Speaker: Dr. Michael Arnold, Professor of Landscape Horticulture and Director of the Gardens, Texas A&M University
Webinar Date: July 17, 2025
Duration: 53:35
NARRATIVE SUMMARY
Dr. Michael Arnold presented a comprehensive analysis of climate variability impacts on landscape plant specifications and management strategies for resilient urban landscapes. With over 40 years of experience in landscape horticulture, Dr. Arnold examined how increased fluctuations in global climate, enhanced severity of weather events, and exacerbated urban heat island effects are fundamentally changing how landscape professionals must approach plant selection and maintenance regimes. His presentation emphasized that climate change is not simply about global warming but rather about massive variations and extremes in temperature, precipitation, wind, and other environmental factors that create serious challenges for modern landscape designs.
Dr. Arnold presented compelling data on the increasing frequency and severity of extreme weather events across the United States, noting that the southeastern and south-central regions have experienced substantial impacts from billion-dollar weather events over the past 25 years. He documented the northward migration of USDA hardiness zones, with Zone 2 no longer appearing in the continental United States and Zone 10 expanding into South Texas and Florida since 1990. His analysis of urban heat island effects revealed temperature differences of up to 27 degrees Fahrenheit between urban centers and surrounding countryside, with cities averaging eight more days per year above 90 degrees Fahrenheit. Dr. Arnold explained the critical importance of this 90-degree threshold, demonstrating how most C3 plants experience peak photosynthesis around 90 degrees, beyond which respiration exceeds photosynthetic capacity, resulting in net negative carbon fixation and plant stress.
The presentation explored complex interactions between temperature stress and biotic factors, including research showing how elevated temperatures increase both insect metabolism and plant defensive compound production. Dr. Arnold shared compelling case studies from Texas, including observations of Eve’s necklace (Sophora affinis) severely damaged by wood-boring insects during drought stress, and bald cypress (Taxodium distichum) mortality patterns based on provenance adaptation. He emphasized the critical distinction between provenance and seed source, explaining that genetic material origin determines adaptation potential regardless of where nursery stock is grown. His discussion of water availability challenges included projections of declining precipitation in the Southwest, rising sea levels causing saltwater intrusion into groundwater, and the need for greater salt-tolerant plant selections.
Dr. Arnold detailed storm damage patterns, explaining how polar vortex deterioration is increasing the frequency of severe winter events in temperate latitudes despite overall warming trends. He presented data showing declining chilling hours in California’s Central Valley while simultaneously warning of more extreme cold snaps in other regions. Management recommendations emphasized low-impact development features including bioswales, rain gardens, and constructed wetlands for stormwater management. Dr. Arnold stressed proper planting depth as critical, sharing research showing that only Vitex survived three-inch deep planting among five tested species. He concluded with extensive plant recommendations emphasizing drought tolerance, heat tolerance, and regional adaptation, including native and adapted species such as buttonbush (Cephalanthus occidentalis), Texas persimmon (Diospyros texana), Mexican buckeye (Ungnadia speciosa), and various oak species selected for specific provenance characteristics. Throughout the presentation, Dr. Arnold emphasized that landscapes are evolving motion pictures rather than static installations, requiring flexible specifications that anticipate future climatic conditions rather than simply addressing current challenges.
YOUTUBE TIMESTAMPS
0:00 Introduction and Speaker Credentials
1:35 Presentation Overview and Texas Weather Context
2:40 Climate Change vs. Climate Variability
4:10 Defining Climate Change Beyond Global Warming
5:05 Financial Impacts of Extreme Weather Events
6:00 25-Year Drought Analysis Across the United States
6:45 Billion-Dollar Weather Events Mapping
7:30 Heat Waves: Frequency and Duration Trends
8:00 Increasing Heat Wave Frequency (60-Year Analysis)
8:45 Heat Wave Season Length Extension
9:30 Maximum Heat Wave Duration Increases
10:10 Precipitation Pattern Changes
10:45 Regional Precipitation Increase Projections
11:15 Southwest Drought Intensification Forecast
12:10 USDA Hardiness Zone Map Evolution (1990-2012)
12:40 Key Hardiness Zone Changes and Implications
13:15 Projected Hardiness Zone Shifts (Next 30 Years)
14:10 Long-Term Planning for Zone Migration
14:45 Urban Heat Island Effect Explained
15:40 Urban Heat Island Temperature Differentials
16:40 Socioeconomic Factors in Urban Vegetation
17:15 Oklahoma City Case Study: 22-Degree Impact
17:50 Urban Heat Island Study Results (60 Cities)
18:35 Eight Additional 90-Degree Days Per Year
19:15 Photosynthesis and Respiration Temperature Response
20:30 The 90-Degree Threshold for C3 Plants
21:15 Net Negative Photosynthesis Above 95 Degrees
22:00 Temperature Impacts on Plant-Pathogen Interactions
23:00 Grain Yield Loss Projections (10-20% Per Degree)
23:25 Tomato Research: Temperature and Insect Interactions
24:00 Jasmonic Acid Defense Response Under Heat Stress
24:45 Temperature Effects on Pest Populations
25:05 Eve’s Necklace Case Study Introduction
25:40 Drought Stress and Borer Infestation Example
26:20 Visual Documentation of Tree Decline
27:15 Multiple Stressor Interactions in Urban Trees
28:00 Water Availability Challenges Overview
28:40 Precipitation Deficit Projections for Southwest
29:20 Sea Level Rise and Saltwater Intrusion
30:00 Need for Salt-Tolerant Plant Selections
30:40 Storm Damage Frequency and Severity
31:10 Gulf Coast Tropical Storm Impacts
31:45 Ice and Snow Storm Damage Patterns
32:30 Wind Damage and Hail Events
33:00 Winter Storm URI and Recent Freeze Events
33:45 Warmer Air Holds More Moisture Principle
34:20 Polar Vortex Deterioration Explained
35:10 Increasing Frequency of Cold Air Outbreaks
35:45 Chilling Hour Accumulation Decline
36:20 California Central Valley Chilling Analysis
36:50 Shorter Cold Snaps Across the Country
37:20 Provenance Importance for Winter Adaptation
37:50 Buttonbush Provenance Example
38:35 Cold Damage from Northern Seed Sources
39:15 Management Strategies Overview
39:50 Drought Management: Irrigation Best Practices
40:20 Cyclic Irrigation on Slopes
40:50 Mulch Depth and Placement Guidelines
41:20 Storm Damage Prevention: Avoid Topping
42:05 Proper Planting Depth Critical Importance
42:40 Vitex Exception to Deep Planting Mortality
43:00 Fertilization and Salinity Management
43:30 Water-Efficient Plant Design by Zones
44:00 Regional Adaptation vs. Native Plant Myth
44:40 Provenance vs. Seed Source Distinction
45:30 Bald Cypress Provenance Adaptation Example
46:15 pH Tolerance and Western Provenances
47:00 Species Selection for Future Conditions
47:40 Low-Impact Development Features Integration
48:20 Recommended Drought-Tolerant Shrubs
48:50 Damiania and Chrysactinia for Rock Gardens
49:10 Buttonbush for Wet Urban Sites
49:35 Little Leaf Cordia Cold Tolerance
49:55 Sparkleberry: Heat and Cold Tolerant Vaccinium
50:10 Eve’s Necklace Multi-Season Interest
50:25 Mexican Buckeye Drought Adaptation
50:40 Texas Persimmon Ornamental Characteristics
51:00 Shin Oak for Desert Conditions
51:20 Large Tree Recommendations: Texas Red Oak
51:35 Cedar Elm and Western Soapberry
51:50 Montezuma Cypress and Quercus polymorphia
52:20 Ground Cover Recommendations
52:40 Zoysia Grasses and Native Ferns
52:55 Question and Answer Session
53:20 Closing Remarks and Thank You
QUESTIONS & ANSWERS
Q: What is the difference between climate change and climate variability, and why does it matter for landscape specifications?
A: Climate change is not simply global warming but rather deviations from typical weather patterns and long-term climatic events for a region. While this can include changes in average temperature or precipitation, landscape professionals should be more concerned with the massive variations and extremes in temperature, precipitation, wind, and other factors. These extreme weather events create the most significant challenges for plant survival and landscape performance in built environments.
Q: How have USDA hardiness zones changed over the past three decades?
A: Significant northward migration of hardiness zones has occurred between 1990 and 2012. Zone 2 no longer appears in the continental United States in large enough areas to map. Zone 9 has expanded substantially into Gulf Coast areas. Texas, which had no Zone 10 areas in 1990, now has an entire section of South Texas designated as Zone 10. Projections suggest these trends will continue over the next 30 years, requiring long-term planning adjustments especially for trees and large woody materials.
Q: What is the urban heat island effect and how severe can it be?
A: Urban heat islands are created by heavily urbanized areas with extensive impervious surfaces and buildings that absorb heat, canyon-like streets with altered airflow, air conditioning units that dump heat into the environment, and reduced vegetation providing less evaporative cooling. Studies of 60 cities found temperature differences of up to 27 degrees Fahrenheit between urban centers and surrounding countryside. On average, these cities experienced 2.5 degrees warmer daytime temperatures and 4 degrees warmer nighttime temperatures, with eight additional days per year exceeding 90 degrees Fahrenheit.
Q: Why is 90 degrees Fahrenheit considered a critical temperature threshold for plants?
A: Most C3 plants have photosynthetic mechanisms that peak around 90 degrees Fahrenheit and then rapidly decline. Plant respiration continuously increases with temperature and burns carbohydrates, while photosynthesis makes carbohydrates. Above approximately 90-95 degrees, respiration exceeds photosynthetic capacity, creating net negative photosynthesis. The more time plants spend above 90 degrees, the more they deplete rather than accumulate carbohydrate reserves, leading to stress, decline, and reduced performance.
Q: How do temperature increases affect plant-pest interactions?
A: Research from Michigan State University on tomatoes demonstrated that increasing temperature elevates both insect metabolism and growth rates, causing insects to feed more frequently and aggressively. Plants respond by producing defensive compounds like jasmonic acid, diverting carbohydrate resources from growth and yield into defense. Combined with increased transpiration for cooling and impaired photosynthesis at high temperatures, plants experience multiple simultaneous stresses. This isn’t just about temperature itself but how temperature changes interact with pathogens and pests in the landscape.
Q: What is the difference between provenance and seed source?
A: Provenance refers to where genetic material originally came from, while seed source indicates where nursery stock was actually grown. For example, if you collect seeds from a native red maple stand in Georgia and grow those seedlings in Texas, they remain Georgia provenance even though they’re Texas seed source. If you then collect seed from those Texas-grown trees, it’s still Georgia provenance. Provenance determines genetic adaptation to climate, soil pH, and other environmental factors, making it critical for selecting plants adapted to specific site conditions and future climate scenarios.
Q: Why are chilling hours important if we’re experiencing warming trends?
A: While overall warming is reducing chilling hour accumulation in areas like California’s Central Valley (critical for fruit and vegetable production), the pattern is complex. Data shows declining chilling hours from the 1950s through projected 2090 levels. However, cold snaps are becoming shorter but potentially more severe due to polar vortex deterioration. This means landscapes must be designed for both reduced winter chilling (affecting species requiring specific chilling for proper growth) and occasional extreme cold events that can damage plants not adapted to sudden temperature drops.
Q: What happened in the buttonbush provenance example?
A: Dr. Arnold was growing buttonbush (Cephalanthus occidentalis) for a bioswale project and needed additional plants, so he obtained seed from a source further north. The seedlings grew wonderfully in the nursery through summer and fall. However, when winter arrived, these northern-provenance plants suffered extensive cold damage because they were genetically programmed for longer, colder winters and did not properly harden off for the shorter, milder Texas winter. This demonstrates that even within a wide-ranging native species, provenance matching to local conditions is essential.
Q: What are the key recommendations for proper planting depth?
A: Research on five different species from five different families showed that planting just three inches too deep resulted in mortality for four of the five species within three years. Only Vitex survived because it produces new adventitious roots above the original root zone on the stem. Other species cannot do this, so deep planting leads to oxygen deficiency, root rot, girdling roots, and increased susceptibility to drought stress. The root flare should be visible at the soil surface, and trees should be planted at or slightly above the grade they were grown at in the nursery.
Q: Why shouldn’t you over-fertilize in water-stressed or salt-affected areas?
A: Over-fertilization exacerbates salinity issues in the root zone. Excess fertilizer salts increase the osmotic potential of soil water, making it harder for plants to extract moisture even when water is present. In drought-prone areas or regions with increasing saltwater intrusion into groundwater, adding unnecessary fertilizer salts compounds the stress plants are already experiencing. This is particularly important in urban areas where irrigation water may already have elevated salt content.
Q: What low-impact development features should be incorporated into landscape designs?
A: Dr. Arnold recommends incorporating bioswales, rain gardens, constructed wetlands, and other stormwater remediation techniques that minimize runoff and damage from storms. These features help manage the increasing precipitation intensity events, reduce non-point source pollution, and create beneficial habitat. When combined with preservation of coastal and riparian ecosystems, these low-impact development strategies contribute significantly to landscape resilience and help mitigate both flooding and drought impacts in built environments.
Q: What makes buttonbush a good choice for urban landscapes?
A: Buttonbush (Cephalanthus occidentalis) is fantastic for urban environments with wet soils or poor drainage conditions. It has a very wide natural distribution across North America, tolerates periodic flooding, provides multi-season interest with unique spherical white flowers attractive to pollinators, and adapts to various soil conditions. Western provenances typically have glossier foliage than eastern types. It’s an excellent choice for bioswales, rain gardens, and other areas where water tends to accumulate.
Q: What trees does Dr. Arnold recommend for drought and heat tolerance in urban environments?
A: Key recommendations include Texas red oak (Quercus texana), Cedar elm (Ulmus crassifolia), Western soapberry (Sapindus saponaria var. drummondii), Montezuma cypress (Taxodium mucronatum), and Quercus polymorphia. For smaller trees, Eve’s necklace (Sophora affinis) offers multi-season interest and urban tolerance, Mexican buckeye (Ungnadia speciosa) provides excellent drought tolerance, and Texas persimmon (Diospyros texana) features attractive bark and extreme heat and drought tolerance. All have proven resilient in urban Texas environments under variable weather conditions.
ADDITIONAL RESOURCES
Climate Data and Tools:
- USDA Plant Hardiness Zone Map (updated periodically)
- National Weather Service Drought Monitor
- Urban Heat Island Resources (EPA)
Plant Selection Resources:
- Texas A&M AgriLife Extension Plant Database
- Regional Native Plant Societies
- Proven Winners and Select Plant Programs (regionally adapted cultivars)
Low-Impact Development:
- EPA Green Infrastructure Resources
- Bioswale and Rain Garden Design Guides
- Stormwater Management Best Practices
Research References:
- Michigan State University tomato-temperature-insect interaction studies (2020)
- Urban heat island analysis across 60 U.S. cities
- Central Valley California chilling hour projections (1950-2090)
- Polar vortex research and winter weather pattern changes
Key Concepts Emphasized:
- Provenance vs. seed source distinction
- 90-degree Fahrenheit photosynthesis threshold for C3 plants
- Net negative photosynthesis under extreme heat
- Jasmonic acid plant defense responses
- Multiple stressor interactions (temperature, drought, pests)
- Landscape as “evolving motion picture” vs. static design
Speaker Contact:
Dr. Michael Arnold
Texas A&M University
Professor of Landscape Horticulture
Director of the Gardens
Note: Plant recommendations focus on Texas and southwestern adaptations but principles apply broadly. Always verify hardiness, heat tolerance, and provenance suitability for your specific region before specifying plants. Consider future climate projections, not just current conditions, when selecting long-lived woody plants.
This webinar was part of the Getting the Best of Pests (GTBOP) educational series hosted by the UGA Center for Urban Agriculture, providing continuing education for pest management and Green Industry professionals.