Welcome to NGA,
@Myrajillrussellyaho ! (whew -- I thought "weedwhacker" was a long screen name...
)
As long as you don't have plants overwintering in the raised bed I see no reason why covering it with a tarp would be harmful. (If you DO have plants in the bed, I think it would be better to use something like "floating row cover" - such as Reemay, Agribond, etc -- that would let water through.)
