Which foods are in-season in Rome right now?

I'm just going from memory, but I think that fresh fig (ficchi) and cactus fruit (ficchi di India) are both popular gelato flavors in September, as well as pear. In Grom and some other shops, it's pretty obvious which are the limited-time special flavors.

Artichokes are probably out of season, so some restaurants won't serve them, while eggplant season seems never to die (unfortunately for me). Tomatoes should be good. Porcini mushroom season might be starting; look for "freschi" on menus and/or actual fresh mushrooms displayed in the window.

P.S. Another fall gelato flavor is cachi - persimmon.

When we were in Italy 3 weeks ago zucchini blossoms were on some menus and lots of dishes with mushrooms.

I was just there and zucchini blossoms , tomatoes, mushrooms and figs as well.