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.
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.
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.
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.
Ryan Boyle’s biography:
Ph.D., Atmospheric Science, North Carolina State University, 2006
Plant Hardiness Zones:
2007 North Carolina Drought:
2016 Drought monitoring map: