Aggressive biennial that may also develop as an annual or winter annual?

• A. Diffuse Knapweed
• B. Scotch Thistle
• C. Plumeless Thistle
• D. Musk Thistle

Answer: (D)

Life cycle variability under different conditions determines whether this weed behaves as a biennial, annual, or winter annual. Musk thistle is classically a biennial, rosette in year one and flowering in year two. But, when seasons are favorable, it can complete its life cycle in one year, or germinate in fall and overwinter as a rosette to flower the next spring (winter annual). This flexibility makes musk thistle more aggressive and harder to predict than species that are typically fixed as biennial. The other species listed are not usually described as having this same range of annual or winter-annual life cycles, so musk thistle best fits the description.