- Four-Leafed Clover: To use to see the Fey folk when they are trying to make themselves invisible to you (by manipulating the electromagnetic field and light around them--it's not like they do become invisible).
- Lily of the Valley: To be avoided at all costs for the Fey use this flower to entice the unwary. But its sweet scent is but a disguise for the poison that lies beneath it and is sure to kill you within three days.
- Saint John's Wort: Excellent Fey-repellent, though doesn't work against the higher-order Sidhe (but it may give the a rash or make them prone to intense sneezing bouts).
- Water Lily: may help reduce or even heal a particular sickness given to you by the Fey folk that turns one to stone (aka paralyzes them completely). To be used with moderation, or you may find your body melting off your bones instead.
As you, I'm sure, in no way doubt, there are many other plants which can be just as dangerous, while others prove to be very useful or even beneficial. It is why Miss Pelletier works so closely with Dr. Cockleburr, the later being always in search of the latest anti-poisons and other remedies against the Fey and their enchantments.
The best would be, of course, to avoid the Fey altogether (they are fickle by nature and quite resentful of our having taken the Earth back from their control, so are prone to helping us find our demise...as long as there is no knight around, of course). So should you see any flowers that presage their presence (1), simply turn around and walk back the way you came from. Better be safe than sorry, right?
Notes:
(1) See Rise of the Fey