Weather forecasters say frost could hit northern Iowa tonight. Iowa StateUniversity extension horticulturist Gary Peterson says the damage done toplants by the frost depends on how far below freezing the temperature drops.Peterson says the temperature must stay below freezing for more than severalminutes to do much damage. He says you can protect tender plants by coveringthem. Peterson says covering the plants helps hold in warm air.Tonight’s frost is predicted for the northern half of Iowa, above Highway30.