Iowa Plant Hardiness Zones Map And Gardening Guide

Summary

Iowa plant hardiness zones range from 4b to 6a. The average minimum extreme temperatures range from -25°F (-31.7°C) to -5°F (-20.6°C). The state has a humid continental climate zone. 

For extreme weather, Iowa experiences below-freezing temperatures in the winters and humid, hot summers with a risk of tornadoes, high humidity, and heat.^1-8

Iowa’s growing season ranges from 170 to 210 days, depending on where you live. Like a lot of the midwestern states, Iowa’s climate is starting to warm and see a longer growing season.^8,16 

Plants that require cold stratification to germinate can thrive in Iowa. Sowing cold-hardy annuals, like native wildflowers, can extend your growing season and increase your chances for earlier bloom times in your garden.^11-14

Plant suggestions include but are not limited to the following:^17-30

• Burr Oak
• Arctic Fire Dogwood
• Spiderwort
• Endive
• Lemon balm
• Garlic 
• Canada Plum
• Wild stonecrop

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Iowa Plant Hardiness Zones

USDA Hardiness Zone	Average Minimum Extreme Winter Temperature Range  Fahrenheit (°F) Celsius (°C)	Average Last Frost Date Range In Spring (Beginning Of The Growing Season)	Average First Frost Date Range In Autumn (End Of The Growing Season)
4b	-25°F to – 20°F -31.7°C to -28.9°C	Early May	Early to mid-October
5a	-20°F to -15°F -28.9°C to -26.1°C	Early May	Early to mid-October
5b	-15°F to -10°F -26.1°C to – 23.3°C	Late April to early May	Early to mid-October
6a	-10°F to -5°F -23.3°C to -20.6°C	Late April to early May	Mid to late October

Iowa’s Growing Conditions

General Climate^8-11

Iowa has a humid continental climate. 

Spring through summer experiences the most precipitation, with extremely humid, hot weather during this season.  

Iowa winters can experience freezing temperatures and snow storms, with extreme blizzard conditions at times. Winter prep is essential in Iowa, as the snowy season lasts from the end of October until April. 

Thunderstorms and tornadoes are most common during the spring season, with an average of 37 tornadoes a year, which can bring heavy winds, rain, and hail. 

Fall is the calmest and most enjoyable season in Iowa. It has the best range of temperatures and is a wonderful sight when all the leaves of the deciduous trees change color. 

Microclimates

USDA plant hardiness zones are an important starting point for your garden, but you’ll also need to consider microclimates. 

Microclimates are areas where specific conditions create a climate different from the climate they’re situated in. 

Buildings, fences, paved areas, or short hills and valleys can create these microclimates. 

They can be as small as a space in your backyard or as large as a city.

In other words, learn about your local conditions from local experts to see if your garden falls into a microclimate.

Extreme Weather^8,11-14

Blizzards 

Iowa experiences cold winter storms and harsh blizzards. This can include heavy snowfall, whiteouts, and high winds. Variations of this extreme weather are dependent on where you are within the state. 

Southern Iowa can experience less snow at an average of 30 inches (76.2 centimeters) or less, while the northern part of the state experiences a larger amount of snowfall at an average of 45 inches (114.3 centimeters) or more per year. 

You can use the cold to your advantage when living in a state with long winters. Some plants require cold stratification in order to germinate. This means they must undergo a period of colder temperatures to sprout and grow! Learning about and growing cold-hardy annuals can lengthen your growing season and mean you will have flowers blooming earlier every year! 

Humidity And Heat

Iowa can experience humid, hot summers with max temperatures ranging in the 90s in Fahrenheit (30s in Celsius) . Humidity can reach into the 90th percentile, meaning Iowa gets sticky and sweaty! 

Humidity can cause mildew and mold in the garden. To mitigate this, remove lower leaves and keep a wider spacing between your plants. This helps to create better airflow, which controls the spread of mold and mildew. 

Tornadoes 

Averaging 46 tornadoes a year, Iowa ranks 6th for the number of tornadoes the United States experiences annually. Tornadoes produce high winds, flash floods, and can be unpredictable and very damaging to properties! 

Regularly fertilizing and mulching your trees can create stronger root systems, making them more likely to withstand tornados. By creating healthy, strong trees, they anchor into the ground and become more resilient to extreme weather. 

Growing Season^8,16

Iowa’s growing season ranges from 170 to 210 days, depending on where you live. 

Like a lot of the midwestern states, Iowa is seeing longer growing seasons due to a warming climate. Over the last 30 years, they have seen an 8-to-12-day increase in the growing season, compared to its historical number.

Although the growing seasons are getting longer, chances of hard frosts still happen in the spring and fall, still making it a gamble to plant things out earlier. 

Iowa Gardening Tips^11-14 

Winter Weed Control 

Weeding in the winter before the snow sets in can help reduce the amount of work you will need to do in the spring when mitigating against weeds. This allows you to focus on other important garden tasks. 

Removing weeds before the seeds have a chance to spread (from snow melt and other forms of seed dispersal) helps to gain more control of the amount of weeds you have in your garden. Combine winter weeding with heavy mulch to prevent weeds from germinating to really ensure your weeds won’t spread.  

Cold-Hardy Plants And Cold Stratification 

Cold stratification is when plants require an overwintering period of below-freezing temperatures in order to germinate. 

In areas where there are seasonal winter climates, cold-hardy plants can help extend the growing season and can provide blooming flowers earlier in the season. Cold-hardy annuals are direct sown in the fall. 

Examples of cold-hardy annuals include larkspur, nigella, lupine, yarrow, and many native wildflowers. 

Cold Frames For Growing Year Round Salad Greens

Cold frames in cold climates help to extend your growing season and are an excellent tool for growing cold-tolerant salad greens all season long. Build the cold frame against the side of your house. The house generates extra heat, helping to keep a more consistent warmer temperature. 

By direct sowing in the fall and in late winter, then early spring, you can ensure salad greens all seasons long. Cold frames help germinate seedlings beautifully, even with below-freezing temps. 

Consult With Local Professionals

Consulting with local gardening professionals allows you to benefit from their experience with your area’s conditions, the plants that do well there, and overall best practices. 

Iowa Plant Suggestions^17-30

Trees

• Burr Oak (Quercus macrocarpa)
• Margaret’s Hawthorn (Crataegus margaretta)
• Common Juniper (Juniperus communis)

Shrubs

• Vanilla Strawberry Panicle Hydrangea (Hydrangea paniculata ‘Renhy’)
• Arctic Fire Dogwood (Cornus stolonifera ‘Farrow’)
• Common Purple Lilac (Syringa vulgaris)

Flowers

• Rattlesnake master (Eryngium yuccafolium)
• Spiderwort (Tradescantia ohioensis)
• Partridge pea (Chamaecrista fasciculata)

Vegetables

• Endive (Cichorium endivia)
• Leek (Allium ampeloprasum)
• Corn (Zea mays)

Herbs

• Lemon balm (Melissa officinalis)
• Dill (Anethum graveolens)
• Comfrey (Symphytum officinale)

Spices

• Garlic (Allium sativum)
• Spicebush (lindera benzoin)
• Ginger (Zingiber officinale)

Fruits

• Canada Plum (Prnus nigra)
• Common Pawpaw (Asimina triloba)
• Common Persimmon (Diospyros virginiana)

Succulents

• Wild stonecrop (Sedum ternatum)
• Hens & Chicks (Sempervivum tectorum)
• Prickly Pear Cactus (Opuntia humifusa)

Disclaimer

Any of the above can change and is not exhaustive. 

Treat anything above like a good starter guide. Then use that as a foundation as you consult with local gardeners, professionals, forecasts, guides, and organizations. 

Sources

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7. “Frost Dates for Ottumwa, IA”. Almanac. Accessed April 11, 2024.
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9. Britannica, The Editors of Encyclopaedia. “Humid continental climate | Temperature, Precipitation & Seasons”. Britannica. January 18, 2023. Accessed 11 April 2024.
10. “Humid continental climate”. Wikipedia. April 3rd, 2024. Accessed April 11, 2024.
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13. “Heat and Humidity Tolerant Plants”.Swallowtail Gardens. Accessed on April 11, 2024. 
14. Protect Your Home by Prepping Trees for Tornadoes and Hurricanes. Davey Tree Service. Accessed April 11, 2024. 
15. “Tornadoes | Iowa”. PBS. Accessed April 11, 2024. 
16. Herrold, Benjamin. “Farmers see longer growing seasons”. Missouri Farmer Today. March 13, 2023. Accessed April 11, 2024. 
17. “The Nativ The Native Trees of Iowa Trees of Iowa”. Iowa State University. 2006. Accessed April 12, 2024. 
18. “Quercus macrocarpa”. Wikipedia. February 18, 2024. Accessed April 11, 20.
19. “Bestselling Iowa Trees & Shrubs”. Nature Hills Nursery. Accessed April 12, 2024.
20. “Cichorium endivia” North Carolina Extension Gardener. Accessed April 12, 2024. 
21. “Leek”. Wikipedia. March 28, 2024. Accessed April 14, 2024.
22. “Sweet corn”. Wikipedia. April 10, 2024. Accessed April 14, 2024.
23. “Lemon balm”. Wikipedia. March 30, 2024. Accessed April 14, 2024.
24. “Dill”. Wikipedia. April 14, 2024. Accessed April 14, 2024.
25. “Symphytum”. Wikipedia. December 3, 2023. Accessed April 14, 2024.
26. Mahr, Susan. “Garlic, Allium sativum”. Wisconsin Horticulture. Accessed April 14, 2024
27. Mahr, Susan. “Ginger, Zingiber officinale”. Wisconsin Horticulture. Accessed April 14, 2024
28. Grant, Bonnie L. “Hens And Chicks”. Gardening know how. March 11, 2021. Accessed March 10, 2024
29. “Sedum ternatum (Wild Stonecrop)”. Gardenia.net. Accessed March 10, 2024.
30. “Eastern Prickly Pear Cactus”. Canada.gov. April 2010. Accessed on March 10, 2024.