Tree trimmers and pruners

John Davey conceives of tree surgery as a systematic profession in Warren, Ohio, after observing the decline of landscape trees and concluding they could be treated with the same methodical care as patients in a hospital. His 1901 book "The Tree Doctor" codifies the emerging science. The Davey Tree Expert Company is incorporated in 1909, and the Davey Institute of Tree Sciences opens that year to train the first generation of professional arborists.

Francis A. Bartlett, a 1905 Massachusetts Agricultural College graduate, founds the F.A. Bartlett Tree Expert Company in Stamford, Connecticut. Bartlett is the first commercial arborist to use power spray equipment to manage landscape pests and the first to develop practical cabling and bracing methods for structurally weak trees. The company remains family-owned for four generations and grows into one of the world's largest scientific tree care companies.

The National Shade Tree Conference is convened on June 5, 1924, at F.A. Bartlett's office in Stamford, Connecticut, organized by the Connecticut Tree Protective Association. Just 36 participants attend. The Conference becomes the International Shade Tree Conference in 1961 (when Canada objected to "national" in the name) and is formally chartered as the International Society of Arboriculture (ISA) on January 1, 1976, to mark the US Bicentennial. The ISA's 1924 founding is cited in arboriculture as the formal beginning of organized professional tree care in North America.

On August 28, 1928, brothers Griffith, Lester, and Carl Asplundh found the Asplundh Tree Expert Company in Pennsylvania, initially to finance their educations after their father's death. Griffith brought arboriculture expertise, Lester electrical engineering, and Carl finance. Their first major contract, with the Philadelphia Electric Company in 1929, establishes the line-clearance model: specialized crews dedicated entirely to trimming vegetation away from utility lines. By 1994 Asplundh becomes the largest US firm in utility line clearance; by 2021 it employs 34,000 workers and generates $4.7 billion in revenue.

Dutch elm disease arrives in the United States in 1930 via elm logs imported from Europe for veneer manufacturing, arriving through at least four ports with initial cases near New York City. The fungus Ceratocystis ulmi, transmitted by elm bark beetles, eventually kills millions of elm trees across 31 states. At peak infestation in the 1970s, hundreds of thousands of elms were lost each year. Local governments and private property owners spend many millions of dollars annually on tree removal work, creating substantial demand for tree trimming and removal crews throughout the 1940s-1970s.

On August 10, 1965, eighteen-year-old Jeffrey Hugg dies while pruning a tree after contacting a 4.8 kV overhead power line. His mother Ethel M. Hugg writes to federal and state authorities demanding safer tree trimming standards. The Z133 Committee forms on April 4, 1968, with the National Arborist Association as secretariat and the ISA as a key participant. The ANSI Z133 safety standard for arboricultural operations is unanimously adopted on July 14, 1971, and approved as an American National Standard on December 20, 1972. OSHA (enacted April 1972) later adopts portions of Z133, making it the legal baseline for the industry.

Alex Shigo, a plant pathologist with the USDA Forest Service, publishes his CODIT model (Compartmentalization of Decay in Trees) after dissecting over 10,000 trees to map how wood-decay fungi spread through wounded trees. Shigo demonstrates that trees actively compartmentalize decay rather than simply succumbing to it, and that the pruning-cut location relative to the branch collar determines whether the tree can wall off the wound or not. The research overturns the industry's longstanding "flush cut" practice and establishes the "branch collar" pruning method that ISA and ANSI standards would later codify. Shigo retires from the Forest Service in 1985 and continues publishing and teaching the science-based approach to tree care.

The International Society of Arboriculture launches the ISA Certified Arborist program, the first national credential for tree care workers, requiring three years of documented experience in arboriculture and passage of a written exam covering tree biology, diagnosis, pruning, cabling, soil management, and safety. The credential is accredited by the American National Standards Institute (ANSI) against ISO 17024. By the mid-2020s, nearly 50,000 ISA credentials have been issued. The certification does not legally restrict who can call themselves an arborist or perform tree work, but it creates a recognized professional standard that municipal government clients and large utility companies increasingly require of contractors.