Which of the following noxious weeds has deep roots and cannot be controlled by digging or tilling up the plants?

Russian knapweed is known for its deep root system, which is a significant characteristic that makes it particularly challenging to control. This perennial plant can develop roots that extend down several feet into the soil, making simple removal methods like digging or tilling ineffective. When attempting to eliminate Russian knapweed by these methods, any remaining root fragments can resprout, allowing the plant to thrive and continue spreading.

In contrast, while some of the other noxious weeds listed may have substantial roots, they do not exhibit the same resilience against physical removal. For instance, curly dock, poison hemlock, and puncture vine might be controlled through certain digging or regular tilling practices, particularly during specific growth stages. However, Russian knapweed's adaptive nature and deep roots necessitate more comprehensive management strategies, including the use of herbicides or integrated control methods that target its growth cycle effectively.