Today my yard helper Paul did this throughout 95% of my center food forest which had been overtaken by two VIGOROUS tall-growing annual grasses. The seed heads were still VERY green and immature so I think that seed production was largely prevented. My plan is to this winter cover the whole area with a foot or more of leaves, which should choke out the vast majority of grass seedlings, while the sweet potatoes should have no problem emerging through the leaf layer. Cool to see in this video some plants I grew in my Denver gardens.

http://www.youtube.com/watch?v=eo3atfTD-N4