4–5 minute read

Temperature is only one of many factors that affect plant performance. For example, USDA Hardiness Zones are drawn solely on winter average annual minimum temperatures. While Seattle, Tucson, and Atlanta are all in the same hardiness zone, their rainfall and summer growing conditions are very different.

Average temperature

• red line: daily average high temperature
• blue line: daily average low temperature
• dark blue or red band: 25–75% likelihood
• pale blue or red band: 10–90% likelihood

Weatherspark: Hillsborough, NC 1980–2016 Temperature

Growing Season

• dark pink band: 90% likelihood
• medium pink bands: 50% likelihood
• pale pink bands: 10% likelihood

Weatherspark: Hillsborough, NC 1980–2016 Time Spent in Various Temperature Bands

The growing season is the longest continuous period of non-freezing temperatures in the year, typically 214 days here, from around April 2 to around November 3.

Microclimates

Within any hardiness zone there will be areas that are slightly warmer or cooler because of elevation, urbanization, wind patterns, and other factors. Examples in your yard are: a spot protected from winds and/or sun by a structure, an elevated area exposed to winds or a low-lying place where cooler air settles. These microclimates may permit you to extend your plant selections by at least a half-zone in either direction.

Seasons

Winter

The Piedmont is partially protected by the mountains from the cold air masses that move southeastward across the central states. When cold waves do cross, they are usually modified by the crossing and the descent on the eastern slopes. However, we are too far from the Atlantic Ocean to benefit from its moderating influence, which raises the average winter temperature and decreases the average day-to-night range in coastal areas. The temperature drops to 10–12°F about once during an average winter in central North Carolina, and temperatures as low as 0°F are rare. The temperature falls below freezing on more than half of the days in winter but rarely remains that low for 24 hours.

• Winter risks to plants include freezing, dessication, and limb breakage.
• Winter also affects the levels of insect populations during the subsequent growing season.

Summer

In most summers our weather is dominated by the Bermuda High pressure system, resulting in calm, virtually cloudless conditions. Morning temperatures in summer average about 20 degrees lower than those in the afternoon. A temperature of 90°F occurs about 50 times from late March until well into October.

• Summer risks to plants include high night temperatures and drought stress.

Autumn

Autumn is the season of most rapidly changing temperature. The drop-off is greatest during October and by the end of November, average daily temperatures are within ~5° of the lowest point of the year.

• Autumn is the best time to plant.
• Early frosts in autumn can also pose challenges. See Frosts for more information.

Frosts & freezes

• See Frosts for maps, frost dates, a discussion of types of frost, and suggestions for preventing damage.
• AgroClimate shows freeze likelihood maps. You input the date range.
• The National Weather Service shows maps for median/earliest/latest dates of 32 degree and 28 degree spring and fall freezes for the most recent 30-year period of record. Because temperature is measured at five feet above the ground, plant surfaces can be colder than the air temperature suggests.

Chill hours

Most temperate plants including fruit trees, berries, and deciduous trees enter a dormant period during late fall and winter. Dormancy enables plants to tolerate freezing temperatures and prevents growth during mid-winter warm spells. Once dormant, plants require accumulated exposure to cool temperatures for resumption of normal growth, referred to as the chilling requirement. Chilling requirement is measured in either of two ways:

chill hours

hours below a chilling temperature threshold

chill units

chill hours that are weighted by temperature for effectiveness in satisfying species/cultivar requirements

Apples need ~1200 chill hours, blueberries need ~600, while peaches vary (some need as little as ~700 hours). Species with low chill requirements (e.g., peaches) can be susceptible to spring frost because once the chill requirement has been satisfied, a short spell of warm weather induces buds to swell and open.

The AgroClimate Chill Hours Calculator monitors and forecasts chill accumulation (temperature is measured at 6.56 feet above the ground) from October 1–April 30 using the weather station that is the closest to the selected county. During the chill season the current accumulation is shown. During the remaining of the year the prediction for the next season, the historical average, and the past season are shown.

• Neutral, El Niño (usually cooler winters), La Niña (usually warmer winters), and average conditions can be selected
• Users can easily view and change the station used for monitoring
• Users can choose monitoring by 2 types of chill hour