A support for mounting a grass or weed trimmer to a deck of a riding mower or tractor includes a bracket for releasably and rotatably receiving a post, a telescoping post, and a cradle for releasably receiving and nesting with a trimmer shaft supported by the post. The cradle is mounted to the post by a spring, allowing front to back pivoting of the trimmer during use. In another aspect, the invention provides a trimming system comprising a trimmer mounted to a mower deck by the support as described.