Speaker: Dr. Damon Abdi, Assistant Professor of Landscape Horticulture, LSU AgCenter Hammond Research Station
Webinar Date: July 17, 2024
Moderator: Dr. Bodie Pennisi, UGA Extension Horticulturist
Duration: 52:51
NARRATIVE SUMMARY
Dr. Damon Abdi from the Louisiana State University AgCenter Hammond Research Station delivered an extensively researched presentation examining the science behind mulch application in landscape beds, transforming a commonly overlooked task into a data-driven management strategy. Drawing from his background as a landscape industry professional and his current research focused on green infrastructure and landscape management, Dr. Abdi presented findings from two comprehensive studies conducted at the Hammond Research Station, located between Baton Rouge and New Orleans. His extension program serves landscape contractors, landscape architects, and hobby horticulturists with practical solutions grounded in rigorous scientific methodology.
The presentation opened with fundamental questions about pine straw depth application, examining how different mulch depths affect soil temperature moderation. Dr. Abdi’s first study, published in 2023 in HortTechnology as an open-access article, monitored replicated plots with zero, two, four, and six inches of pine straw using subsurface soil sensors over a three-month spring period. The research revealed that bare soil surfaces experience dramatic temperature fluctuations ranging from 42 to 77 degrees Fahrenheit, while increasing mulch depths progressively dampened these extremes. A critical finding showed that two inches of pine straw provided significant temperature moderation benefits, but the research identified diminishing returns between four and six inches of depth, with both treatments producing statistically equivalent insulation effects. This economic consideration proved particularly valuable for landscape contractors managing material costs while maintaining plant health benefits.
The second and more extensive study addressed a persistent industry question about colored dye mulches and their thermal properties. Using locally sourced shredded hardwood mulch dyed black, brown, or red, Dr. Abdi’s team installed 21 experimental plots in March 2024, monitoring both surface and subsurface temperatures continuously at 30-minute intervals over an entire calendar year. The research design incorporated two application depths of two and four inches, with sensors positioned three inches below the soil surface in a standardized landscape bed mix composed of bark fines, coarse bark, and sand. The study measured temperatures at two critical depths: surface sensors that experienced direct environmental exposure and subsurface sensors positioned in the root zone where plant health is most impacted.
Data analysis from representative weeks in March and July 2024 revealed substantial findings. Surface temperatures on bare plots reached 122 degrees Fahrenheit during peak July heat, demonstrating the extreme thermal stress that unprotected landscape beds endure in humid subtropical climates. Mulch application dramatically moderated these extremes, with all colored mulches providing significant temperature buffering. Surprisingly, mulch color showed minimal impact on soil temperatures, contrary to common assumptions that darker mulches would absorb more heat. The research demonstrated that application depth proved far more influential than color choice, with four-inch applications providing superior insulation compared to two-inch depths across all color treatments. Both surface and subsurface measurements confirmed that mulch acts as an effective thermal jacket for soil and root systems, maintaining more stable growing conditions throughout seasonal temperature fluctuations.
Dr. Abdi expanded the discussion beyond temperature moderation to address broader landscape management considerations including aesthetics, weed suppression, moisture retention, and practical application strategies. He discussed alternative mulch materials including stone applications, lava rock, and river rock, noting their advantages for low-maintenance commercial landscapes, security applications, and specific aesthetic designs such as Japanese gardens. However, he acknowledged that stone mulches present unique challenges regarding heat retention and reflection that require further investigation. Dr. Abdi announced upcoming research with a master’s student examining landscape stones and their effects on soil temperatures and plant physiology, addressing audience concerns about potential root zone overheating and light reflection stress from white or light-colored stone mulches.
The presentation concluded with integrated pest management perspectives, emphasizing that mulch serves as one tool within a comprehensive weed control strategy that includes cultural practices, routine hand weeding, chemical applications, and equipment sanitation to prevent weed seed and disease spread. Dr. Abdi stressed the importance of creating healthy, sustainable landscapes where mulch selection and application should follow the principle of “right mulch, right application” similar to the established concept of “right plant, right place.” His practical approach resonated throughout the presentation, bridging rigorous research methodology with real-world application guidance that landscape professionals can immediately implement.
YOUTUBE TIMESTAMPS
0:00 Introduction and Speaker Background
2:14 Welcome and Technical Setup
2:40 Dr. Damon Abdi Introduction
2:48 LSU AgCenter Hammond Research Station Overview
3:03 Extension and Research Program Focus
3:15 Industry Experience with Landscape Mulching
3:27 Why Do We Mulch Landscapes?
4:02 Problems with Bare Soil Surfaces
4:14 Mulch as Insulation and Aesthetic Feature
4:23 Pine Straw: Popular Regional Mulch
4:36 Common Questions About Mulch Depth
5:07 Pine Straw Depth Study Design
5:30 Experimental Plot Setup and Sensors
6:03 Temperature Data: February to May 2023
7:04 Interpreting Temperature Fluctuation Charts
7:27 Bare Soil vs. Mulched Plot Comparisons
8:04 Key Takeaway: Mulch Depth Effects
8:14 Published Research: HortTechnology 2023
8:30 Pine Straw Study Conclusions
9:01 Diminishing Returns: 4 vs. 6 Inches
9:26 Economic Considerations for Contractors
9:33 Colored Dye Mulch Research Introduction
10:10 Does Mulch Color Affect Temperature?
10:28 Study Design: Black, Brown, and Red Mulch
10:51 Application Depths: 2 vs. 4 Inches
11:09 Temperature Sensor Placement Strategy
11:30 Experimental Site Preparation
11:43 Soil Type Standardization
12:07 Landscape Bed Mix Composition
12:33 Plot Installation: March 12, 2024
12:52 Surface vs. Subsurface Temperature Sensors
13:17 Continuous Data Collection: 30-Minute Intervals
13:27 Hula Hoop Installation Method
14:37 Cross-Section: Soil and Sensor Layout
15:20 Study Duration: Full Calendar Year
15:32 Data Analysis Strategy
15:55 Model Months: March and July 2024
16:07 March Conditions: Fresh Installation
16:20 July Conditions: Peak Heat Stress
16:43 Unmulched Plots: March Temperature Data
17:02 Surface vs. Subsurface Comparison
17:25 Temperature Buffering Three Inches Deep
17:48 July Unmulched Plot Data
18:11 March and July Side-by-Side Comparison
18:33 Extreme Temperatures: 122°F Surface Heat
18:56 Soil Temperature Ranges in Louisiana
19:14 Surface Fluctuations vs. Subsurface Stability
19:33 Research Questions: Depth and Color Effects
20:00 March 2024: Two-Inch Mulch Depth Results
21:14 Black, Brown, and Red Mulch Comparisons
22:30 July 2024: Two-Inch Mulch Depth Results
23:47 Four-Inch Mulch Depth: March Results
25:04 Four-Inch Mulch Depth: July Results
26:21 Key Finding: Depth Matters More Than Color
27:38 Surface Temperature Analysis: All Treatments
29:39 Subsurface Temperature: Minimal Color Differences
31:25 Practical Takeaways for Color Selection
32:35 Aesthetics vs. Temperature Performance
33:45 Additional Mulch Benefits: Weed Suppression
34:28 Moisture Management Considerations
35:15 Stone Mulch Applications in Landscapes
36:45 Commercial vs. Residential Stone Use
37:55 Lava Rock and River Rock Features
39:01 Japanese Garden and Aesthetic Applications
39:57 Stone Mulch: Low-Maintenance Option
40:35 Practical Application Guidance
41:03 Depth Selection: Aesthetics and Function
41:23 Integrated Weed Management Strategies
41:38 Equipment Sanitation and Cultural Practices
41:58 Summary: Creating Healthy Landscapes
42:17 Contact Information and Questions
42:38 Q&A Session Begins
42:50 Q: Moisture Retention – Mulch vs. Pine Straw
43:36 Q: Mulch Near House – Termites and Roaches
44:42 Follow-up: Foundation Clearance Recommendation
45:03 Q: Stone Mulch and Root Zone Heat
45:23 Upcoming Research: Stone Temperature Effects
45:33 Drought Observations: Stone vs. Organic Mulch
46:05 Dr. Pennisi: Stone Color and Light Reflection
46:48 Reflective Stress from White Stones
47:30 Discussion: Light Angle and Plant Stress
48:15 Future Research: Stone Colors and Types
48:46 Water Infiltration Through Stone Mulches
49:27 Right Mulch, Right Application Concept
49:52 Urban Stormwater and Mulch Selection
50:12 Mixed Mulch Systems: Research Possibilities
50:35 Final Discussion: Publication Timeline
51:15 HortTechnology Submission Plans
51:40 Mulch Flammability Demonstration Discussion
52:23 Closing Remarks
52:51 End of Presentation
QUESTIONS & ANSWERS
Q: Can mulch keep moisture in the soil better than pine straw?
A: The research presented did not directly compare moisture retention between different mulch materials, so Dr. Abdi cannot definitively state which material performs better. In the pine straw study, soil moisture was measured, but frequent irrigation prevented observable differences between treatments. Moisture management effectiveness depends on multiple factors including rainfall patterns, irrigation frequency, and site-specific conditions, making this an interesting area for future research.
Q: Which mulch is best to put next to a house foundation that does not attract roaches and termites?
A: Inorganic stone materials such as gravel are recommended around house foundations rather than organic wood-based mulches. Stone mulches eliminate concerns about attracting wood-destroying insects while providing practical benefits including easier weed visibility and removal, reduced maintenance near the foundation, and better access around air conditioning units and other mechanical equipment. However, stone is not necessarily the absolute solution for all situations. A follow-up comment suggested maintaining a few inches of clearance between any ground cover and the foundation for additional protection.
Q: Isn’t using stone mulch going to get the root zones too hot and stress plants?
A: This is exactly what Dr. Abdi’s next research project will investigate. A master’s student beginning work in fall 2024 will study landscape stones and their effects on soil temperatures and plant physiology. Anecdotal observations from the severe 2023 drought in Louisiana suggested that plants in stone mulches and rubber mulches showed more stress than those in organic mulches during extreme heat conditions. The upcoming research will compare different stone types and colors against organic mulch materials to provide evidence-based recommendations. This study will address whether darker stones retain more heat and whether lighter-colored stones create plant stress through light reflection.
Q: What are your thoughts on the color of stone mulches, particularly regarding light reflection and plant stress?
A: Dr. Pennisi observed installations using large white stones that appeared to cause plant stress, possibly including earlier dormancy in trees, potentially due to intense light reflection creating stress even on lower leaf surfaces that normally receive less direct sunlight. Dr. Abdi agreed this is a valid concern, noting that reflected light hits plants from unusual angles rather than just from above. The upcoming stone mulch research will specifically investigate whether darker stones warm up and retain more heat versus whether lighter stones create stress through color reflection. The study will compare various stone types alongside organic controls like pine straw and shredded wood to provide comprehensive comparisons.
Q: How does water percolation work with stone mulches, especially during heavy rainfall events?
A: Stone mulches facilitate rapid water infiltration, which can be beneficial in urban environments experiencing quick, heavy rainfall events where promoting infiltration is desirable. However, the smooth channels between stones may allow water to move through so quickly that mature landscapes without irrigation systems might not receive adequate moisture during intense downpours. This presents a trade-off where stones prevent washout and matting problems associated with organic mulches, but organic materials may provide better moisture retention for plant roots. The concept of “right mulch, right application” should guide selection based on site-specific factors including rainfall patterns, irrigation availability, and landscape maturity.
Q: Could mixing gravel with organic and inorganic materials in different ratios provide benefits?
A: This represents an intriguing research possibility. Mixed mulch systems could potentially combine the benefits of different materials, such as stone stability with organic moisture retention and soil improvement. Dr. Abdi acknowledged that once researchers begin exploring these questions, the possibilities become limitless. While not part of current research plans, investigating optimal ratios of mixed mulch systems could provide valuable guidance for landscape professionals seeking to optimize multiple benefits simultaneously.
Q: What is the publication timeline for the colored mulch temperature study?
A: Data collection concluded in March 2024, providing one complete calendar year of measurements from 21 plots with readings every 30 minutes at two depths. The extensive dataset is currently being analyzed for publication submission to HortTechnology or another American Society for Horticultural Science journal, with anticipated publication later in 2024. The enormous dataset offers numerous analysis possibilities, though Dr. Abdi limited the presentation to key findings to maintain audience engagement while providing valuable preliminary insights that professionals can apply immediately.
Q: Were the mulch flammability demonstrations shown in the videos part of publishable research?
A: The flammability demonstrations were conducted as an informal trial for educational purposes rather than formal research intended for publication. Dr. Abdi explained he would not be comfortable publishing safety recommendations about flammability without more controlled experimental conditions. The demonstrations were included in the presentation primarily to maintain audience engagement after presenting extensive temperature data charts, serving as an interesting visual element while acknowledging the many environmental factors affecting fire behavior including wind conditions.
Q: What are the key practical takeaways landscape professionals should remember?
A: Mulch depth has a far greater impact on temperature moderation than color selection, so contractors should prioritize adequate application depth over color concerns. Two inches of mulch provides significant benefits, while four inches offers superior performance with diminishing additional returns beyond that depth. Color selection can be based primarily on aesthetic preferences and client desires since temperature effects are minimal. Mulch serves multiple functions beyond temperature control including weed suppression, moisture management, and aesthetics, so application strategies should consider all these factors. The principle of “right mulch, right application” should guide decisions based on site-specific conditions, similar to “right plant, right place” philosophy. Finally, mulch represents one component of integrated landscape management requiring combination with cultural practices, sanitation, and other control methods for optimal results.
Q: What are Dr. Abdi’s recommendations for mulch depth application depths?
A: Based on the research findings, two inches of mulch provides substantial temperature moderation benefits and represents a practical minimum for most applications. Four inches provides enhanced protection and weed suppression while staying below the point of significant diminishing returns. Applications beyond four inches offer minimal additional temperature benefits while increasing material costs. Landscape professionals should balance these scientific findings with aesthetic considerations, as clients typically request enough mulch to achieve desired visual appearance rather than measuring specific depths. The research provides evidence that allows contractors to confidently recommend depths that optimize plant health benefits while managing material expenses.
Q: How should landscape professionals approach weed management with mulch?
A: Mulch is an important weed suppression tool but should be integrated with other management strategies including cultural practices, routine hand weeding, mowing, and appropriate chemical applications. Supporting practices like cleaning equipment to remove weed seeds and thatch buildup and sanitizing pruners to limit disease movement enhance overall landscape health. A comprehensive approach combining multiple tactics provides more sustainable and effective weed control than relying solely on mulch depth. Professional applicators should educate clients that mulch contributes to weed management but does not eliminate the need for ongoing maintenance and integrated pest management strategies.
ADDITIONAL RESOURCES
Dr. Damon Abdi Contact:
LSU AgCenter Hammond Research Station
Email: Available through LSU AgCenter directory
Specialization: Landscape Horticulture, Green Infrastructure, Landscape Management
Published Research:
Abdi, D. et al. (2023). “Pine Straw Depth and Soil Temperature Moderation.” HortTechnology (Open Access, 2 pages)
Note: Colored mulch temperature study publication pending (2024)
Research Station Location:
Hammond Research Station, Louisiana (between Baton Rouge and New Orleans)
Climate Zone: Humid Subtropical (similar to coastal Georgia and southeastern U.S.)
Extension Audiences:
- Landscape contractors
- Landscape architects
- Hobby horticulturists
- Homeowners and garden enthusiasts
Upcoming Research:
- Stone mulch temperature effects (beginning Fall 2024)
- Plant physiological responses to stone mulches
- Comparative analysis: Stone vs. organic mulch materials
- Color effects of different stone types
GTBOP Program:
Getting the Best of Pests Webinar Series
UGA Center for Urban Agriculture
Continuing Education for Pest Management and Green Industry Professionals
For questions about landscape mulch management and research findings, contact Dr. Damon Abdi at the LSU AgCenter Hammond Research Station.