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Shadows cast by a proposed 39-story development in Crown Heights would destroy half of the Brooklyn Botanic Garden’s collection of rare and exotic plants within 10 years, according to one of the horticultural museum’s chief green thumbs, who urged members of the City Planning Commission to seriously consider the project’s shadow impact.

“The rezoning proposal would cause serious, tangible damage to the gardens,” Rowan Blaik, director of living collections at Brooklyn Botanic Garden, said during a Tuesday public-scoping meeting about the development.

Developer Continuum Company is seeking permission to rezone the property at 960 Franklin Ave., the site of an old spice factory, to pave the way for two 39-story mixed-use towers, which together would host a whopping 1,578 residential units split evenly between luxury and below-market-rate housing.

Garden honchos have for months claimed that the development will bathe the green space in as many as three hours of additional darkness per day. And Blaik stressed the effect those shadows would have on critical growing facilities located within the green space, which house a stunning 18,500 plants — including 150 endangered species — and are necessary to breed replacements for the approximately five percent of plants that die off naturally around the garden every year.

“These propagation and growing facilities are the heart of the garden,” said Blaik.

And because the garden’s grow houses heavily depend on sunlight to function, compromising their ability to soak up rays could result in a rapid, catastrophic loss of plant life, according to Blaik, who said that importing new plants — which could carry pests and diseases — to replace dead ones is not an option.

“Should we lose propagation growing facilities, more than half of our collection will be gone in a decade,” he said. “There are simply no commercial alternatives to on-site propagation facilities for botanic gardens.”

A spokeswoman for the builder, however, insisted that although it has yet to complete a full environmental-impact study of the project, early analysis shows that the development’s impact on the garden would be negligible, citing research conducted by Continuum’s own environmental experts.

“Based on preliminary findings, no significant adverse shadow impacts are anticipated on the Brooklyn Botanic Garden,” said Lupe Todd-Medina.

Todd-Medina further alleged that her bosses at Continuum Company attempted to speak to garden brass about the development, but that the horticulturists routinely brushed their proffered olive branch aside.

“To date, all outreach and requests to meet have been rejected,” she said.

City Planning bigwigs are accepting written public comments on the rezoning proposal through March 25, after which

Continuum honchos must compile their environmental-impact statement for the upzoning, which will then begin its journey through the lengthy Uniform Land Use Review Procedure, where it will again go before the City Planning Commission, as well as the local community board, Council, and ultimately Mayor DeBlasio.

The public can submit written comments about the Franklin Avenue rezoning request to the Department of City Planning via e-mail at 19DCP095K_DL@pl annin g.nyc.gov .