I just had my sewer line cleaned. Should I wait to use this now or wait (how long?) so roots will be there to absorb it - does it stop new growth?

Yes. You may use the Zep Root Kill after cleaning to dissolve the remaining root hairs. Applications may be made each year in the spring after plant growth begins, during late summer or early fall, any time there's a reduction in water flow. I hope this information is of help. We appreciate your business and opportunity to service you. Sincerely, Zep Inc. Retail Technical Support Team