What type of weed is characterized by spiny, notched leaves and typically occurs when turf is mowed?

Thistle is indeed characterized by spiny, notched leaves, which are a key identifying feature of this type of weed. These leaves often have a sharp, prickly texture that makes them stand out in a turf environment. Thistles are commonly found in lawns and grass areas, particularly when mowing occurs, as this can stress the turf and create favorable conditions for their growth.

In contrast, the other plants listed have distinct characteristics that do not align with the description. For instance, violets have heart-shaped leaves and typically present a different flowering structure, while common chickweed has smooth, ovate leaves with a different growth habit. Ground ivy, also known as creeping charlie, has a distinct square stem and roundish leaves, which are not spiny or notched, making it easily distinguishable from thistle.