I was/am hoping some of the experts would/will chime in on this subject since I would like a definitive answer as well. What I DO know is that Concrete curing slows drastically at 50 degrees and stops entirely at 40 degrees (that is the temperature of the concrete itself though and Not Air temperature and I am not sure how to correlate the two). Since Hypertufa is basically a special blend of the same concept (Cement and Aggregate), I would think that the same temperature requirements would apply. If no one else replies with an expert opinion, perhaps a detailed search of the archives will give us the answer?

Great Answer. Consistent temps around 50 degrees F or above is best if you want to the hypertufa to cure properly, otherwise the planters will crack. That was my first big mistake many planters ago.
Mike

Here is a link that might be useful: My Blog