Cold-hardiness zone map, heat zone map and fruit chill hour requirements

 

Our pond iced over.
Photo Ezra Freeman

I was alarmed to hear on NPR that the USDA had issued a new map of winter-hardiness zones. It seemed so recent that we got an update! Relax! (Sort of!) It’s the same 2023 map that I reported on here in November 2023. What is new and worth a visit is the interactive map posted by NPR .

2023 USDA Cold-hardiness zones map

Enter your location and see a map with your zone on the previous 2012 map. Scroll down (it was not immediately obvious to me to do this. . . ) and you can see what zone that area is in now. Scroll further down and you can see the amount the 30-year minimum temperature average has changed since the 2012 map (which used the data from 1976-2005). In my case, Louisa, Virginia has got 3 Fahrenheit degrees warmer in winter on average, than it used to be. Good for keeping some perennials and annuals alive over winter; not so good for fruits requiring a certain number of chill hours, such as apricots.

Our grape rows from the north.
Photo Kathryn Simmons

Fruit and Nut Tree Chill Hours Explained

Apple trees and other fruit and nut trees need cold temperatures to be able to set fruit the following season. Chill hours are the cumulative number of hours during the winter, below a temperature of 45ºF. Nut and fruit trees need a specific number of chill hours each winter to regulate their growth. It is a cumulative total for the whole of the dormant season, whenever they happen. Chill hours are further explained on the Stark Brothers website where there is also this map.Click the link for the ability to zoom in on your location.

Chilling Hours map of the US, for fruit and nut tree growers. MRCC Vegetation Impact Program

Citrus fruits do not need any chill hours. At our farm, we get around 700-800 chill hours each winter. Any tree needing less than 700 will be well served by our winters! Redhaven peaches need 800 – a bit of a gamble. Honeycrisp apples need 800-1000, so they are not for us!

Deke Arndt, director of the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information says that winters are warming at a faster pace than other seasons.

Also, an increase in the quantity and quality of data collected at weather stations across the nation in recent years has helped to increase the overall accuracy of temperature readings. The 2021 data are more accurate than the 2012 data, so the difference (change) over that time is a bit uncertain.

Immature frosty cabbage. Photo Lori Katz

Scroll down the NPR page further, and after a reminder that your zone measurement is an average of the coldest yearly temperature in your area over the past 30 years, you can see a scatter chart of the actual coldest temperature each year from 1991 to 2020. The coldest here was -13ºF in 1996. In that same period, seven winters have had a minimum that was warmer than 10ºF. When you average those coldest nights in each of the 30 years, the average comes out at 4F here. That is, the average coldest night over 30 years was 4ºF.

Scrolling down further we see that puts us in zone 7a, where the average lowest temperature each winter is in the range 0ºF-5ºF. Next comes a reminder that this is only useful for plants that have to survive the winter.

The site focuses on ornamental perennials: You plant them once and they come back after each winter if they’re given the right environment to survive. Think things like trees, shrubs and woody plants. Hydrangeas, Azaleas and Lavender can survive our winters. The Windmill Palm grows in zones 7-11. It might survive here. But, anyway, I’m more interested in vegetables.

A stormy winter day, garlic, rowcovered spinach beds and our hoophouse.
Photo Wren Vile

However, it’s also true that the lowest winter temperature affects annuals that are overwintered to produce spring vegetable crops. Combine the information about your winter-hardiness zone with my Winter-Kill Temperatures of Cold-Hardy Crops list to see what vegetables it is worthwhile keeping in your garden over winter, and which ones will probably survive with rowcover. And which would need a hoophouse or greenhouse to survive.

On its own, your cold-hardiness zone can’t tell you what to grow in your area. For example, parts of Juneau, Alaska; Boston, Mass.; and Santa Fe, N.M are all in USDA’s Zone 7a, as are we in Louisa, VA.

Juneau has relatively temperate winters that are extremely wet, averaging over 80″ snow a year. Santa Fe is extremely dry, with much hotter summers than Juneau. Boston has temperate winters and temperate summers. It gets rain, but not nearly enough for Juneau’s rainforest plants to thrive. It gets heat in the summer, but is colder and wetter in the winter, preventing desert plants like cactuses and other succulents from thriving.

Because all these three cities rarely get below 0ºF each winter, they are all classified as zone 7a.

Vates kale with a freeze-killed center January 19 2018.
Photo Pam Dawling

Here are some things to keep in mind about winter-hardiness zones:

  • Your winter temperature can still dip below your hardiness zone. Remember, your zone is a measure of the average coldest night in 30 years. Some years are above average, others are colder!! Even much colder! In 2014, the temperature in St Louis (zone 7a) dipped three half zones below St. Louis’ hardiness zone, to -10º F. So be alert to the threat of extreme cold snaps. And find a way to keep your plants warmer until the snap passes.
  • The hardiness map says nothing about the frequency of extreme cold weather. A plant or an animal can survive an occasional short dip to a temperature much colder than usual. But an extended period of very cold weather is more deathly.
  • Sweet potatoes in storage. An ideal crop for winter meals, as they store at room temperature for a long time, maybe seven or eight months.
    Photo Pam Dawling
  • The hardiness map can’t tell you if your plants will survive the summer.  I have had people ask me what zone sweet potatoes can grow in. My reply is that winter-hardiness zones will not affect sweet potatoes, because they are not out there in the soil in the winter! They are in your basement or a root cellar or a storage room, above 55ºF.
  • AHS Heat Zone Map 1997
  • For thinking about summer temperatures, consult the 1997 American Horticultural Society heat zone map that measured the average number of times per year that the temperature of an area exceeds 86ºF. The heat zone map is on the NPR site, if you scroll far enough. With 60-90 days a year with a high temperature above 86ºF, Louisa, VA is in heat zone 7 of the 12 created. I hope an update of this map comes soon.
  • If your winter-hardiness zone has changed, you could plant some new things, or leave plants such as figs, or spinach, that you previously wrapped each winter, unwrapped.
  • Warm microclimates such as containers on paved surfaces or near brick buildings absorb a lot of radiant heat during the day and hold it into the night.
  • Remember the cold-hardiness zone calculations are using the past 30 years’ data. If you notice temperatures are continuing to climb, year after year, you could experiment with less-hardy plants from the half-zone warmer than yours.
  • Your local Extension Service may be able to help with more local information.