The first major breakthrough in the commercial
feasibility of TGRs on a large scale was the
formulation in the late 1970s of the cell elongation
inhibitors, paclobutrazol, uniconazole, and
flurprimidol for trunk injection. Due to their low water
solubility, it was considered necessary to dissolve
the new generation of growth retardants in either
methyl or isopropyl alcohol. The active ingredients
of these formulations were unquestionably effective
in reducing tree growth. After several years of use
throughout the United States in the 1980s, problems
associated with trunk injection begin to appear. Cracks
in the bark and cambium, weeping from injection
holes, and internal wood discoloration due to the
alcohol carriers led to disenchanted utility arborists
and their customers.