Alien plants can come in in various ways. Well known examples of inadvertent introductions are, for instance, weedy species that are introduced as contaminants in cereals (so-called grain aliens) or pasture species that are introduced in sheep wool (so-called wool aliens). Likewise, the nursery trade is also responsible for the introduction and spread of weed species. Other sources may be bark or timber, granite, coal, sand, etc. but these vectors are obviously of a lesser importance in Belgium than are cereals these days or was wool in the past.