I believe Hamburg parsley is biennial, just like leaf parsley. Since it's usually the root one eats on Hamburg parsley whether or not it's a perennial is rather academic. Were you intending to use it for the leaves only?

I grow Italian flat leaf in the open garden and use huge amounts. I don't think a 16" pot would be enough. My parsley patch is a couple of square metres.

Hamburg rooting parsley is just a variant of regular parsley (radicosum group, aka Petroselinum crispum var. tuberosum) that produces fat, parsnip-like roots that are popular in eastern European cuisine. It is also a biennial.

Seeds for this are available at various outlets, including Baker Creek. Not sure why it would be considered any more reliable or longer lived than regular curly or flat leaf parsley in a hot, humid climate........

From Underwood Gardens: "Parsley, Hamburg Root Seeds - (Petroselinum crispum var. tuberosum) SKU: H1080 Seed Count: Approx 100 seeds per pack Type: Perennial Organic: Yes Days to Maturity: 75 days Status: Heirloom, Non-Hybrid, Non-GMO seeds."

Underwood Gardens says that its a perennial and I've seen that elsewhere as well. I was planning to eat the leaves, but perhaps the root also. Obviously if I harvest the root, that would be the end of the plant.

They can call it whatever the want but it is still a biennial, just like any other parsley variety. And there are several dozen websites that will attest to that. At most, it might be considered a short lived perennial in a suitable climate as several biennials tend to be but I would not expect it to survive more than a couple/three years. And once it flowers and goes to seed, that's the end of it.

FWIW, if I can collect seeds from a plant that will easily produce more of that same plant, I consider it as good as a true perennial, since you only need to buy those seeds once.

There are others such as this site that list it is a biennial.

https://pfaf.org/user/Plant.aspx?LatinName=Petroselinum+crispum+tuberosum