Fruit Crop Cold Hardiness Zones: The Dream and the Reality

The cold hardiness of fruit trees and woody plants in general is somewhat debatable.  One reason is that if you ask at what temperature a particular fruit cultivar will incur damage, there are a number of factors that go into that.   

Generally, the least cold hardy part of a fruit plant are the fruit buds, while the vegetative parts of the plant are more cold hardy.  So, the question of whether or not a fruit species or cultivar will reliably survive somewhere is often a different question than whether it will reliably produce fruit in that area.

Cold Hardiness Can Vary By Location

If we use peaches as an example, 'Reliance' is generally regarded as the most cold-hardy cultivar of peaches. There may be some seedling varieties that are more cold hardy, but of the named cultivars, 'Reliance' is the standard to try when growing in marginal peach production areas.  When I performed an internet search in Google for 'Reliance peach hardiness zone', eight of the top 10 sites list 'Reliance' as hardy to zone 4 and two of the sites list it as hardy to zone 5. 

Which ones are correct? Good question.  Personally, I'm risk averse when recommending planting marginally hardy fruit cultivars and species.  The cost of the trees is high, they take several years to bear a full crop, and there are only a few years when they bear a full crop. Additionally, species like peaches are short-lived trees in our region.

Sources indicate differing levels of hardiness for peach buds.  Some states, like Michigan, suggest that bud damage will begin at -13F, and generally in Iowa some articles suggest that significant bud loss occurs at -18F.  Why the differences?  One reason is that we are growing different cultivars; some are more cold hardy than others. 

The other reason is that plants will attain different levels of hardiness in different regions.  One example is that some grape cultivars are listed as hardy to -30F in Minnesota, but studies in the Finger Lakes region of New York will have those same cultivars only hardy to -20F in the middle of winter.  This is because plants can somewhat respond to their environment.  In that area of New York, the temperatures rarely get below -5F. In that setting, the plants do not develop maximum cold hardiness since they are not exposed to very cold temperatures, while the same cultivar in Minnesota experiences much colder temperatures, and so develops more cold hardiness.

Does this mean a peach tree will be able to withstand Iowa winters reliably?  Not really.  These plants have genetic and physiological limitations that prevent their fruit buds from being cold hardy to something like -25F.

So where could/should we plant peaches in Iowa?  If we were looking at this from the perspective of needing to get a crop every year, there are very few areas I would plant them.  But in your backyard, getting a crop of fresh homegrown peaches once every 3 years or so, with the trees only living 15 years, may be acceptable.  From a business perspective, if you were selling peaches at a wholesale price of $1/lb, this would be a horrible investment. Let's look at some numbers to back this up.

Plant Hardiness Zones Don't Tell You Everything

The plant hardiness zones 4b, 5a, 5b, 6a are found in Iowa (Figure 1).  Plant hardiness zones tell you the average annual extreme minimum winter temperature over a historical 30-year period (Table 1).  The most recent hardiness zone map is from 2023. It's based on the average annual coldest temperature from 1991-2020. 

We need to keep in mind that hardiness zones do not tell you what are the lowest temperatures that can occur each year, it's the average low temperature that occurred in that 30-year period.  For example, if we looked at a 2-year period, if there was a low of -30F and -10F, the average would be -20F, but plants don't live on averages!

To further tell the story, I looked at 4 locations in Iowa and overlayed them on the current USDA Plant hardiness zone map (Figure 1).  The northwest location is Orange City, the southwest is Atlantic, the northeast is Decorah, and the southeast is Oskaloosa.  I looked up the low temperatures from each location from Iowa Environmental Mesonet.  I put those temperatures in three categories (Table 2).  High risk (less than -18F), medium risk (between -18-13F), and low risk (above -13F).  The percent listed is how many years the annual low temperature was in that range from 2010 to 2024. 

Keep in mind that the topography and other factors around the weather station can greatly impact the temperature.  For example, if the weather station is at the top of a hill or a river valley, the river valley will be colder.  So don't focus too much on each exact temperature.  In the news the windchill temperature, or as they call it now, 'feels like temperature,' makes the press, but plants basically respond to the actual temperature, with the windchill having minimal impact.

Depending on what temperature we use for bud damage on peaches (-13 or -18), you can see that there are very few years were there is a low risk of bud damage.  In Decorah (NE) it's as low as 7% of the time in those 15 years, and in Oskaloosa it was 27%. 

When Can You Expect to See Damage?

Why don't we know exactly at what temperatures peach buds will die?  As was mentioned before, there are large cultivar differences.  'Reliance' is likely hardier than a California or Georgia peach cultivar.  The time of year also matters (Figure 2).  If we get down to -5 in October, we will likely see fruit bud damage on peaches. But those same temperatures in January will likely not incur damage since maximum hardiness is in the middle of winter. In the fall and spring, the buds are less hardy.

So, is a peach like 'Reliance' cold hardy to zone 4 like 80% of the nurseries stated?  Yes and no.  My experience from the peaches at my place suggests that at temperatures between -13 to -18F I would be losing some fruit buds. Keep in mind that peaches produce more flower buds than we need, so we can lose a few with minimal yield loss.  One note of concern is that as temperatures approach -20F we are very likely receiving damage to the woody parts of the tree, and temperatures of -25F could even be lethal to the entire tree.  In 3 of the 4 locations in Iowa I looked at, we were below -25F at least once in those 15 years. (Table 3)

So What Can We Expect?

So, would a peach tree survive in zone 4b?  I'm sure that it will for a while, but the risk of premature death is high, and the likelihood of a reliable crop is very low.  I would also suggest that this is the case for planting peach trees in zone 5a and maybe zone 5b as well.

Additionally, consider a 'recency' bias factor when selecting fruit trees.  At the Oskaloosa site, from 2020-2023, temperatures were not recorded below -18, but from 2017-2019 each year, it was recorded below -18. The early 2000's were hypothetically good years for peaches there, but the preceding 3 years were not.

My take-home message is to take reported hardiness zones for some fruit trees with a bit of skepticism and realism.  If I want a relatively consistent crop most years, don't assume that if a fruit listed as just hardy to your zone that it will do this.  Also keep in mind that your site can have a large impact on these absolute temperatures.  By not planting in low-lying areas, you increase your odds of not receiving those ultra-low winter temperatures where cold air will settle on a still night. At my own home in rural Mahaska County, I consider it a win if I get a crop of peaches every other year, but acknowledge that my peach trees, on average, have lasted less than 15 years of healthy growth.

More Information

• What affects will the cold temperatures have on my fruit trees?
• Cold damage to peaches from Michigan State University