By Calvin Chong, University of Guelph, Department of Plant Agriculture, Vineland Station, Ontario and Glen P. Lumis, University of Guelph, Department of Plant Agriculture, Guelph, Ontario

Introduction
Container shade tree production has increased dramatically during the past 15 years. In the 1980s, above-ground wire basket (4) and in-ground fabric (7, 11, 15) container growing were popular. Today, it is pot-in-pot.

     The pot-in-pot system was first established in Oklahoma in the mid 1980s (5) and is now used by many nurseries in the U.S. and Canada (3, 8, 13). It involves growing a potted tree in a second socket container of equal or larger size pre-sunken in the ground (12, 14). It combines the benefits of field and container growing but requires intensive management of fertilizer, substrate and irrigation. Marketable trees are produced faster than when field-grown. The socket container prevents trees from falling over, enables tree harvest and replacement at anytime without digging, and insulates the roots from summer heat and winter cold.

     Traditionally, most container substrates include various amounts of peat and/or bark. Previously, we investigated pot-in-pot production in 38- and 76-L (10- and 25-gal) containers filled with various mixtures of pine bark (20-100% by volume), sphagnum peat (15-30%), and/or top soil (5-100%) (9, 10). We also showed that farm and industrial waste by-products, such as spent mushroom compost, recycled municipal waste and paper mill waste, can be used successfully as alternative and inexpensive amendments for growing nursery shrubs in smaller 6-L (2 gal) containers (1). Such use offers great potential for diverting these wastes from landfills, but there has been no similar research with container-grown shade trees.

     In this study, described in detail in a recent scientific publication (6), we examined various organic waste-derived substrates for growing container shade trees.

Procedure
In the spring of 1994, seedling whips of green ash (Fraxinus pennsylvanica), Japanese birch (Betula platyphylla var. japonica) and silver maple (Acer saccharinum) were grown for two seasons in 76-L (25 gal) containers and in-ground socket containers of similar size, spaced 3.8 m between and 1.5 m within rows. Substrate treatments included a control nursery mix (50% by volume of pine bark:15% compost:35% top soil) and nine other mixes classified into three groups: Group I (25, 50 or 75% bark mixed with 50, 25 or 0% wood chips, and 25% paper mill sludge); Group II (25, 50 or 75% bark; 50, 25 or 0% wood chips; and 25% peat); Group III (25, 50 or 75% peat; 50, 25 or 0% wood chips; and 25% paper mill sludge).

     A two-year controlled-release fertilizer (Nutri­cote 18-6-8) was incorporated into each substrate at the start. A black plastic sleeve (4 mil, 35-cm wide) placed like a florist’s sleeve around each inner container suppressed weeds and water loss (2). When required, the same quantity of water (usually between 8 and 12 L per container) was supplied by trickle until all substrates were saturated. Selected physical properties of the substrates were determined at the start of the experiment. Electrical conductivity, a measure of the soluble salts concentration, using 1:2 substrate: water by vol. extracts, were measured periodically during the two seasons.

Results

• In both years, trunk diameter of the three species were highest with Group III substrates, intermediate with Group II, and least with Group I (Fig. 1, data shown for second year only).
• Trunk diameters of Group II and III trees were equal to, or slightly exceeded (10-12%), those of the control nursery mix.
• Trunk growth (Fig. 1) was positively related to water retention porosity, which ranged from 42-57%, 38-42%, and 20-27% for Groups III, II, and I, respectively (Fig. 2).
• The high-peat (50 and 75%) Group III substrates marginally but consistently produced trees with the largest trunk diameters (Fig. 1), although with birch (not the other species) shorter trees resulted as the peat content increased (data not shown).

Discussion
This study provides new and important information about using waste-derived substrates for pot-in-pot grown shade trees. An important underlying objective of the research was to demonstrate and recommend the use of selected wastes and their combinations for use as alternative amendments or substitutes for traditionally-reliable peat moss.

     Group III substrates, especially the two with the highest proportions of peat (50 and 75%), were marginally but consistently the best of the 10 substrates. The high water retention capacity of these peat-based substrates was a key to this result. Although quite heavier, and of quite different composition, the control nursery mix had comparable water retention porosity (49%), generally the highest content of soluble salts, and produced only marginally less growth (Fig. 1). This result indicates that substrates with widely different physical and chemical composition can be used.

     If waste products can be substituted and used with positive effects or, in the worst case situation, small diminutive effects on growth that seem to be of little economic importance as in this study, then there may be an overall benefit to using them, particularly if they are readily available or less expensive than peat.

Award-winning research
Landscape Trades congratulates Drs. Calvin Chong and Glen Lumis on receiving the Plant Products Co. Ltd. Award from the Canadian Society for Horticultural Science (CSHS) for this paper, judged to be the best published in the Canadian Journal of Plant Sciences in 2000.

Acknowledgments
Financial assistance for this research was provided in part by: Donohue Inc. (Now Abitibi Corporation); Landscape Ontario Growers’ Group and Horticultural Trades Foundation; and The National Research Council of Canada, Industrial Research Assistance Program (IRAP). Material assistance was provided by: Plant Products Co. Ltd., Nursery Supply Co., Vanden Bussche Irrigation Equipment Limited, Waterdown Garden Supplies Ltd., and M. Putzer (Hornby) Nursery Ltd.

References

1. Chong, C. 1999. Experience with the utilization of wastes in nursery potting mixes and as field soil amendments. Canadian Journal of Plant Science 79:139-148.
2. Chong, C. 1999. Turning ideas into reality for the nursery industry. Landscape Trades 24(4):24-29.
3. Chong, C. and B. Hamersma. 1994. Putzer nursery adopts pot-in-pot shade tree technique. Horticulture Review 12(8):7-9.
4. Chong, C., B. Hamersma and P. Braun. 1990. Shade tree production in above-ground wire basket containers. Landscape Trades 12(9):10, 12-14.
5. Chong, C. and H. Mathers. 1989. Revolutionary Oklahoma technique produces shade trees in two years. Horticulture Review 7(19):8-10.
6. Chong, C. and G.P. Lumis. 2000. Mixtures of paper mill sludge, wood chips, bark, and peat in substrates for pot-in-pot shade tree production. Canadian Journal of Plant Science 80:669-675.
7. Chong, C., G.P. Lumis and R.A. Cline. 1989. Effects of fabric containers. American Nurseryman 170(11):51, 53-55.
8. Haydu, J.J. 1997. To bag or to pot. American Nurseryman 185(8):40-42, 44-47.
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10. Murray, C.L., G.P. Lumis and C. Chong. 1997. Response of shade trees grown in in-ground containers to three container substrates. Journal of Environmental Horticulture 15:183-186.
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15. Whitcomb, C.E. 1988. Plant production in containers. Lacebark Publications, Stillwater, OK. 633pp.