Can I grow flowering tobacco varieties, such as Nicotiana sylvestris, and harvest the leaves for smoking?
Nicotiana species are in the Family Solanaceae. Nicotiana sylvestris is a parent of cultivated tobacco, N. tabacum. You can surmise that the cultivated tobacco plant was bred for characteristics that the ornamental plants were not—that is, use of the leaves for smoking without (immediate, anyway!) dire toxic consequences. All Nicotiana species have toxic properties, but levels of those substances may vary from species to species, so it would be unwise to assume that leaves from the other varieties are ‘safe’ to smoke. For example Nicotiana glauca, a weedy species also called tree tobacco, does not contain nicotine but instead anabasine, which is extremely toxic to humans and animals, according to this weed report from Weed Control in Natural Areas in the Western United States.
According to The North American Guide to Common Poisonous Plants and Mushrooms (Nancy J. Turner and Patrick von Aderkas, Timber Press 2009), “all tobaccos should be considered poisonous to consume (smoking brings its own risks); some have caused fatalities. […] Poisoning through intentional or accidental misuse of nicotine and products containing it is a relatively common occurrence. Related species may contain other toxic alkaloids, chemically similar to nicotine.” For this reason, we suggest that you enjoy Nicotiana sylvestris, N. alata, and other ornamental species for their flowers only. Also avoid growing Nicotiana near plants like tomatoes and others in the Solanaceae which are susceptible to tobacco mosaic virus (in fact, don’t touch those plants after handling Nicotiana, or smoking tobacco products).