It's gray and grim outside, but inside one of Boston's most treasured museums — the Isabella Stewart Gardner Museum — there’s a celebration in orange taking place. The nasturtiums are back, streaming from the balconies overlooking the lush greenery of the courtyard.

Gardner had a passion for paintings, tapestries and sculpture. She had a passion for plants, too — chrysanthemums and azaleas, but especially nasturtiums — as another form of art. Gardner first displayed the nasturtiums in 1903, right before Easter and another celebration.

"Mrs. Gardner's birthday is April 14th. So we like to have them hanging from the balconies for her birthday," says Stan Kozak. He’s the chief horticulturist at the museum — the Gardner's gardener. He’s been there for 47 years, and he grows the plants that once a year take the spotlight in the magnificent courtyard. That includes hydrangeas in the summer, and grasses and berries in the fall.

It all starts at the museum's greenhouse in Hingham. That's where Kozak and crew cultivate 10,000 to 15,000 plants and trees for the museum over the course of the year.

When we visited in early February, the greenhouse was filled with light and lined with nasturtiums in terra cotta pots. Kozak started the plants from seeds last June. Now they're almost 20 feet long. They're clipped onto tall stakes, and they creep all the way up the wall and then across the ceiling. They're orange with round green leaves and spectacular.

Gardner gardener Stan Kozak attends to the nasturtiums at the museum's greenhouse in Hingham. (Robin Lubbock/WBUR)

Kozak picks off old leaves and dead flowers as he talks. He tends to the nasturtiums' every need, as if he were dealing with some kind of blooming diva about to make her debut.

The nasturtiums are susceptible to flies and viruses, as well as cold temperatures. Kozak explains the worst possible scenario for the flowering vines: "Having the heat go off in the greenhouse at night. It's happened ... so if it's ever on a cold night — like last year we had a couple nights in a row where it was 8 or 9 [degrees] below zero — I stayed here the night."

Delicate Transport

When it comes time for Kozak and his crew to finally move the finicky flora from Hingham to the Gardner Museum in a big box truck in late March, the pressure mounts. But the veteran gardener doesn't show it. The truck pulls up on a sidewalk. A museum employee removes a thick chain and padlock from the front gate.

A crew of landscapers gently carries each nasturtium vine from the truck to the sidewalk, and then into the museum's stone cloisters around the courtyard garden. The blossoms aren't looking their best today. They're scraggly and a little withered after their journey. But they'll recover after getting some sunlight.

Corey Roche, 21, leads a group of four staffers carrying a string of nasturtiums through a doorway into the Gardner Museum. (Robin Lubbock/WBUR)

The workers give them a final pruning.

"OK, well that's as good as it's going to get until we hang it," Kozak says after looking over one of the plants.

And the final part of this rite of spring begins.

One of the crew leaders picks up a pot, which weighs about 30 pounds. Then three other workers gently take a portion of the vine and silently start the slow procession upstairs — one flight, then three more — step by step past marble arches, a sarcophagus, Titian's "Europa" and other notable artworks.

It takes 18 trips to transport the nasturtiums to the third floor, where the pots are placed on benches and the vines dangled over the balconies.

"Every time it happens, it's exhilarating, it's exciting, and to me, it really does signal spring is on its way," says Corinne Zimmermann, the director of visitor learning at the Gardner. She sees the installation of the nasturtiums much like a processional or performance, she says.

"They're beautiful, and they're surprising. And I think that that's a real hallmark of the Gardner Museum," she says. "It's this multi-sensory environment where you encounter things that you wouldn't see anywhere else."

Kozak kneels by two nasturtium plants as staff lay them out in the museum cloisters and carefully step over them. (Robin Lubbock/WBUR)

Labor Of Love

And it's a labor of love — emphasis on "labor."

So it's known that nasturtiums were one of Isabella Stewart Gardner's favorite plants. But for Kozak, the museum horticulturalist?

"Not so much," he acknowledges. "I do not dislike them. I do not dislike any plant. But I do have my favorites. ... The nasturtiums are very stressful. They're a very delicate plant."

Kozak works methodically as he drapes one of the vines over a balcony.

"Hopefully, it doesn't take a quick exit from our hands down into the courtyard," he says.

Kozak and Leland Eglin gently pass one of the nasturtium plants over a balcony into the museum courtyard. (Robin Lubbock/WBUR)

That has happened before, so he always has a few extras on hand.

He has help this day from Braintree landscaper Dick Roche, who stands on the opposite balcony to direct Kozak on placing and turning each plant. They want the flowers facing outward, to put on their best show.

The nasturtiums last only a few weeks. So the nine-month endeavor has a short-term payoff for Kozak. But the real reward is that he's fulfilling Isabella Stewart Gardner's wish.

In a couple of months, he'll plant the seeds for next year's crop.