A rotary walk behind lawn mower (2) has a wheeled cutting deck (4) with a downwardly facing cutting chamber (16). A rotatable cutting blade (20) is contained inside the cutting chamber (16) above a lower edge of a peripheral sidewall (24) and beneath a central hub (26), an annular channel (28) being formed between the sidewall (24) and the hub (26) of the cutting chamber (16) for receiving grass clippings from the cutting blade (20). A portion of the peripheral sidewall (24) of the cutting chamber (16) above the level of the cutting blade (20) and a portion of a top wall (22) of the cutting chamber (16) form a pivotal mulch door (44) for opening and closing the annular channel (28) in which the grass clippings are moving.
