The term wool alien is used for any plant species whose occurrence at a particular site is due to transportation there as a result of the making of wool products.
The most common source of wool aliens is when a plant, seed, or bur attaches to the wool of a sheep or other wool-producing animal prior to shearing.
After the animal is shorn and the raw wool is transported to a refinery mill, the impurities are discarded along with the attached seed, which then successfully germinates.
[1] Wool aliens are typically found on waste ground near woollen mills.
Stork's-bills (Erodium) are particularly prone to appearing as wool aliens in Britain.