Unusually cold temperatures Monday night caused a frost in Story County, and freeze advisories were issued in counties north of Ames.

Crops yet to be harvested could be damaged by the frost, said Dale Farnham, assistant professor of agronomy extension.

Only 5 percent of the corn crop and 2 percent of the soybean crop have been harvested, he said.

A lot of crops were planted late in the spring due to cold and wet conditions early in the season, Farnham said. This makes the crops’ maturity date later and closer to the first frost date, he said. Typically, Oct. 5 is the first frost in central Iowa, Farnham said.

“The seed will be smaller which affects the yield of the crop,” he said.

A smaller yield means a smaller profit for the farmer, Farnham said.

There isn’t anything to prevent the frost from harming the crop, he said. Harvesting the crop before the frost happens is the only way to prevent it, Farnham said.

Frost doesn’t always have to end the growing season, though. If the frost only burns the top part of the flower on soybeans, the seed will continue to grow, he said.

However, if the frost gets to the base of the plant, it kills it, Farnham said.

Frost harms the outer surface of the plant while freezes can go deep inside the plant. Hard freezes end the growing season, said Shane Searcy, meteorologist for the Des Moines National Weather Office.

When dew on the surface of a plant freezes, frost develops, he said.

Frost occurs most often with light winds, clear skies and surface temperatures around 32 degrees, Searcy said.

“With either a freeze or frost, tender vegetation will be damaged,” he said.

The damage will depend on the thickness of the ice on the plant and how long it stays on it, he said.

Gardeners can protect their plants by covering them with a towel or blanket or bringing them indoors, Searcy said.