Which of the following noxious weeds can be controlled by digging and tilling up the plants?

The correct answer is smooth dock. This plant, along with certain other perennial weeds, can be effectively managed by physically removing it from the soil. Digging and tilling serve to disrupt the root systems of these plants. In the case of smooth dock, which has a deep taproot, it is crucial to ensure that the entire root is removed to prevent regrowth.

This method is particularly effective during the growing season when the plant is actively using resources to fuel its growth, allowing for more complete eradication. Additionally, repeated tillage can help in managing seed banks in the soil, increasing the likelihood of successful control over time.

In contrast, Canada thistle and field bindweed can regenerate from small root fragments, making physical removal alone less effective. Teasel, while it can be dug up, often requires additional management techniques to control effectively due to its rapid growth and seed production. Thus, smooth dock stands out as a weed where digging and tilling are reliable methods for control.