USDA Hardiness Zones in Ohio

Anything you plant in Ohio in the landscape must endure temps as cold as -15°F to a (somewhat) balmier -5°F, depending on your location.

The USDA hardiness zones in Ohio are 5b, 6a and 6b, with the temperatures getting warmer as the zone number increases. Ohio used to include a zone 5a at the colder end of the spectrum, but the USDA, along with other sources like the Arbor Day Foundation, now omit 5a.

What do these zones mean, and how do they affect what you plant? Let's find out.

About USDA Hardiness Zones

When you find out that your hardiness zone is 6a, which is defined as -10 to -5°F, can you assume that's as cold as it gets? No. The zone reports the average lowest winter temperature in a location over a certain amount of time, according to Oregon State University. This temperature is influenced by elevation, latitude, wind, any nearby body of water and other factors.

The map was last updated in 2012, based on data from 1976 to 2005. The zones are likely outdated at this point, primarily due to climate change. Experts know this because the 2012 data itself reported many areas shifting warmer by as much as a half zone from the prior zone map. In fact, it was with the 2012 map that most of Ohio moved from zone 5 to zone 6.

Nonetheless, it's an excellent guide as long as you know how to use it. There are a few factors to consider, such as the existence of microclimates in any given area and whether your plants are situated directly in the soil or in pots.

USDA Hardiness Zone 5b

Zone 5b comprises just a small part of the state. For example, the Ohio zone map provided by Gilmour reports that the largest area in this colder zone is south of Mansfield surrounding Mount Vernon. Other pockets of the state remain in zone 5, including the very northeast and west of Dayton.

This zone's average minimum temperature is -15 to -10°F, with an average last frost date of May 14 and an average fall frost date of October 1, although these dates vary from year to year.

Hardiness Zones 6a and 6b

USDA hardiness zone 6a covers central Ohio, including the areas around Columbus. Zone 6b runs along the state's northern border along Lake Erie, including Cleveland, and along the south and eastern borders along the Ohio River and other locales.

Cincinnati bridges the two zones, so your zone will differ depending on where you live in that city. For example, downtown is within 6b, but Price Hill to the west falls into 6a.

The average winter temps between the two zones don't differ all that much, however: Zone 6a's average winter temps range from -10 to -5°F, while 6b is in Ohio's banana belt at -5 to 0°F .

Microclimate Considerations

So you live in zone 6b. Does this mean you can depend on the temperature range published for this zone? Nope. Other factors influence an area's climate; one area could be measurably colder or warmer than another area close by.

The tags on nursery plants use the USDA hardiness zones but can't consider the potential microclimate of a given area. This is where you have to do some observation and research. One obvious variable is the elevation: the higher the elevation, the colder the temperature. In fact, on average, the temperature decreases 3.5 to 5°F for every 1,000-foot increase in elevation.

Other factors, including wind, winter sunshine, snow and humidity, play a part. You might think that the presence of snow would mean that plants are more susceptible to cold, but snow can insulate against extreme cold, covering the root systems of plants that are dormant.

To determine whether your area has its own microclimate, observe the angle of the winter sun, the location of your garden in regards to structures that provide shade (whether natural or man-made) and how exposed to wind your area is. Talk to local experts at your agricultural extension office and local nurseries to determine the likely microclimate and potential cold tolerance of any plants you want to incorporate into your landscape.

Above all, don't just note the USDA hardiness zone on a plant tag to determine definitively what you can plant in your Ohio garden.