephedrin ephedrin: epheda or ma huang MDMA(Extacy): chemical alteration of sasafras oil(root beer flavouring) Psylocybin: cubesis family of mushrooms Muscinol: amanita mushrooms LSD: chemical alteration of morningglory seed extract or ergot mold Barbituates: chemical alteration of Barbituric acid, inorganic. Ketamine and PCP are both chemicaly created as well with no natural sources for similar molecules.