“All of a sudden the apartment felt so cold and empty,” said Oakes, 33. “I needed to find a way to make the space feel warm and full of life again.”

Her solution? A fiddle leaf fig tree; the first of nearly 700 houseplants — spanning 400 species — that Oakes, founder of Homestead Brooklyn, would eventually buy for her 1,200-square-foot apartment.

Her indoor forest features everything from a subirrigated living wall in her bedroom, which is a wall of greenery that is essentially a self-watering planter with a built-in reservoir; a vertical garden made out of Mason jars mounted to the living-room wall with wooden boards and hose clamps; and a closet-turned-kitchen grow garden with edible plants (ranging from herbs and greens to pineapple plants and curry leaves).

“I didn’t set out to build a jungle,” Oakes said. “I just saw how much energy and life the plants brought to the space and kept going.”

It’s a sentiment that more and more young people seem to be echoing in their own apartments. Wellness-minded millennials, especially ones in large urban environments that lack natural greenery, are opting to fill their voids — both decorative and emotional — with houseplants.

“Millennials were responsible for 31 percent of houseplant sales in 2016,” according to Ian Baldwin, a business adviser for the gardening industry. The 2016 National Gardening survey found that of the 6 million Americans who took up gardening that year, 5 million were ages 18 to 34. “This group has more college debt and, as a result, are renting homes instead of buying,” Baldwin said. “Houseplants are a low-cost way to have a green space at home.”

Meanwhile, Greenery NYC, a botanic design company, has increased its clientele by 6,500 percent since it was founded in 2010; developers are finding ways to include gardens as an amenity for residents; and more people — like Oakes — are turning what little spare space they have in their apartments into indoor gardens.

“Our sales have doubled each year,” said Rebecca Bullene, the founder of Greenery NYC. “And I attribute that mostly to businesses that want to attract millennial talent and millennials themselves who want more nature in their lives.”

Inside her 1,800-square-foot apartment in Brooklyn, Bullene, 37, cares for more than 100 plants. She has installed a green divider wall — a 6-foot-by-6-foot steel shelving unit filled with a dozen wooden planter boxes and more than 50 plants — that separates her living room from her in-home office, as well as a terrarium and several other large-scale plants, including an 11-foot-tall Ficus Audrey tree, to help break up the open layout of the space.

But for Bullene, the plants do more than help define the apartment; they make her home healthier, too.“Plants boost serotonin levels and dissolve volatile airborne chemicals,” she said. “They actually make healthier spaces for humans to inhabit.” She cited a 2010 study from Washington State University that breaks down the benefits of indoor plants, including cleaner air and lowered stress levels.

Along with her floor-to-ceiling plant divider wall in the living room, she also employed a combination of plants that release oxygen at night in her bedroom — including aloe vera and sansevieria — so that she and her husband can breathe cleaner air while they sleep.

Millennial-minded companies are also going to great lengths to integrate greenery into their offices.

The Etsy headquarters in Brooklyn, for example, could easily be mistaken for an indoor botanical garden. Spanning nine floors and more than 200,000 square feet, the office is home to more than 11,000 plants, including dozens of large-scale plant displays and living walls installed and maintained by Bullene and Greenery NYC.

“Every employee has a sight line to greenery,” said Hilary Young, Etsy’s sustainability manager, who helps the company seek ways to conserve the environment. “It’s a beautiful space that inspires and boosts productivity.” Greenery NYC and the architects at Gensler worked closely to create a state-of-the-art rainwater-harvesting and irrigation system at Etsy’s headquarters, which is considered the largest commercial “living building” in the world. It allows all the office plants to be watered with recycled stormwater.

With the increasing number of young people searching for access to greenery in their residences, real estate developers have also jumped on the trend.

At the ARC in Long Island City — a new 428-unit “industrial-inspired” luxury rental building developed by the Lightstone Group — residents have access to a 1,100-square-foot glass greenhouse, where they are free to plant and grow their own vegetables and herbs. “It’s been a tremendous selling point to prospective tenants,” said Scott Avram, senior vice president for development at Lightstone.

Greg Garunov, 33, said: “One factor of my decision to rent in the ARC was the beautiful courtyard and greenhouse. There is something to having a green oasis at your fingertips in the steel city of New York.”

And over at the Margo, in Brooklyn, residents enjoy a living wall in the lobby as well as a rooftop garden with plots that tenants can adopt for their own gardens.

“Wellness is a priority for our millennial-aged residents,” said Dave Maundrell, executive vice president for new developments for Brooklyn and Queens at Citi Habitats. “They’re willing to pay more for access to a green space.”

For instance, Oakes has managed to make the bulk of her indoor garden self-regulating and, perhaps more impressively, removable.

Thanks to several do-it-yourself irrigation systems she hacked throughout her home, including two irrigation units she created using a 150-foot hose that connects to pipes under her kitchen sink, Oakes has to spend only about a half-hour a day tending to her plants.

“New York City is tough,” she said. “My plants gave me a sanctuary to come home to.”