Dustin Dorsey
TVWBB Hall of Fame
Celery seed has nitrite (or nitrate?) in it so it will partially cure the meat. I'd think you might want to put it on in advance for it to work. If you put it on the night before you'd might get way too much smoke ring. I've seen pictures people post where it's clear they put tenderquick on it and it doesn't look right. Some of my best briskets had no smoke ring, but it sure looks nice if it's there.