ciated with TGRs is thought to contrib- ute to this response (Sherald 2007). APPLYING TGRS CORRECTLY As with any plant health care product, proper application is essential. Always follow label instructions and manufac- turer guidelines to ensure both safety and efficacy. Species response varies: some, such as certain Cornus species as well as Japanese maple ( Acer palmatum ), are very sensitive, while others (notably many pines) respond minimally. Research indicates the best growth regulation occurs when TGRs are ap- plied within 90 days before or after pruning. However, significant eco- nomic benefits have also been ob- served when applications are made within 180 days of pruning. SUMMARY Tree growth regulators offer arborists a versatile, research-supported tool for
Norway Maple (variegated variety): (Left) Example of untreated tree affected by tar spot. (Right) Example of treatment with paclobutrazol displaying reduced symptoms of tar spot.
managing trees in the built environ- ment. They are not a standalone solu- tion, but when integrated into a broader tree health care plan, TGRs can reduce maintenance needs, improve safety, and enhance tree resilience to drought,
disease, and urban stressors.
Used responsibly, they expand the ar- borist’s ability to maintain healthy, structurally sound trees in the places where people live and work.
ArborTIMES ™ Winter 2026 | 63
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