Potential for eucalyptus mulch used as a component of potting mixes for foliage plant production

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University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report, RH-91-13

R.T. Poole and C.A. Conover*

Pine bark is currently the most widely utilized lumber industry product for potting media. Since pine bark is a versatile commodity utilized by more than one industry, competition sometimes makes it unavailable and more substitutes are needed. Several other types of tree products such as hardwood bark, woodchips, melaleuca bark and shredded melaleuca trees are also successfully used as potting mix ingredients.

Many acres of eucalyptus trees, native to Australia, were planted for timber, on an experimental basis, in southwestern Florida. Unfortunately, the hard freezes Florida experienced in the last decade reduced the value of much of this crop. While the timber is not suitable for use as lumber, many stands are being converted into a high quality mulch used by the environmental plant industry. Whole trees are shredded, during the manufacturing process, into particles small enough to pass through 1 inch screens. The following experiment compared the effects of Florida sedge peat, pine bark and eucalyptus mulch (in various combinations to give nine media), using 5 replications per medium tested on growth of three foliage plant species.

Materials and Methods

Experiments began on 16 April 1991 when starter plants of Nephrolepis exaltata 'Bostoniensis Compacta' (compact Boston fern), Musa acuminata 'Dwarf Cavendish' (dwarf banana) and Maranta leuconeura erythroneura (red-vein prayer plant) were transplanted into 6 inch containers using the nine media described in Table 1. All media were amended with 7 lbs dolomite and 1 lb Micromax/yd³ (Grace/Sierra Co., Milpitas, CA 95035) during the mixing process. Plants were grown in a greenhouse where minimum and maximum recorded temperatures were 65°F and 95°F, respectively, and light intensity did not exceed 1500 ft-c. Since containers dried at different rates depending on medium composition and plant material, they were watered 2, 3 or 4 times per week as needed. Osmocote 19-6-12 (Grace/Sierra Co., Milpitas, CA 95035) 3 month release rate fertilizer was surface applied (7 g/6 inch pot), on 19 April 1991.

The initial electrical conductivity and pH of media leachate from pots containing compact Boston ferns were measured on 30 April 1991. Data collected on 9 July 1991, when the experiment was terminated, included height and width (measured in cm), and plant grade. Plants were graded based on a scale of 1 = poor quality, unsalable, 3 = fair quality, salable and 5 = excellent quality plants.

Results and Discussion

The three plants responded differently to the various potting mixes. Compact Boston ferns receiving the highest grades were grown in the mixes made up completely or mostly of pine bark, although statistically they were no different than ferns grown in all the mixes composed of eucalyptus mulch and Florida sedge peat. Plants in the mix composed entirely of eucalyptus mulch received the lowest grades (Table 1). Dwarf bananas and red-veined prayer plants grew best in mixes composed of at least 2:1 (by volume) Florida sedge peat regardless of the other components making up the mix. Dwarf bananas and red-veined prayer plants grown in mixes containing less than 2:1 by volume or no Florida sedge peat received the lowest plant grades. All plants grown were determined to be of salable quality.

Dwarf bananas and red-veined prayer plants generally grew tallest in the mixes containing at least 2:1 (by volume) Florida sedge peat and mixes composed Florida sedge peat and pine bark (Table 2). Overall, tallest compact Boston fern were generally grown in media containing Florida peat sedge and eucalyptus bark. The medium made up entirely of coarse eucalyptus mulch produced the shortest compact Boston ferns and dwarf bananas, while the shortest red-veined prayer plants were grown in 100% pine bark.

The mixes containing at least 2:1 (by volume) bark and/or eucalyptus mulch had higher pH and lower electrical conductivity readings than mixes composed of over 2:1 (by volume) Florida sedge peat (Table 3). The best and worst quality ferns were grown in 100% pine bark and 100% coarse eucalyptus, respectively-the two mixes with the lowest leachate electrical conductivity readings. While pH and electrical conductivity measurements can be good indicators of medium fertility and nutrient availability, medium physical characteristics also influence plant growth. The mix composed of 100% coarse eucalyptus bark was very porous and did not retain enough water to prevent drying out between waterings.

Several types of composted wood products are currently utilized as medium amendments in combination with sedge peat in order to moderate the acidity of the peat (raise the pH), to provide some aeration and to reduce costs. The results of this experiment indicate that eucalyptus mulch can be successfully used as a component in potting mixes and may serve as a substitute for pine bark. An earlier study (5) indicated eucalyptus bark may contain toxic substances and should be composted before use, but the eucalyptus mulch used in this test did not seem to contain any substances harmful to the three foliage plants tested.

*Professor, Plant Physiology, and Professor, Center Director respectively. University of Florida, IFAS, CFREC-Apopka, 2807 Binion Road, Apopka, FL 32703-8504

Additional Reading

1. 1. Conover, C.A. and R.T. Poole. 1983. Utilization of Melaleuca quinquenervia as a potting medium component for greenhouse production of foliage plants. HortScience 18(6):886-888.

   2. Forster, W.J., R.D. Wright, M.M. Alley and T.H. Yeager. 1983. Ammonium adsorption on a pine-bark growing medium. J. Amer. Soc. Hort. Sci. 108(4):548-551.

   3. Gartner, J.B., T.D. Huges and J.E. Klett. 1972. Using hardwood bark in container growing mediums. Amer. Nurseryman 135(12): 10-11, 77-79.

   4. Ingram, D.L. and C.R. Johnson. 1983. Melaleuca: an alternative container media component for woody ornamentals. Proc. Fla. State Hort. Soc. 96:254-256.

   5. Lunt, O.R. and H.C. Kohl. 1959. Horticultural applications for bark and wood fragments. Forest Products J. 9:39A-42A.

   6. Poole, R.T., C.A. Conover and J.N. Joiner. 1981. Soils and potting mixtures. Chapter in textbook, Foliage Plant Production, J.N. Joiner, editor, Prentice-Hall Inc. Englewood Cliffs, NJ 07632. pp 179-202.

   7. Poole, R.T. and W.E. Waters. 1972. Evaluation of various potting media for growth of foliage plants. Proc. Fla. State Hort. Soc. 85:395-398.

   8. Sterrett, S.B. and T.A. Fretz. 1977. Effect of nitrogen source and rate on composted hardwood bark media and subsequent growth of cotoneaster. J. Amer. Soc. Hort. Sci. 102(5):677-680.

^zPlants were graded on a scale of 1 = poor quality, unsalable, 3 = fair quality, salable and 5 = excellent quality.
^yFP = Florida sedge peat, PB = pine bark, EUC-1 = fine eucalyptus mulch (particles passed through a inch screen), EUC-2 = coarse eucalyptus mulch (particles passed through a 1 inch screen).
^xMean separation in rows by Duncan's multiple range test, 5% level.

^zFP = Florida sedge peat, PB = pine bark, EUC-1 = fine eucalyptus mulch (particles passed through a 1/2 inch screen), EUC-2 = coarse eucalyptus mulch (particles passed through a 1 inch screen).
^yMean separation in rows by Duncan's multiple range test, 5% level.

^zFP = Florida sedge peat, PB = pine bark, EUC-1 = fine eucalyptus mulch (particles passed through a 1/2 inch screen), EUC-2 = coarse eucalyptus mulch (particles passed through a 1 inch screen).
^yMean separation in rows by Duncan's multiple range test, 5% level.