Heat domes and plant stress

July 2024

Richard Martinson, Ph.D.

Director, High Desert Horticultural Center

One of the questions we hear frequently at the nursery is if it’s okay to plant during the heat and drought conditions typical of this time of year in central Oregon.  In this brief article, I’ll summarize my thoughts about that question and explain some of the science behind my response.

Weather patterns in central Oregon have been changing significantly over the past decade or so.  If you’ve been in this area for any amount of time, you’ve probably noticed a shift in the timing and duration of our storm events and winter weather.  While there’s always been a fair amount of variability in local weather (“if you don’t like the weather, wait five minutes -- it’ll change”), there has been a notable change in climatic conditions.  Climate differs from weather in scale.  Weather can be quite variable and change quickly; Climate refers to the long-term trends in environmental conditions.

One of the main differences we’re seeing in climatic conditions is precipitation timing and intensity.  Rainstorms (and winter snow) are occurring less frequently but with greater intensity.  This pattern affects soil moisture levels significantly by allowing soils to dry out at greater depths before groundwater is replenished by rain or snow.  Even though many of our native plants are very deeply rooted, moisture in those deep soil layers is no longer maintaining relatively stable levels.  Plants, even deeply rooted species, are stressed as a result of overall declining available moisture. 

But many of our native species are physiologically adapted to drought conditions and can acclimate through various mechanisms such as summer dormancy (plants that go dormant as a response to heat and drought), shedding leaves and other body parts to reduce transpiration rates, early seed development, increased rooting, changes in hormone balance and gene expression, and other structural, physiological, and biochemical changes.

How does all this apply to an urban landscape where environmental conditions are normally mediated through irrigation, increased nutrient availability, and other maintenance practices?  Good question.

There have been several academic studies that look at the response of many xeric (dryland) native plants to irrigation (increased water availability).  Results suggest that many native species still exhibit those adaptive traits even when they’re under different rates of irrigation.  Most species that were included in these studies (such as sagebrush or bluebunch wheatgrass; two primary native species found in central Oregon) respond more to changes in temperature, humidity, and wind than to changes in soil moisture levels.  In more technical terms, this means that plants are more affected by “vapor pressure deficit” than they are by “evapotranspiration” (see sidebar).  Recent studies completed in central Oregon confirm those findings.

Heat domes have become a common climatic event in central Oregon over the past few years.  These events have higher temperatures and lower humidity than average -- sometimes significantly different than average.  For example, in July 2024, our temperatures exceeded daily averages and humidity was significantly lower than average for the first half of the month (Figure 1).  Both of these factors increase stress for all vegetation in the region – not just natives.  In essence, the high heat and unusually low humidity levels robs all plants of the moisture necessary for photosynthesis, respiration, and physical structure.  Even native plants begin to wilt, not in response to water stress (because many species are already dormant or have entered into a summer dormancy), but because of the high Vapor Pressure Deficit affecting cellular structure and function. 

Back to our original question.  Is it okay to plant during these hot, dry weather events?  Our answer is no.  Even though most of the species we grow at WinterCreek are well adapted to our climate, plants at the nursery have been grown under ideal conditions and have not developed the root structure or physiological responses to heat and drought that those same species develop in nature.  Xeric plants are generally highly drought adapted, but not when they have been grown in a nursery setting.  They need a period of growth and acclimation in landscapes or restoration sites before developing the characteristics that help them survive.  This is true even in managed systems with supplemental irrigation.  Water availability for these plants generally isn’t an issue in a landscape, but the extremely hot and dry air that surrounds each plant leaf is.

So, after all that, the bottom line is that it’s physiologically advantageous to wait until temperatures lower a bit and humidity levels increase to reduce the stress on plants.  Moving plants out of the nursery and into a landscape is stressful enough without adding the additional strain of planting them in extreme conditions.

We’ll be getting into a little more detail on plant physiology and adaptive strategies in future posts.  Stay tuned!