What's really up on Beacon Hill

High above the roof decks of Beacon Hill, a green stem that looks like something out of ``Jack and the Beanstalk" is growing heavenward, unfurling its narrow, budding branches with the city skyline as its backdrop.

The 50-year-old houseplant, a blue agave normally found in desert climes, was just a 7-foot cactus-like houseplant for most of its life, planted in a pot on the top floor of a yellow brick townhouse, just a few steps from the bathroom, under an arch of greenhouse glass.

But this spring, after years of loving devotion by its owner, the plant began its spectacular, once-in-a-lifetime towering bloom. From a cluster of spiny leaves, a stalk sprouted and shot up 30 feet in height and is now growing out of a hole in the top of the greenhouse.

Within weeks, the buds on the stalk's branches will turn to flowers, the flowers will fall, and the agave will die. That will be the end of the telephone pole-sized curiosity that has recently captivated neighbors, plant specialists, and even the guys who work at the local hardware store.

``It's kind of touching to watch the plant put forth this effort," said Frans Lawaetz, a freelance technology consultant who has been renting the apartment and taking care of the agave since last fall. ``The plant is doing so much at the end of its life. It's running out of steam; it has done enough."

Blue agaves aren't rare: they dot the landscape in Mexico, where the heart of the plant is harvested, mashed, and fermented to make tequila.

Wild agaves send up towering stalks, and long after the flowers are pollinated by bats and the seeds are dispersed, the dead, woody stems can be seen in the desert. In northern latitudes, the plants are sometimes grown in greenhouses.

But there are only sporadic reports of other agaves that have blossomed in captivity.

In 2001, another species of agave sent up a 20-foot stalk that punched through the greenhouse ceiling at Cambridge University Botanic Garden in the United Kingdom.

``Growing one in Massachusetts; growing one in Boston; growing one in Beacon Hill -- and then having it flower -- it's pretty one in a million," said Fred Drenckhahn, who brought the plant to Boston in 1967, when he came to attend the Massachusetts Institute of Technology.

A team from Arnold Arboretum came to admire the plant last month, at Lawaetz's invitation, and said the plant was in exceptional health.

``I think it is a lifetime event for me," said Irina Kadis, a curatorial assistant at the arboretum. ``. . . This was the first time in my life I saw a flowering agave, and it was a one-time possibility."

The plant was thriving, she said. ``I guess that it's a matter of love more than anything else. I think this is a beloved plant. You can tell it at a glance just by watching it."

Agaves aren't easy to love. Even the agave's most ardent admirers have their reservations.

``They're a pain in the neck to work with," said Wendy Hodgson, herbarium curator at the Desert Botanical Garden in Phoenix, who has studied the plants since the 1980s. ``They're spiny, they're dangerous, they don't flower, or you can't predict flowering. But they're beautiful."

In the coming weeks, the buds will sprout pale yellow flowers that will attract insects, birds, and bees, she said.

Drenckhahn, an engineer, has always treated his plant like an ornery, but endearing family member.

He gripes a little about its spiky leaves. He acknowledges the plant can be a hassle. He has had to rig up a pulley system to lift the hefty plant so he could change pots when the roots needed more room every few years.

But two decades ago, when he moved to the Beacon Hill apartment that is part greenhouse, part loft, the agave became the crown jewel in a plant collection that now includes rare orchids, cactuses, and a citrus tree bearing small oranges.

When he started building a house in the Berkshires, he began to rack up speeding tickets rushing to Boston to check on his plant family.

These days, Lawaetz, who rents the Garden Street apartment from Drenckhahn, is the one who lavishes attention on the plant.

When he saw the stalk crash into the greenhouse ceiling in April, he removed the pane of glass overhead and let rain pour into the apartment until he devised a solution -- a piece of plexiglass with a hole cut through the center just a little bigger than the 4-inch-wide stalk.

He documents the agave's growth meticulously and has been giving it a lot of water, because, he says, ``who cares about root rot at this stage of the game."

At two house parties Lawaetz has thrown for the agave, about 50 neighbors and friends have come to crane their heads to gawk at the plant, and he plans a final tequila-themed farewell party in the next few weeks.

`` Wow," said Todd Stevenson , a friend, at a recent ``Agave Friday" party. He peered up to where the tip of the plant seemed to burrow into the sky. ``I saw [these plants] when I was in Austin. They're there. But something like this in Massachusetts. In Beacon Hill. Are there zoning restrictions?"

Meanwhile, Drenckhahn is finishing up a 22-acre arboretum, house, greenhouse, and atrium in Western Massachusetts where he eventually hopes to transplant his Boston plant family.

Before the agave began its final crescendo, he planned to rent a crane to get it down from its top-floor perch and hire movers to truck it to the Berkshires.

Now that the plant is dying, he plans to saw down the stalk and mount it on a wall.

``People have their trophies; I may have one of my own now," he said. ``A lot of weird things have happened in my life, and this is one."

But this isn't the end of the story.

On a shelf above the shower, a pup -- a five-year-old agave that budded off the roots of the agave -- is growing a little bigger every day.

Carolyn Y. Johnson can be reached at cjohnson@globe.com.