What is needed to kill annuals or biennials, and when is it easiest to do so?

• A. To kill annuals or biennials, you must burn the field; easiest when seeds are present.
• B. To kill annuals or biennials you must remove roots only; easiest in winter.
• C. To kill annuals or biennials you only need to cut the top; easiest mid-summer.
• D. To kill annuals or biennials, you must kill the whole shoot; tillage or herbicides will kill them and stop seed production.

Answer: (D)

The main idea is that annuals and biennials must be killed as whole plants to prevent them from coming back or setting seeds. If you only cut the top, or remove the roots, or burn while seeds are present, you may leave viable roots or seed in the soil, so the plants can regrow or spread again. Tillage physically destroys the entire root and crown, and herbicides can move through the plant to kill it completely, stopping seed production. The easiest time to do this is when the plants are actively growing and before they have produced seeds, so killing the entire shoot prevents them from seeding and establishes better long-term control.