Urgent weed problem in lawn

At a local church of mine i am responsible for maintaining the garden of rememberance. Currently, it is a large lawn area, but in the middle there is a large cross cut out of the grass. In the garden we are currently having alot of problem of not only weeds growing on the lawn area, but alot of weeds growing in the cut out cross.
What i want to know is does anyone have any solutions to the weed problem. The cross has about an inch and half thickness of bark over the top, but because the cross is in use for the burial of ashes, we cant put a plastic layer underneath as access is regularly needed.
(p.s im not sure if this post is in the right place as im abit of a newbie)
Thanks in advance. Matt

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