The Rowan Cross
An Old European Charm

Dried rowan leaves & berries
A rowan cross is crafted with two pieces of rowan (Mountain Ash) of equal length and bound in the center with red yarn or string. Rowan (Gaelic rudha-an) is the Scottish name for the mountain ash meaning, “The Red One”. This type of cross is hung in the shape of an x not a +.
Rowan tree and red thread
Will put witches to their speed. -Allan Ramsay

Strung dried rowan berries
Rowan crosses are used for protection. You can place it over your door, in your car, wear it on your person, place it in your desk at work, in your back pack or purse, or in another place you need to protect. It is said when the rowan cross breaks it did its job and a new one should take its place.
In 1618 in the town of Irvine, Ayrshire, Scotland, Margaret Barclay was brought to trial for witchcraft for having in her possession a piece of rowan tied with colored thread. She crafted this charm to help her cow give milk.
So potent is the flower or berry or wood of the rowan or witchwood or quicken or whicken-tree or mountain ash against the wiles of the elf-folk, that dairymaids use it for cream-stirrers and cowherds for a switch. -Walter De La Mare
The Celtic tree calendar lists Rowan (also known as Luis or Luisliu meaning flame or delight of the eye) as the ruler from January 21st to February 17th. It's a popular activity during Beltaine to craft rowan crosses.
Resources
The Witches' Almanac Spring 1996 to Spring 1997
The Witches' Almanac Spring 2009 to Spring 2010
Tales of the Taibhsear by Scott Richardson-Read
The Irvine Witch Trail of 1618
Beltane: Rituals, Recipes & Lore for May Day 2015 by Melanie Marquis