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one day i'll have a house with a lawn and there's gonna be so much damn native grass...
(beehaw.org)
[alt text: Text that says, "People [say] 'I never see butterflies or lightning bugs in my yard. Their yard: (colon)". Below the text is a photo of a birds-eye view of a large house with an equally large yard. The lawn is covered in standard turfgrass (probably Kentucky bluegrass) that has been recently mowed.]
The plywood was to provide extra weight for crimping the grasses, which helps to keep them acting as a cover for the next round of grasses and herbaceous plants coming up instead of standing back up. Some operations use a weighted crimper/roller on a tractor, some folks use railroad ties, Ray Archuleta used to use his daughter's old honda civic... I had leftover plywood from a project.