Mustard has a sharp or keen taste. "Keen as mustard" means being sharp-minded.
As keen as mustard or as keen as a kitten
Food
The phrase "as keen as mustard" combines "keen," meaning enthusiastic or eager, with "mustard," which historically refers to the sharpness and zest of the condiment. The expression likely originated in 19th-century England, where mustard was associated with a strong, vibrant flavor, symbolizing enthusiasm and zest for life. Thus, the phrase conveys a sense of lively eagerness, capturing the essence of being intensely enthusiastic or alert.
"Keen" can mean either sharp or eager.
Mean Mr. Mustard was created on 1969-09-26.
The word "keen" rhymes with mean and can also mean to scream loudly or shout enthusiastically.
mustard gas
sheen, lean, mean, bean, green, jean, keen, ween
The word "keen" can mean sharp in the context of physical attributes, such as a keen blade or keen senses. It also conveys a sense of intensity or eagerness, such as having a keen interest or enthusiasm for something. In both cases, it suggests a heightened level of perception or ability.
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Keen can have several meaning depending on its place in language, is can mean 'sharp and having a fine cutting edge', or highly developed, or eager or zealous, it can mean sever or acute (as in a keen or acute sense of hearing), it can also mean closely contested or strong. Also 'to be keen on' can mean to have a strong affection for.
Presuming you mean mustard plants, then presumably, yes. They only need moisture to grow.