Landscape trees benefit from potassium-based superabsorbent polymer-amended backfill soil
Weiner (1975) put it aptly when he stated that “However rewarding the act of tree planting may be, watching a young tree slowly die can be spiritually defeating.” Landscape plants, noted Richard Harris (1983) in his well-known “Arboriculture” book, “probably suffer more from moisture-related problems than from any other cause.” The success of tree planting programs must be measured by the health of the plants after planting and in years that follow. Tree survival can be assured only if soil moisture and nutrients are maintained at adequate levels. To accomplish this, any product or conservation technique with potential for reduction in water amount and irrigation frequency while not adversely affecting tree establishment and growth, should be given serious consideration.
Many professional tree transplanting enterprises praise the benefits of superabsorbent polymers for tree survival when evenly dispersed in the planting hole and/or mixed with the backfill soil at transplanting time. However, limited research has been conducted and only cursory notice has been given to utilization of polymers for trees in the urban tree plantings. We reported earlier (Dehgan, et al., 1994) that at least for a fast growing species, such as Photinia X fraseri, amending the growth with 0.75% (by volume) or potassium (K) based Terawet’s TeraGel (T-200) superabsorbent polymer resulted in application of 50% less irrigation water. Other studies (Johnson and Leah, 1990) have observed greater efficiency of utilization of gel-stored moisture when various plants were grown in polyacrylamide polymer-amended media. Polymer-amended media provide a buffer against temporary drought stress and reduce the risk of failure during the establishment of the plant. Evidence has been provided (Ouchi, et al., 1989) that indicates water held by the polymer is almost completely available to the plants, though sodium (Na) - based polymers reportedly bind certain nutrients, making them unavailable, and result in reduced hydration capacity of the media (Bowman, et al., 1990). In fact, it has been demonstrated in recent studies (Ouchi, et at., 1990) that gel-stored moisture is utilized with greater efficiency than conventional forms of water. Moreover, addition of polymer to the soil has been shown to reduce evapo-transpiration by restriction of water movement from the subsurface to the surface layer (Ouchi, et al., 1990). Our results as well as those of others have also presented evidence of increased tissue nutrient levels concomitant with reduced leaching of some nutrients (Dehgan, ie al., 1994). Thus, utilization of polymers may be a means of reducing nutrient runoff at nurseries (Sweet, 1994).
In our most recent studies, nearly all of the 10 tree species [Podocarpus macrophylla (podocarpus), Juniperus silicicole (red cedar), X Cupressocyparis leylandii (Leyland cypress), Phoenix canariensis (Canary Island palm), Sabal palmetto (cabbage palm), Washingtonia robuste (Washington palm), liez Z-attenuate “Savannah” (Savannah Holly), Photinia X fraseri (Fraser photinia), Quercus schumardii (Schumard Red Oak), Lagarstroemia indica (Lagarstroemia)], as well as Zamia floridana (= Z. pumilia, Florida coontie) which were planted at the Urban Tree Teaching and Research Unit of the University of Florida at Gainesville, benefited from Terawet’s TeraGel (T-200) superabsorbent polymer-amended backfill soil. This occurred despite a drastically reduced irrigation frequency which began with 7.5 gallons of water every two, four, and seven days for the first two weeks and was ultimately reduced to once every seven, 14, and 28 days, for a total of six months. Surprisingly, plants of red cedar, a presumably drought tolerant species, were the only ones to severely decline or die as a result of water stress. Plants of all other species survived and in most cases were, to a greater or lesser extent, aided by the polymer in their establishment and growth. In this regard two points should be noted. First, although enhanced appearance was the obvious expected benefit when polymer was used, each of the species responded differently, with some having larger calipers (e. g., podocarpus) while others greater height (e. g., Leyland cypress) and still others with both greater height and caliper (e. g., Savannah holly). Second, despite absence of any irrigation in more than two years after project’s termination, even the remaining plants of red cedar, which as noted earlier suffered from lack of water, have since recovered and exhibit superior growth.
Corresponding to the above experimental results, uniform distribution of Terawet’s TeraGel (T- 200) polymer to the periphery of planting holes may have been a contributing factor in survival of 100% of the transplanted 5-8 inch caliper live oak trees on the campus of the University of Florida. In this ccase, the urgency of the assignment necessitated root pruning of the trees only tow weeks prior to transplanting, but ample care was give to reduce shock during removal and transplanting process.
In general, the intent of polymer and other irrigation frequency/amount studies at the University of Florida, Department of Environmental Horticulture is to determine, demonstrate, and furnish guidelines for maintenance of optimal soil moisture levels for transplanted trees in the urban environment. Despite genuine interest and consideration for well-being of trees, those most concerned with their planting and management are often perplexed by the contradictory irrigation frequency information presented by “experts” and in the literature. The seemingly simple question of irrigation frequency requirement remains an enigma. However, based on this and related research, it is becoming increasingly apparent that, though there may be some exceptions, frequent, protracted irrigation or application of excessive water amounts may be unnecessary for establishment of the vast majority of landscape trees. Amending the backfill soil with K-based superabsorbent polymers assists in maintenance of sufficient soil moisture, when irrigated judiciously, during tree establishment period. In general, results of these experiments indicate that while nearly all trees, irrespective of species or growth habit, may benefit from addition of superabsorbent polymers, several factors, and influence effectiveness of the polymer. However, logic does not necessarily dictate that because a small amount of polymer assists in tree survival and establishment a larger amount is more beneficial. It is advisable that the manufacturer’s recommendations be adhered to.
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