A flexible tall crop knockdown barrier for use with rotary cutting headers in harvesting specialty crops such as grass seed. The knockdown barrier comprises a support frame mounted to the front of the header that extends forwardly and upwardly ahead of the header. A flexible barrier supported by the frame extends downwardly during idle conditions and downwardly and rearwardly as the header is propelled through the crop. The flexible curtain may be rolled around a lower spool and stowed on the support frame when not in use to improve visibility. The knockdown barrier serves to gently lean the crop forward before cutting so as to minimize seed shatter due whipping of the seed head.