A comparison of various mulches; temperature relationships and plant growth

Dr. Curtis E. Swift, Colorado State University Extension (Horticulture Agent)

 

cedar mulch jpeg Minibark mulch jpeg
Cedar Mulch
Minibark Mulch
Pallet mulch Rubberific mulch
Pallet Mulch
'Rubberific' Mulch

Abstract:
A rubber-based mulch was compared with three bark/wood mulches and bare non-mulched soil to determine the surface temperatures of each, the soil temperature below each mulch, as well as the leaf temperature and overall growth of plants in the mulched areas. Surface mulch temperatures as high as 189 degrees Fahrenheit were recorded. Plant growth was not negatively affected at these temperatures. All mulches examined maintained soil temperatures consistently cooler than occurred in non-mulched soil.

Introduction:
The use of mulch is recommended as a means to retain soil moisture and prevent excessive heat build up in the underlying root zone of plants. Wood mulches are the norm, however, rubber-based mulch products are used.

Dark mulches are expected to absorb more heat when exposed to the direct rays of the sun. It is a common belief this heat is transferred to the soil beneath heating the soil to a point where root growth is adversely affected. Soil temperatures above 96 degrees F. are reported to stop root growth and development adversely affecting the growth of plants (Craul, P. J. 1992. Urban Soil in Landscape Design, John Wiley & Sons, Inc. New York. NY. p. 34).

This project was undertaken to determine the temperature differences between mulches commonly used in Western Colorado, an un-mulched area, and a rubber-based product.

Materials and Methods:
A raised garden built in the early summer of 2003 was planted with roses. The soil used in this garden was a blended planting mix. A collection of roses was planted in this garden. Plants were uniformed fertilized. Hand watering was done to ensure plant establishment at which time watering was accomplished by means of a bubbler placed near the base of each plant. All plants were well watered throughout the experiment. All mulches were pulled away from the base of the plant to avoid root rot problems common to plants that are not allowed to dry out at the base, and to prevent high temperature damage from hot mulch next to the rose stem.

This garden was mulched with a rubber-based mulch product ('Rubberific'), a light colored mulch derived from ground up pallets, a cedar mulch, and a mulch consisting of small ( - inch) dark bark particles. The rubber-based mulch was provided by Grand Junction Pipe and Supply, the pallet mulch by the Mesa County Composting Facility, and the Mini Bark and Cedar Mulch were provided by SouthWest Soils of Olathe. Mulches were applied to a depth of three inches. One portion of the garden was left free of mulch for comparison. The Mini Bark and Cedar Mulch were applied on August 1st. Data was taken within 1 hour of applying these two mulches. The 'Rubberific' and pallet mulch had been applied approximately one week prior to the collection of this first set of data.

Data was collected in July and August to determine the surface temperature of each mulch, the surface temperature of the soil in the non-mulched area, the temperature 1 - inches below the soil surface, the leaf temperature of plants within each mulched areas, and the growth of roses in the mulched areas. Leaf temperatures were collected on young tissue on the southwest side (full sun exposure) of one plant within each mulched area. An infrared scanning thermometer (Raytek ST20 Pro) and a Spectrum digital soil thermometer were used to determine the mulch/soil surface and leaf temperatures, and soil temperatures respectively.

Results:
The recorded surface temperature of the rubber-based mulch was usually higher than the temperature of the bare soil or bark/wood mulches. The temperature under this mulch, however, was within the range maintained by the bark/wood mulches indicating these high temperatures were not transmitted down to nor did they result in elevated temperatures in the underlying soil. The soil temperature under each mulch seldom exceeded 90 degree F. On several days the surface temperature of the mini bark mulch (dark in color) and cedar mulch exceeded the surface temperature of the rubber-based mulch. This temperature, however, appeared to have no effect on the growth and health of the roses.

The surface temperature of the bare ground plot was no higher than the temperature of the mulches examined. However, the soil temperature measured 1 - inches below the soil surface of the non-mulched area were as high as 101.6 degrees F. as compared to the 80’s for mulched areas. This data indicates the application of mulch in high light, high summer temperature areas is critical to root health. Neglecting to apply a layer of mulch can have a negative impact on root growth and development causing plant stress and possible plant death.

The leaf temperature differences among the mulched plots were insignificant as demonstrated by the new and vigorous growth exhibited by plants within each mulched area.

Discussion:
The application of mulch to planted areas is critical to soil moisture retention and maintaining soil temperatures appropriate for root growth and development. The type of mulch used, whether wood, bark, or rubber-based, even when surface temperatures exceed 180 degrees F., does not appear to affect plant growth in a negative manner, as long as the plants are in well-prepared soil and properly watered.

The mulches examined in this project would be appropriate in areas where soil moisture retention and cool soil temperatures are required. Further research is needed to determine temperature relationships when lighter colored rubber mulch products and light colored wood and bark mulches are used.

Note: Financial support for this project was provided by Deep Creek Landscaping and the Desert Vista Garden Club.

 

Date
7/31
8/1/03
8/6/03
8/8/03
8/12/03
8/13/03
Ambient Air
97.5 o
104o
93.5 o
96 o
98.2 o
Rubber Mulch
135 o – 147 o
147 o – 153 o
154 o – 163 o
127 o
123 o
159 o - 172 o
Soil temp *
88 o
86 o
86 o – 91 o
84 o
83.3 o
89.2 o
Leaf Temp
91 o – 101 o
94 o – 91 o
107 o – 115 o
81 o
83 o – 86 o
104 o
Pallet Mulch
119 o – 131 o
122 o – 130 o
104 o – 110 o
109 o
106 o – 112 o
150 o - 156 o
Soil temp *
129 o – 141 o
86 o
84 o – 87 o
86.3 o
85.2 o
86 o
Leaf Temp
Not taken
Not taken
100 o
84 o
87 o – 87.5 o
103 o
Minibark
Not applied
139 o – 146 o
150 o – 170 o
120 o
117 o – 124 o
159 o - 173 o
Soil temp *
87 o - 89 o
84 o
83.6 o
83.3 o
Leaf Temp
99 o – 102 o
86 o
86 o – 87 o
106 o
Cedar Mulch
Not applied
104 o – 122 o
125 o – 135 o
116 o
123 o – 132 o
149 o - 189 o
Soil temp *
85 o
81.6 o
83.4 o
83 o
Leaf Temp
98 o
85 o
88 o – 90 o
101 o
Bare Ground
129 o – 141 o
137 o - 140 o
124 o – 130 o
117 o
122 o – 128 o
152 o - 166 o
Soil temp *
Not taken
97.8 o
97 o
98 o
100.2 o
101.6 o
Asphalt Parking Lot
127 o – 128 o
142 o
Not taken
135 o
134 o
158.5 o

All temperatures are reported in Fahrenheit

* Taken 1 - inch below soil surface

Placed on the Internet August 27, 2003 10:06 PM

WebMaster Dr. Curtis E. Swift, Area Extension Agent, Horticulture
Colorado State University Extension
2775 US Hwy 50, Grand Junction, CO. 81503
voice: 970-244-1834
fax: 970-244-1700