the root of it

garden carrots || photo by Lauren Mandel

This year’s Thanksgiving dinner was something to write home about.  A menagerie of vegetable dishes flanked my mother’s heavenly, rosemary-infused turkey: roasted bliss potatoes, braised fennel, sautéed Brussels sprouts, butternut squash soufflé.  But the one side that got me thinking was the roasted rainbow carrots.

I grew these carrots from seed in my backyard garden.  The colorful crop produced tall greens and long, spindly tap roots that plunged into the soil below the raised bed in which the carrots grew.  But what about root crops that don’t have the luxury of growing endlessly downward?  As depth-limited environments with thin soil profiles, rooftop farms and gardens present a unique set of challenges.  Thankfully, we can pluck some useful knowledge from the green roofing world.

For decades, green roof designers have experimented with growing deep-rooted plants – even trees! – on rooftops.  The key is selecting hardy cultivars, providing at least the minimum thickness of soil in which the plants can survive, and ensuring that their nutritional needs are met.  Irrigation is also key in cultivating plants in depth-limited environments.  Proper irrigation techniques often allow deep-rooted plants, like ornamental grasses, to flourish in unusually thin soil profiles.

In the case of rooftop carrots and other root crops, follow these 6 steps:

1| Make sure your roof contains adequate root protection (i.e. a polyethylene or polyvinyl chloride root barrier)

2| Select hardy cultivars that won’t grow excessively deep

3| Blend or purchase well-drained growth media

4| Mound enough media for each crop

5| Pay close attention to each crop’s nutritional needs

6| Provide sufficient drip irrigation

What’s been your experience with root crops?  Have you tried growing them in a container or other depth-limited environment?  We want to hear all about it!

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that is one hot potato.

To the newbie, designing and building a rooftop farm may seem like a cakewalk.  What’s the big deal?  You just plop a regular farm on top of a building and it’s business as usual, right?  Wrong.  Planning a successful rooftop farm requires careful consideration of siting (where the farm will be located), infrastructure, project goals, and long-term financing.  Coordination with the building’s architect is also crucial, particularly when designing a rooftop farm for a building that has not yet been built.

Proper coordination can avoid one serious rooftop threat that ground-level farmers will never face: reflectivity.  Light that reflects off of surrounding surfaces can scorch spinach and burn brassicas.  Highly reflective glass on neighboring buildings, or even on surrounding levels of the farm’s own building, can devastate crops.

Here are two anecdotes that illustrate the power of reflectivity:

1|  The green roof firm for which I work designed and built a courtyard shade garden for an important Philadelphia client.  The courtyard is surrounded by taller building stories, which are faced with glass so that workers can enjoy the garden view.  The garden’s ferns, heuchera, and other shade plants performed well at first, until strong summer rays reflected off of the windows and fried some of the plants!  What seemed at first to be a shaded haven had become a seasonal hotbox.  The most sensitive plants were replaced with sun-loving species, and next time we will coordinate with the architect to avoid a similar mishap.

2|  In 2010 a Las Vegas hotel found itself in a sticky situation when poolside guests were burned by light reflecting off of the hotel’s glass façade.  For 90 minutes each day, the concave building reflects light that is hot enough to melt plastic and burn hair.  Guests have the ability to seek refuge under patio umbrellas, but replace people with vegetable plants, and you’d have some fried green tomatoes on your hands.

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a financial giant and a pitchfork

While young metro-agrarian trend setters around the country dive elbow deep into compost, one financial giant wants to get its hands dirty too.  This multinational investment banking and securities firm (that prefers to remain anonymous), is constructing a raised bed production area on the roof of the firm’s newly acquired hotel in Lower Manhattan.  The hotel is located in the affluent Battery Park City neighborhood, and will contain a restaurant that aims to incorporate rooftop produce into its menu.

Cedar for raised beds || Photo by Lauren Mandel

The sophisticated rooftop design incorporates swaths of extensive green roof, decorative crushed glass, and rubber pavers in addition to the farm plots.  Ken Smith Landscape Architect designed this layout, and hired Roofmeadow (the green roof firm for which I work) as the project’s green roof consultant.  Roofmeadow maintains a national network of licensed green roof contractors, one of whom, the Sponzilli Landscape Group, is currently installing this project.  I had the good fortune of visiting the site this morning to perform construction oversight, and was able to see the farm plots first hand.

The raised beds themselves are much like those on the ground plain, except that these beds more intricately address the threshold between soil and underlying substrate.  Rooftop raised beds require a well-planned drainage system in order to maintain proper soil moisture in each plot.  A bed that does not drain effectively can become waterlogged and kill the plants growing within.  A bed that drains too quickly will require significant levels of irrigation, and will therefore produce weak plants while wasting water.  The Embassy Suits Hotel raised beds utilize specially engineered growing media that holds a specific amount of water between its particles.  This media sits on top of a sheet drain, which was selected for its specific transmissivity (or horizontal flow) rate.

Installing raised beds on a roof is much more expensive than on the ground, due mostly to costs associated with rooftop access and labor.  This means that the beds themselves should ideally last much longer on a roof than on the ground, so that they do not need to be replaced very often.  What material should be deployed, then?  Untreated wood typically lasts 3-4 years when used for raised beds in the northeastern climate.  Treated wood lasts longer, but proves toxic when containing edible crops.  Cinder blocks and brick are sturdy options, except that they can easily exceed rooftop weight limits.

Ken Smith Landscape Architect chose cedar for the raised beds at the Embassy Suits Hotel.  This wood naturally resists decay, and therefore acts as an ideal candidate for long-lasting rooftop raised beds.  The downside?  The specific wood used for this project may not have been sustainably harvested, and it was also extremely expensive.  This is a wood that may be prohibitively expensive for most professional projects, and certainly would exceed the budgets of most at-home installations.

So what is the best material to use for rooftop raised beds?  That’s up for debate.  What do you think?

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standing room only

The packed audience sat in anticipation as I introduced three panelists at the international CitiesAlive  conference on Friday.

The conference, held this year in Philadelphia,  is organized annually by the Toronto-based Green Roof Professional (GRP) accreditation organization Green Roofs for Healthy Cities.  Unlike other international trade conferences, CitiesAlive is unique in its ability to attract a multidisciplinary audience.  The three-day event acted as a magnet for designers, policy makers, scientists, material vendors, rooftop farmers, green roof enthusiasts, and industry leaders.  The diversity was moving, as was each presenter’s ability to capture the interest of these audiences simultaneously.

Out of the conference’s 12 panel discussions, two focused on rooftop agriculture.  The discussion that I was asked to moderate, Design for Food Production and Biodiversity, highlighted built projects that promote either rooftop farming or green roof biodiversity.  While other sessions on green roof policy and key international projects were moderately attended, this panel discussion was filled to the brim, with standing room only.  Rooftop agriculture is one hot topic.

Scott Torrance, Founding Principal of Scott Torrance Landscape Architect Inc, discussed his process of ecological green roof design.  He developed a biodiversity checklist for these rooftop environments, which considers species diversity, nesting habitat, water sources, and other elements that promote wildlife occupation.  Scott’s talk posed interesting questions with regard to the development of pollinator habitat and pollinator corridors on rooftop farms.

Next, Keith Agoada, President of Urban-Ag, energetically described a demonstration greenhouse that he built on top of a nightclub in California.  The structure was designed and erected with a “guerrilla” approach, without the assistance of an architect or waterproofing consultant.  The small greenhouse is visible from an adjacent elevated highway, and is visited regularly by guests and performers from the nightclub below.

Lastly, Anna Suardini, a Technical Sales Coordinator for American Hydrotech, Inc., discussed the Gary Comer Youth Center in Chicago.  This community center houses a 7,400 sf rooftop row farm that is completely enclosed by higher levels of the surrounding building.  The farm is shielded from the elements, and provides children from the surrounding compromised neighborhood with a chance to work with plants and soil in a safe environment.  The rooftop farm’s success quickly lead to the development  of a larger, at-grade farm near the building.  These community agricultural nodes are effectively fostering a positive food production ethic, whereby children and adults alike become reconnected with their food.

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