In Mary Stewart's novel Thornyhold, the heroine Geillis Ramsey, known as Gilly, inherits an isolated but magical home from her cousin. When she looks through her new home, she finds a "still room." On the center table is a bowl containing four herbs: trefoil, John's Wort, Vervain, Dill. Then she remembers the old rhyme: "Trefoil, John's-wort, Vervain, Dill, hinder witches of their will."
This gentle protection charm is easy to make with ingredients from the kitchen, the yard, or the local herbal store. Trefoil is common three leaf clover. Dill is usually in everyone's kitchen. St. John's Wort and Blue Vervain are available in most health food stores that stock herbs.
Mix up a lovely batch of sweet-smelling herbs and keep in your home, office, or car: anywhere for fresh scent and legendary protection.
"[Trefoil] has a long tradition of use as a protection against magic: an old rhyme says, "Vervaine and dill/ Hinder witches of their will." (Another version says, "Trefoil, vervain, St. John's Wort, dill/ Hinder witches of their will." In either case, this is specifically meant against evil witches: the healers actually used all of these herbs.) http://windspirit57.tripod.com/id4.html
Here are some hints and guidelines when gathering herbs.
Before picking trefoil, make sure to have either a small amount of milk or some ginger, fresh or powdered. Leave the milk or ginger as gifts for the Little People to thank them for the herb. This practice keeps humans on good terms with these faery folk.
Be careful about stepping on St. John's Wort after the sun sets:
"...true it is alleged to be that, if on the Isle of Man you tread on St. John's wort after sunset, a fairy horse will rise from the earth and bear you about the whole long night, only leaving you when dawn comes."
Dill seeds are said to help with love, to help with court cases, and to help with health.
Vervain requires honey, which should be left after picking. Many people wait until the new moon, to pick this herb.
http://www.janih.com/lady/herbs/magick/V.html
Resources
http://books.google.com/books?id=2JYF1Uj2o2QC&pg=PA237&lpg=PA237&dq=trefoil+johns+wort+vervain+dill&source=bl&ots=XBcWh-dn8N&sig=CnPfP-8fX_eaVPh1XHdCUCqztcw&hl=en&sa=X&ei=UhXXUq7POM-QyQHO8YHQCA&ved=0CEUQ6AEwAg#v=onepage&q=trefoil%20johns%20wort%20vervain%20dill&f=false
