Our Local Climate Has Changed

USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov/.  Click the link for access to state and regional interactive maps.

I’m prompted to write this post to alert growers to the publication of the revised USDA Plant Hardiness Zone Map (often incorrectly called the Cold-Hardiness Zone Map). Canada and Mexico are now included. Most areas are a quarter-zone warmer in winter than in 2012.

Click this link for more on the history of the USDA Plant Hardiness Map. Note that the original intention of this map, which shows the average annual minimum winter temperature, was to indicate which perennial plants (such as fruit trees or shrubs) would survive the winter in each location. It does also indicate which biennial and annual plants can successfully overwinter (on average). There are lots of things it was never designed for, such as indicating how long the winter lasts, or how many days will be below a certain temperature. It is very valuable for its purpose: showing which plants will survive in an average winter.

Zones 10 and 11 were added in 2012, and Zones 12 and 13 are added to the new 2023 map. Zones 12 and 13 are presently only found in Hawaii and Puerto Rico, where the average annual minimum temperatures are above 50°F (10°C) and 60°F (15°C) respectively. The new interactive map has closer definition between neighboring areas than previously, and can be examined at a finer scale. A city or town with heat-absorbing concrete, can be seen as warmer than surrounding countryside.

Comparison of plant hardiness zones mapped for the 11 largest Metropolitan Statistical Areas [MSAs (U.S. Census Bureau, 2003)] in the 1960, 1990, and recently revised Plant Hardiness Zone Maps.

This technical article by Widrlechner and others in HortTechnology in 2012 gives a short history of the Map, and explains how it can be used to predict winter-injury to plants, and avoid damage by changing shipment dates or adding plant protection. It explains how freeze injury can occur during three stages in the annual cycle of plants: firstly, extreme cold occurring during the fall, before plants have adapted to such temperature levels; secondly, during mid-winter when extreme cold can still overwhelm the plants’ adaptive survival mechanisms; thirdly, during late winter and early spring of plants “deharden” after satisfying their minimum physiological rest requirements, in unseasonably warm spells.

The article also discusses attempts in various regions to add in the effects of other weather features such as cloud cover, wind speed and snow cover. It also discusses the 1997 AHS heat zone map, and shows graphs correlating the Plant Hardiness Zone Map and the Heat Zone Map for three parts of the US. Note the date of publication: references to the “updated” map refer to the 2012 version, not 2023.

Pablo Batavian lettuce. The batavians are our most bolt-resistant heat-tolerant lettuces.
Photo Nina Gentle

For summer weather, the American Horticultural Society in 1997 created a Heat Zone Map to show how many summer days are likely to exceed a certain temperature. It is not the mirror image of the USDA Plant Hardiness Map – it does not show the yearly average hottest temperature. I think an update of a heat zone map is due!

AHS Heat Zone Map 1997

According to the AHS map, I am in Zone 7 for summer temperatures (61-90 days a year reaching 86°F (30°C) or more) — right in the middle of the range. According to Weatherspark.com, from June 11 to August 30, our average daily high temperature is 86°F (30°C) or more. Summer nighttime temperatures are also an important feature of the weather, for us and our crops.

Twin Oaks is in USDA Plant Hardiness Zone 7a. (our zone remains unchanged after the November 2023 update, but we did move 5 Fahrenheit degrees milder from our 1990’s 6b to 7a in the 2012 revision). The average annual minimum winter temperature in zone 7a is 0°F–5°F (–18°C to –15°C). I have noticed that our local climate has changed on average, over the past ten-twenty years, to become drier, with milder winters and hotter summer nights.

Savoy cabbage with frost.
Photo Lori Katz

Here are other features of our weather at Twin Oaks, Louisa, central Virginia:

  • Our average rainfall for a year is 37″ (100 cm), fairly evenly distributed throughout the year, at 2.2″–3.6″(5.6-9.1 cm) per month. February is the driest, May the wettest.
  • Our average daily maximum temperatures are 49°F (9.4°C) in December and January, 89°F (31.7°C) in July. The average night low temperatures are 29°F (–1.7°C) in January, 69°F (20.5°C) in July.
  • Our season from last frost to first frost is around 211 days. The average date of the last spring frost is April 24 (later than May 7 only happens one year in ten); the average date of the first fall frost is Oct 14 (earlier than Oct 1 only happens one year in ten).

    Such grey weather! But will it rain?
    Photo Wren Vile
  • Our climate is controlled by three weather systems, mainly by moisture from the Gulf of Mexico, but also by the Bermuda High Pressure area in summer and the recurrent waves of cold Canadian air in winter. Due to the erratic movement of thunderstorms, some parts of our area may experience long periods of drought. September–November is the dry season but also the hurricane season.
  • Our latitude is 38° N, which is very relevant to onion growing and to daylight length.
  • Our period when daylight length is less than ten hours and little plant growth occurs, lasts from Nov 21 to Jan 21.
Climate Summary from Weatherspark.com

To learn some of your other weather details, see Weatherspark.com.

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