Now would be good... As its a bit late in the grow season, I would recommend the kind that you disolved into water and spray on the foliage or drench the ground with. I've recently lost a good dozen plants by being foolish and putting granuals too close to the stem. It was definitely a "duh!" moment when I realized what I did, as I really should have known better.

If you plan on storing the tubers over winter, it is recommended that you stop fertilizing early Sept, as absorbed nitrogen tends to keep tubers from storing well.