climate – Extension Master Gardener Volunteers of Durham County

by Marcia Kirinus, EMGV

Gardeners often rely on seasonal cues or the calendar to determine when to perform specific gardening tasks. For example, they may plant nasturtiums on St. Patrick’s Day or set out tomato plants on Mother’s Day. However, what happens when you move to a new climatic zone and your gardening timing is thrown off? And what if weather patterns change to the extent that plants that once thrived now struggle?

Unpredictable weather patterns or a relocation to a different climate can make it challenging to assess a plant’s adaptability to the local conditions. Fortunately, there are two valuable tools at your disposal: the USDA Cold Hardy Zone designation and the Horticultural Society’s Heat Zone designation.

Image: Tropaeolum majus (Nasturtium)
Sebastian Dario CC BY-NC 2.0

These designations provide vital information about a plant’s ability to tolerate both winter cold and summer heat. Most gardeners are familiar with the USDA hardiness zone system, which rates zones on a scale of 1 to 12. The Horticultural Society’s Heat Zone system parallels this scale, with higher numbers representing regions experiencing more hot days.

In Durham, for instance, the heat zone map places it at a temperate 7, while the hardiness zone map designates it as 7b. The USDA Plant Hardiness Zone indicates the lowest winter temperature a plant can withstand before facing potential death. In Durham’s case, being in Hardiness Zone 7b means plants in this zone can survive temperatures as low as 5-10°F. If you travel south, the numbers increase, and if you go north, they decrease.

However, the USDA hardiness map does not provide insight into a plant’s ability to endure the heat of summers. For that, the Horticultural Society’s Heat Zone number comes into play. It indicates a plant’s tolerance to heat and specifies the temperature threshold at which it starts to suffer and becomes unable to efficiently process water for normal functions. In Durham, our Heat Zone is 7, suggesting that plants should not be exposed to a cumulative total of 61 to 90 heat days above 86°F. Given that we experience many days higher than 86°F, it is crucial to choose perennials wisely based on the heat zone index number.

To accurately describe Durham’s local climate, we can use the format 7b/7, representing the cold hardiness zone followed by the heat zone designation. This combination of numbers provides a clear picture of the climatic conditions suitable for your plants. You can typically find these numbers listed on informational tags at reputable plant nurseries.

Consider incorporating native plants into your landscape. Native plants are well adapted to the local environment, offering numerous benefits such as enhanced climatic resilience, support for local wildlife, and reduced maintenance requirements. Here are some native shrubs that can thrive in Durham’s summers and winters, typically associated with USDA Zone 7b and Heat Zone 7:

Summersweet Clethra (Clethra alnifolia)
Fragrant Sumac (Rhus aromatica)
Sweetshrub (Calycanthus floridus)
Dwarf Fothergilla (Fothergilla gardenii)
Virginia Sweetspire (Itea virginica)

*Clethra alnifolia* flower spike
*Tom PotterfieldCC BY-NC-SA 2.0*

This is a short list, and it is advisable to consult your local Cooperative Extension Master Gardeners office for a comprehensive selection of suitable native plants, shrubs, and trees. You can also use the “Find a Plant” tool on the North Carolina Extension Gardener Plant Toolbox to narrow down native plants by your site conditions and needs.

While the heat zone and hardiness zone systems are essential factors to consider when choosing plants for your garden or landscape, other variables like soil type, sunlight exposure, humidity, and drought tolerance should also be considered to ensure successful plant cultivation.

References:

USDA Plant Hardiness Map: https://planthardiness.ars.usda.gov/
American Horticultural Society https://ahsgardening.org/gardening-resources/gardening-maps/
“Site Considerations When Selecting Plants”: https://hgic.clemson.edu/factsheet/choosing-a-planting-location/
University of Arizona Cooperative Extension: “Heat Zone Map for Arizona” (https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1673-2021.pdf)
Dr. Bryce Lane lecture

by Wendy Diaz

At the Sarah P. Duke Gardens on November 15, 2016, I attended a Durham Garden Forum Talk entitled “Gardening Under Future Weather Conditions” by Dr. Ryan Boyles, director of the State Climate Office of North Carolina. I thought it a very timely talk indeed, as North Carolina this fall has already experienced its own share of extreme weather with the extensive flooding in the southeast from Hurricane Matthew and Exceptional Drought and Forest Fires in the western part of the state.

unknown-1 — Dr. Ryan Boyles (from the internet)

During his talk, he explained the difference between weather (short-term occurrence) and climate (long-term weather frequency or average of the weather). In addition, he briefly discussed the “drivers’’ of our climate variability such as the location of the “Bermuda High”, Atlantic Multidecadal Oscillation (Atlantic Ocean Temperatures), El Nino (wet winter) and La Nina (dry winter) and the Jet Stream oscillations. For example, the 2007 drought was the result of The Bermuda High shifting from its usual location off the Atlantic Coast to over the Gulf of Mexico (New Orleans) deflecting tropical storm systems away from the region over the summer resulting in a cessation of our usual evening thunderstorms. Dr. Boyles described what we could expect for long-term changes to our climate as a result of the increase in greenhouse gases (e.g. CO2 and methane) into our atmosphere. Of course the affects of the increase in greenhouse gases is variable in our complex system and depends on the future emission levels, land use, vegetative cover, polar ice cover, cloud cover etc. I am not a climate scientist so this is the limit of my explanation.

He explained that his predictions are based on Climate models, which are in turn, are based on physics. With the aid of ever more powerful computers climate models (GCM’s) don’t try to predict the weather on any given day but instead predict how the weather, on average, will change. In North Carolina there will be long term warming of the climate and increase in variability of the weather (amplification of the hydrological cycle).

Future winters in North Carolina

Probably more warm days in the winter.
May lose one month of winter by 2070.
Probably fewer very cold days.
Probably fewer days with snow at lower elevations.

Future summers in North Carolina

Probably more hot days and hot nights in summer.
Perhaps fewer days with rain.
Probably are more days of intense rainfall and perhaps more drought.
May gain one month of summer by 2070

What does this mean for the Durham Gardener?

Durham County currently falls on the border of Plant Hardiness Zone of 7 and 8 (7b). By mid-century, Durham may be Zone 8 (low end 8a or high 8b) and at the end of the century 9a.

future-durham-plant-hardiness2-1

Winter

Climate and not just weather affects plants. There will probably be fewer cold nights, which will lower pest die offs, and slow chill accumulation and more warm days. There will be increase soil microbe activity and emergence dates will change. Because of polar ice loss (arctic thaw impacts) we may have colder November and Decembers. There will probably be more warm and fewer cold days in the spring and possibly more intense thunderstorms. Because there may be more warm days in February and March, early flower emergence may not be timed with pollinators and there may be earlier emergence of pests and because it may not necessarily cause earlier ‘last freezes’ there may be increases in ‘last freeze’ damage.

Summer

Plants will experience heat and moisture stress under these conditions and the timing and frequency of rain (increase variability) will increase the need for irrigation. Hotter nights will impact the flowering of plants and increase transpiration at night (can’t rest & failure in pollination and/or fertilization). This has implications for tomatoes, our most popular home garden vegetable. Blossom Drop is caused when night temperatures are above 70 degrees F. resulting in reduced flowering and fruit set. The optimal night temperature range for setting fruit is 59 to 68 degrees F. There is also the expectation of increase in pests (fungal pressure from increase humidity). When it rains it will pour and this will cause ponding in fields and soil erosion. There may be a lower fungal risk only if we are having a drier summer. And finally there is expected to be more hurricanes in autumn.

Gardeners will need better soil preparation and drainage to prepare for these anticipated changes to our climate. We should probably plant trees, due to their longer life spans, that can withstand warmer conditions such as zone 8 or 9a hardiness. Gardeners should plan for water management with anticipated water restrictions as populations continue to grow and install storage systems. We should improve our soil amendment strategies to make them the more resilient to these new conditions (cover crops, improve drainage, etc.)

Dr. Boyles informed me that climate, not just weather, affects plants and that what happens in the oceans and the far away polar regions affects our local growing conditions as well. These are things to keep in mind for the future as we make our gardening decisions now.