Living walls are best left to pros and ex-presidents

When you view a life-size portrait of some notable person, your gaze tends to go first to the subject's face and then to the pose of the body and finally to the background.

The newly unveiled oil painting of former President Barack Obama messes with this visual sequence. The visitors who have been lining up at the National Portrait Gallery in Washington to see the artwork by Kehinde Wiley are gripped by a strange and compelling element — a backdrop of vegetation that seems to embrace Obama and the antique Federal chair on which he sits.

Wiley is known to draw wryly on the imagery and symbolism of long-dead European portraitists, but with African-American figures in heroic array. The vegetation in the Obama portrait makes me think of the way artists, in both fine and decorative art, have turned flora into two-dimensional patterns. The wallpaper of William Morris is an obvious example. In his painting, Wiley has given the viewer such a wall, but then allowed it to stray a little into three dimensions.

The flowers on the canvas bring their own message, some of them disembodied from their plants. The Arabian jasmine recalls the garden plants of Obama's native Hawaii; the chrysanthemum is the official flower of Chicago; and the African lily, or agapanthus, an allusion to his Kenyan forebears. The cascade of foliage appears to belong to the jasmine. It would have been nice to have seen something of Obama's Irish ancestry — Hedera hibernica, perhaps?

Whatever the symbolism, there is something magical about a wall of lush vegetation — whether unexpected on a canvas or cosseting in a garden. A vertical, living tapestry has the power to clear the mind, lower the blood pressure and transport you to a place of delight. Achieving that ideal, like painting a portrait, is a lot harder than it may appear.

The desire to clothe buildings, fences, walls or any vertical plane with plants is a long-held ambition of gardeners and goes back at least to the Hanging Gardens of Babylon. Such effects hide ugly or dull surfaces with charming beauty. For gardeners in tight spaces, the vegetated wall retrieves a barren area and promises an effective screen.

Traditionally, the way to do this is to plant a vine in the ground and let it climb up a wall or fence. The difficulty is that many vigorous climbers — desired for full and rapid coverage — have turned out to be ravening weeds. These include English ivy, Asian wisteria, porcelain berry and five-leaf akebia. Even if you find a vigorous vine that isn't invasive, keeping it pruned, trained and generally groomed takes you into the realm of the hobby gardener. This neediness extends to rambling roses, by the way.

In recent years, another type of vegetative art has emerged, called living walls or green walls, and which relies on pockets to hold the roots. Under the guiding hand of a bold designer, they can take a repertoire of fairly ordinary plants and turn them into a tapestry of stunning effect. The best are extremely photogenic and have thus infiltrated social media. This makes them coveted by folks who have little idea, I suspect, of how expensive and demanding they can be.

In spite of this, the best are truly amazing. One of the champions of this horticultural art form is the French designer Patrick Blanc, whose commissions include the side of a five-story building in Paris transformed from bare stucco to a vertical jungle named L'Oasis d'Aboukir.

Many living walls are constructed either in balmy places such as Florida or indoors, allowing the use of tropical plants, including some of the leafier houseplants.

Building one outdoors in a temperate climate forces the use of hardy plants. In his Paris project, Blanc used not just hardy perennials but also a few shrubs. The palette includes bergenias, pinks, cranesbill, hypericum, liriope, hostas, fatsia, and even nandina and yucca.

Such vertical gardens require expertly engineered systems of support and ways of delivering water and nutrients. They also require large quantities of plants.

“I don't think it's a do-it-yourself sort of thing,” said Ed Snodgrass, a grower of green-roof plants in Street, Maryland. “You have to be pretty slick to take on a do-it-yourself pocket wall, with all the systems it requires.”

The curtain of vegetation veils its workings: Custom armature, a hive of planting cells, a computerized irrigation system and supplemental lighting. Many skip the soil — the plants grow in synthetic felt or the like and are watered and fed hydroponically.

If you want a sense of the challenges and rewards of the vertical garden, consider the Green Wall at Longwood Gardens in Kennett Square, Pennsylvania. It was the largest living wall in the United States when it was built in 2010 and is used to decorate a curving corridor of individual restrooms. The garden consists of two walls — they face each other — and each is 13 feet high and 360 feet long.

Originally planted in coconut fiber, which proved too acidic over time, it was replanted in inert rock wool with fresh plants. Most are tender ferns, but the palette includes spider plants, philodendrons, spathiphyllums and the fluffy asparagus fern. The wall is a beguiling composition of 33,000 plants in contrasting textures and shades of greens. By using mostly ferns, flowering is kept to a bare minimum. It's designed as a break from the floral extravaganza in the East Conservatory. “It's the sorbet in a great meal,” said its gardener, Lauren Jenkins. “This space is a respite.”

For the viewer that is, not Jenkins. With her team of four volunteers, she is continually grooming and trimming and making sure that the plants are happy. At this time of year, they are watered once a day and fed twice a week with irrigation tubes, but the plants nearer the glass roof are warmer, thirstier and watered longer.

Companies have emerged to cater to this market. Employers see value in living walls as ways of beautifying office lobbies and bestowing tranquillity in an otherwise harried environment.

Michael Furbish, of Furbish in Baltimore, has installed approximately 40 living walls over the past eight years. “We have a lot of people who see our walls in commercial applications and say, ‘What if I want one, 2 feet by 2 feet, or 3 by 3, in my kitchen. Can I do that?' “ But scaling down doesn't negate the need for custom installation, sophisticated irrigation and other systems, so “it becomes prohibitively expensive,” he said.

He is developing a more simple setup that he hopes to market later this year. “We are working on prototypes, and we think we are on the path to something that is reasonably priced, lower maintenance and pretty reliable,” he said.

Meanwhile, you can get your living wall fix by visiting the National Portrait Gallery. Pictures don't do justice to Obama's portrait, or the mesmerizing vitality of the surrounding verdure.