The last average frost date in Iowa is what all spring planting is based on; the first frost is what all fall cleanup is based on. It’s helpful to know both.
Spring: The Last Average Frost Date
Climatologists have collected data for decades and determined, approximately, when the last frost of spring usually happens in very specific areas. This is an important date for gardeners to know. While many things we plant in the spring can tolerate some frost (many trees, shrubs, and perennials), some plants will be severely damaged by frost, such as tomatoes, impatiens, basil, marigolds, and other “tender” annual plants. So don’t plant those until after your area’s last average frost date. In Iowa, that date is:
Northern Iowa: May 15
Southern Iowa: May 10
Central Iowa: Somewhere in between
Click here for a detailed information from Iowa State Horticulture Extension on the average last frost dates for various Iowa cities and counties. However, please note that the May 10 through May 15 dates listed here are those dates on which you can reliably plant those annuals in your Iowa garden that might be killed by frost. Do it any earlier and you risk severe frost damage.
So about a month before your area’s last average frost date, you can plant:
• Trees and shrubs
• Perennial flowers
• Perennial herbs, such rosemary, tarragon, oregano, mint, and thyme
• Roses, especially bare-root types
• Cool-season flowers, such as pansies, violas, snapdragon, annual lobelia,
annual centauria, and Iceland poppies.
• Cool-season herbs and vegetables, including parsley, cilantro, lettuces,
radishes, spinach, broccoli, cauliflower, and kale.
Note: Spring bulbs purchased already in bloom in pots also are frost-tolerant. These include tulips, crocuses, hyacinths, daffodils, and many others.
Fall: The First Average Frost Date
The first average frost date in Iowa is calculated in a far looser manner than the last average frost date in spring, perhaps because the stakes are lower.
After the first frost in fall, the foliage of annuals and perennials in the garden are damaged and dying. That signals the beginning of fall garden cleanup. Unlike in spring, nothing will die if you don’t time it right.
In northwest Iowa, the first frost often can be the last week of September. In central Iowa, it tends to be the second or third week of October. And in southeastern Iowa, it tends to be at the end of October. That Iowa State Extension site above also has first fall
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