So far as I'm aware, you earth up the whole bulb, up to the topgrowth - it makes the bulb more tender and keeps it nice and white. That's what they do on the allotment where I live anyway, I've never grown it myself - I'm in the UK, and they start earthing up end of August, early September, when the bulb is visibly starting to swell. It also protects the bulb from the possibility of early frost before its been harvested, which might not be an issue where you are anyway. Certainly, descriptions of earthing up liken it to earthing up potatoes, which definitely are earthed up over the stems of the topgrowth.