Growing season has ended in Iowa City, IA

Iowa City's average last spring frost is around Apr 29, and its first fall frost around Oct 9 — a growing season of roughly 163 days. These figures come from the nearest NOAA station with long-term records (Iowa City, IA).

A first frost sorts the garden quickly. Basil, tomatoes, peppers, squash, beans and cucumbers are usually finished overnight, while kale, collards, carrots, leeks, parsnips and Brussels sprouts shrug off a light frost — several of them turn noticeably sweeter afterward, as cold pushes the plant to convert starches to sugars.

Frost forms most readily on clear, calm nights, when heat radiates away from the ground and the air just above the soil ends up colder than the temperature reported at the weather station. That is why a low of 36°F in the forecast can still mean frost in low spots and open beds. Cloud cover or a breeze usually prevents it.

Row cover, an old sheet, or even cardboard draped over tender plants before sunset traps enough radiated heat to buy a few degrees — enough to carry a crop through the first light frost to the warm stretch that often follows. Remove it in the morning so plants do not cook.