You may not have a 'ridge' from end to end you only need a 'flat' section with the ridge that allows the planer to work the far side. Ideally, the 'flat' would extend to at least 2 inches from each end (cutter to roller distance where snipe can occur anyway).
Be sure to have the convex side down on the jointer. I know that is contrary to normal jointing but in this case you are not really jointing, you are flattening. The light cuts on the planer will get rid of the concave ends.