From: A Floral Grimoire by Patricia Telesco

Now I know some of you may be wondering why on earth one would want to know about the plants and flowers that were considered anti-magick agents. The reason is that many plants were and are still used metaphysically for many purposes. So, if you use one in a spell or ritual and things seem to go askew, it might be because the plant's overall energy was best suited for anti-magick. It simply did its job! Give something else a try!

Alyssum: Protects the bearer from charm and fascination.
Angelica: Cures bewitchment and curses, or deflects them from you.
Anise: A sure-fire ward against the evil eye.
Ash: Protection against sorcery and conjure.
Bamboo: Used for hex-breaking and banishing.
Basil: This purges an area of magickal influences.
Beans (any dried kind): Turns away magicakally generated misfortune.
Broom: Hung in the home or placed across the doorway this will turn back witches.
Chamomile: Averts any kind of spell.
Chrysanthemum: Protects the home against evoked spirits and also averts magickal mayhem.
Cinquefoil: Washes away hexes.
Coriander: Protective herb that fights off mystical evils.
Dill: Protects the bearer from witchery when carried over the heart.
Elder: Used to identify whether or not a person is a real witch
Fennel: Hung in the doorway on a midsummer's day it will turn away any mal-intended magick
Garlic: A good all-around preventative against magickal creatures and unwanted spells.
Holly: Protection against evil eye, no matter how powerful the caster
Hyacinth: Cures fascination
Ivy (ground): When fashioned into a cross, this was thought helpful to witch-hunters.
Mallow: Fights off black-magick
Marjoram: German witches cannot tolerate this herb.
Mistletoe: Carry this to avert evil spells
Nettle: Dispels magickal curses
Oats: Protects horses and other animals from curses, especially the evil eye
Rowan: Bound with red thread, it becomes a powerful anti-magick charm
Sage: Protects against curses cast with a wicked glance.
St John's Wort: Forces a witch to confess the truth
Valerian: Hung in the home it protects all within from malevolent magick
Vervain: When combined with dill this makes a very potent anti-witchcraft amulet
Walnut: If you put this under a true witch's seat, he or she will be unable to move

For every superstition aimed at banishing Pagan influences, there were others that augmented or honored those influences, even somewhat indirectly:

Agrimony: Reverses spells placed upon a mage
Anise: An overall magickal helpmate, especially to protect a high magician
from angry, invoked spirits
Birch: The traditional wood for a witch's broom handle
Catnip: Excellent to help witches improve their rapport with cat familiars
Celery Seed: This was once thought to help witches fly
Chamomile: An indispensable herb for the green-thumbed witch, this energizes
the whole garden with magick
Clove: Improves overall magickal energy
Coriander: A powerful herb spirit that energizes magickal gardens
Cypress: Increases the power of a witch's invocations
Dragon's blood: Increases overall magick power
Echinacea: Used to invoke spirits to help with magickal workings
Elder: Can house a witch's spirit
Eyebright: Improves a magician's psychic powers
Fig: A natural charming agent
Gardenia: Increases positive vibrations for witchcraft
Geranium: Warns witches of approaching guests, especially strangers
Ginger: A power enhancing herb
Hawthorn : Witches can turn themselves into this tree for protection
Hemlock: This helps the witch travel astrally
Juniper: Carry this to improve magickal and spiritual awareness
Lemon: The witch's alternative to a poppet in sympathetic magic
Mugwort: Improves psychic awareness
Mulberry: Used by witches to safeguard spell books
Myrrh: Acts as a magickal blessings and clears the sacred space of unwanted energies before a ritual or spell
Periwinkle: The folk name of Sorcerer's violet says a lot. This is an overall pro-magick herb, especially useful for witchery.
Poplar: A tree used for water witching (as a divining rod) indicating its inherent magickal power for aiding a witch with fortune telling.
Valerian: Used to improve a witch's rapport with spirits
Willow: Used to bind a witch's broom