Want farmland or garden space in the city? Look up.

NEW YORK – Looking out across an urban or even some suburban skylines, most people see an expanse of rooftops.

A few enterprising gardeners, however, see something different: wasted acreage just waiting to be planted.

Rooftops are an underutilized resource “in precisely the places where space is everything and fertile land is most scarce,” says Annie Novak, author of a new book, “The Rooftop Growing Guide” (Ten Speed Press). “It makes you want to roll up your sleeves and get growing.”

Novak is co-founder and head farmer of the Eagle Street Rooftop Farm, in Brooklyn. Her book is filled with images of rooftop gardens and farms in cities and suburbs across the country.

“Rooftop growing is nothing new,” Novak says.

“It’s actually an ancient tradition that has been largely overlooked. Urban people in tight spaces have always made room for rooftop gardens.”

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She offers a 5,000-year timeline that includes Scottish sod-roof houses dating to around 3000 B.C.; Mesopotamian ziggurats featuring rooftop shrubs and trees; the Hanging Gardens of Babylon; European rooftop gardens of the 17th and 18th century; and recent green-roof projects in Chicago, Portland, New York and Montreal.

“It’s important to reconnect with that long tradition and realize that you don’t need to reinvent the wheel. You just need to try to improve upon it,” Novak says. “There’s a big community of expert rooftop gardeners out there, with lots of experience and wisdom to share.”

She advises beginners to search online for “rooftop,” ”garden” and their city’s name to find like-minded gardeners.

Successful rooftop gardeners, she added, must be energetic and unafraid of standing out in a crowd.

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“You have hard work ahead of you,” she writes in the book’s introduction. “You will find yourself climbing multiple flights of stairs, or riding up in an elevator in which you are the only person carrying a bag of potting mix, your flats of lettuce next to someone else’s briefcase.”

In addition to bringing fresh food and jobs to urban centres and helping the environment, some types of rooftop growing also help keep buildings cooler in summer and warmer in winter, proponents say. “Green roof” growing systems, in which the rooftop is protected by waterproofing membrane and layered with a relatively lightweight soil mix engineered for rooftop use, can add to a flat roof’s longevity by reducing UV light exposure and helping with common problems like stormwater runoff.

Not all rooftops are created equal, though, and the first step is to check your building’s specifications and how much weight the roof can handle, along with local laws. There must be safe access to the roof and some sort of wall around the top so that no one falls off.

“Rooftop growing requires permission, practicality and patience,” Novak writes.

Consider building codes, zoning, climate and wind (a major challenge in this type of farming).

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John Stoddard, whose Higher Ground Farm in Boston serves restaurants and farmers markets, says rooftop growing is unlike other kinds of gardening in many ways.

“We don’t have rabbits to contend with, but we have seagulls here and we have wind, so we need to stake our tomatoes accordingly,” he says.

On the upside, “Bees somehow find their way up here. We’re on the ninth floor, but I’ve seen dragonflies and plenty of ladybugs.”

There are three main systems used for rooftop gardening: container gardens (easiest for novices or gardeners on a budget), greenhouse gardens (more common in four-season climates where they can be productive even in winter), and green roof growing.

“Unlike the other two, the green roof has environmental benefits, provides an insulating layer to the building, holds storm water and can allow for gardening much the way it’s done on the ground,” Novak says.

Her guide includes sections on soil, seeds, pests, microbiology (“those teeny guys are important”) and even business 101 for when it comes time to sell the rooftop produce.

Nicole Baum of Gotham Greens, a hydroponic greenhouse grower with a staff of 120 and a total of 4.5 acres of rooftop greenhouse space in Brooklyn, Queens and Chicago, says, “People can easily farm in cities, be it on the windowsill or on a commercial scale like us. We bring our produce down on the elevator every morning. It’s way fresher and tastier this way, and there’s plenty of potential for more.”