Ecologically minded gardens are in vogue, which is all to the good. One of the hallmarks of a successful landscape is that it is in harmony with its place, in its flora, planting aesthetic and general mood.

But it is worth knowing that the contemporary broad interest in plant naturalism is a relatively recent impulse. For the past century or more, gardeners have wanted to grow plants that were decidedly exotic and spectacularly different; the list includes non-native versions of roses, azaleas, hostas and lilies, for example.

Sometimes the craving for exotica meant creating whole new growing environments, none more adventurous than the rock garden. Done well, a rock garden evokes a high country scree. Done badly, that is, hesitantly, it looks as though a dump truck just delivered a load of rubble.

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Botanically, a rock garden opens up a whole new world. There are some curious and lovely native wildflowers for such a place, though rock gardeners have been drawn to grow difficult far-off alpine plants or desert flora.

But hang on to your prickly pears. I’ve discovered there is something even more adventurous than a rock garden, a more extreme version called a crevice garden.

A while ago, Tony Avent sent me pictures of the crevice garden at his Plant Delights Nursery at Juniper Level Botanic Garden in Raleigh, N.C.

A rock garden typically is formed from soil and grit and gravel piled between boulders. A crevice garden, however, uses slabs of stone set vertically and close together. There is less expanse of growing space and virtually no soil as such. Avent’s is remarkable for its size — some 300 feet long and about six feet high — and for its material. Instead of using quarried stone, he used broken up concrete slabs from a property he was clearing for a new home. This saved hauling 70 cubic yards of material to the landfill. But once he started in 2017, he got crevice fever and in February finished the whole installation using salvaged concrete from four other sites. He estimates the concrete pieces weigh 400,000 pounds.

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In the photos, the garden looked a bit industrial, and I thought I should actually see it before passing judgment. So I did, and the verdict? It’s wonderful.

The concrete is clearly not natural, but its rhythms and textures are read as an abstract version of a rock formation. This is down to the design sensibilities of Avent’s collaborators, Jeremy Schmidt, the botanic garden’s supervisor of grounds and research, and rock garden designer Kenton Seth, of Fruita, Colo.

Where once a line of dense old holly trees stood, the crevice garden has given Avent a whole new playground for his unusual plants. He is known for the breadth as well as the depth of his rare plant passions, many of them arid-region plants at home in the crevice garden. These include yuccas, agaves and, yes, prickly pears.

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Some of his choices will grow only in such a dryscape, but others are found in more conventional garden settings. The latter group includes hardy cactuses and succulents as well as other plants that you might not expect in a rock garden setting, including species of dianthus, penstemon, germander and daphnes.

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What these plants lack in soil, they gain in the elongation of their roots. A plant that might be eight inches high, huddled between two slabs, could have root strands reaching down several feet.

“It forces the roots much deeper,” said Seth, owner of Paintbrush Gardens. Their length increases the plant’s access to moisture and nutrients, often in a partnership with bacteria. “A lot of plants are adapted to absorbing mineral nutrients rather than organic, humic nutrients,” he said. “They get their minerals straight from the rocks.”

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In the first sections, they used no soil at all, just great quantities of a horticultural-grade aggregate named PermaTill. Many of the plants — 588 hardy cactuses and agaves — did not make it through the winter. This was surprising because the one thing that gets marginally hardy plants through an East Coast winter is the absence of wet soil. This high mortality rate was not down to record levels of rainfall in 2018. Avent and Schmidt discovered that the growing medium was too open, and frigid air was reaching deep into the ground.

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In winter, the soil temperature 18 inches down is nine degrees warmer than the air temperature, but in the crevice garden, it was only four degrees warmer, Avent said. In subsequent sections they changed the growing mix to include compost and some native soil to fix the problem. Today, 1,500 plants are thriving in the garden, including more agaves and hardy cactuses, delosperma, arabis and draba — all of which would probably not survive in normal garden beds in central North Carolina.

The crevice slabs undulate in waves that give movement to an otherwise unyielding medium. To place them, Schmidt said he set them down with a skid-steer loader above their allotted space and then used gravity to help position them.

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There is a pleasing tension seeing seemingly delicate, novel beauties such as globularia, with its chivelike blossoms, growing amid such hulking surroundings.

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When he is designing crevice gardens, Seth says he conjures a mental image “of a certain mountain range I’ve visited and give it that vibe.” But when tackling this concrete version, he imagined another scenario: A post-apocalyptic city where the plants were taking over the ruins. “A sort of Mad Max city after humanity,” he said.

This was an organizing idea for the design, but Avent says his crevice garden actually offers a model of urban sustainability — a way of creating beauty in the recycled concrete jungle.

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“I think it’s a great way to green cities while keeping stuff out of the landfill and exposing people to a whole new palette of plants,” he said. “I hope this will be an inspiration to cities around the world.”

Tip of the Week

The most effective way to deal with mosquitoes is not with chemical sprays but in removing sources of standing water. Look for trapped rain water in children’s toys and play equipment, blocked gutters, plant pots, soda cans and tarpaulin covers.

— Adrian Higgins

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