When European colonizers brought crops and animals to their new settlements, they carried with them a diverse segment of the European flora in the form of both crops and weeds. According to Crosby the weeds may have been even more important than the intentionally transported species in transforming much of the world into European adjacent ecosystems.

To define why weeds were so important in ecologically colonizing the globe, the text first defines exactly what it means by a weed. Weeds are not necessarily undesirable; many “weed” plants have been considered important cultivars in certain areas. Weeds are defined by their hardiness and their rapid reproduction rates, and for these reasons they were perfectly suited to the recently invaded lands in the New World. European expansion necessitated the establishment of European infrastructure and agriculture across the globe. To achieve this, the European settlers had to remove huge swathes of native plants, tilling the soil and planting European species in their place, building roads and cities, and otherwise ravaging the landscape. This disturbed land was the perfect place for certain plants that thrive in harsh conditions to flourish. European plants like plantain and stinging nettle, weeds that now live worldwide, grow best in disturbed soil and are able to survive trampling and other interference by humans and livestock.