Weeds that are legally regulated and cannot be present in any quantity in seed for sale are referred to as:

Weeds that are legally regulated and prohibited from being present in any quantity in seed for sale are referred to as noxious weeds. These weeds are deemed harmful to agricultural crops, ecosystems, or human health. Regulations vary by jurisdiction, but the designation of a weed as "noxious" typically arises from its potential to cause significant economic damage or ecological disruption.

Noxious weeds often compete aggressively with crops for resources such as water, nutrients, and light, significantly hindering crop production. Therefore, stringent measures are often put in place to prevent their spread, including laws that restrict or regulate their presence in commercially sold seeds. The classification as noxious prompts the need for management and control strategies to mitigate their impact on agriculture.