Genetics: There are several names for the greenhouse cucumber (Cucumis sativus) including European, Dutch, English or burpless cucumbers (Fig. 1). Aside from generally being long and slender, there are several ways in which greenhouse cucumber varieties differ from field-grown cultivars. First, they are generally considered to have a milder flavor and less bitterness. Second, the skin on fruits is very thin and, as a result, peeling is not required prior to eating. Third, fruits are also parthenocarpic, or seedless. Beit Alpha is another type of greenhouse cucumber that is popular. They are similar to the European/Dutch cultivars in that they are mild-tasting and seedless and, though the skin is thicker than standard European varieties, it is still thin enough not to require peeling. However, they are smaller than the European/Dutch cultivars when mature and harvested, when fruits are 6 to 8 inches long.

Production systems: The most common production systems used for cucumbers are those systems used for other vining or high-wire crops, including Dutch buckets or a gutter system with slabs or bags of substrate.

Propagation and young plant production: Greenhouse cucumbers are propagated by seed. The seed for greenhouse cucumber varieties is expensive, approaching $1 per seed, due to the fact that the genetics are gynoecious and parthenocarpic (seedless). Seeds may be sown directly into 4-inch blocks of stone wool, coconut coir, or other substrate that will be transplanted into production systems. The air temperature should be maintained at 84° F for the first few days for optimal germination. After the radicle has emerged, lower temperatures to mid-70s° F. Supplemental lighting can be used to keep seedlings compact. Once transplants have three to four true leaves, they are ready to transplant.

Nutrient solution: Greenhouse cucumbers are a heavy-feeding crop. Complete nutrient solutions with an electrical conductivity (EC) between 2.0 and 3.0 mS/cm and a target pH between 5.5 and 6.0 should be used. The ratio of nitrogen to potassium (K) should be around 1:1.5; elevated levels of K are used for this heavy-fruiting crop. Avoid elevated nutrient solution and root-zone EC, as the high concentration of salts can damage roots and cause root rots to occur.

Temperature: Cucumbers are warm-growing compared to other greenhouse and hydroponically grown crops. Daytime air temperatures should be maintained between 75 and 80° F, but should not exceed 85 to 90° F. Temperatures during the night should not drop below 65° F.

Light: Cucumbers are, generally speaking, a high-light crop. There are some varieties that will perform better under lower-light conditions, but all will respond positively to increased light intensities. Supplemental light can be used to enhance growth and productivity, but only if the investments in lights and operating costs are economical.

CO2: Cucumbers benefit from carbon dioxide (CO2) enrichment in the greenhouse. Supplemental CO2 from liquid injection or burners up to 1,000 ppm can boost productivity, especially during periods where ventilation is reduced.

Pollination: Since nearly all the cucumber cultivars grown hydroponically are parthenocarpic and form fruits without ovules being fertilized, no effort to pollinate flowers is required.

Pruning and training: As with other vine crops, cucumbers require pruning and training. As with tomatoes, “suckers,” the shoots arising from leaf axils, are removed to maintain a single, primary stem. Cucumbers also produce tendrils, which should be removed. If left on the plant, they will wrap around other plants and infrastructure, causing problems. Fruit thinning should also occur for standard-sized cucumbers. If more than one fruit is allowed to develop at each leaf axil, the fruit load will be too great and the quality of individual fruits will be diminished.

Cucumbers need trellising since they are a vine crop, and there are several methods that may be used.

The drape method is where the stem is trellised on twine up the suspended wire, then allowed to drape down over the wire and grow toward the ground.

The umbrella method is similar to the drape method in that the stems are trained up to the wire. However, once stems grow above or over the wire, they are pruned back to the send leaf after the wire and two main stems are allowed to grow and drape over support wires, growing down toward the ground.

The V-cordon method is somewhat similar to the drape method, in that the vine is trellised up to the wire and then grows back down toward the ground. However, the high-wire is not directly above the plants in a V-cordon. Instead, the wire runs a few feet out from the plant into the aisle, requiring stems to be trellised at an angle. With one row leaning into an aisle and the other row running toward the other aisle in a double aisle, it allows more light to be intercepted by each individual plant by reducing the shading from neighboring plants.

Some growers are also staring to use the lean-and-lower method (Fig. 2), similar to what is used for tomato. As plants near the support wire, the trellis each stem is trained to is lowered so the growing point is below the support wire, and then moved down the wire several inches.

Pests: Spider mites, thrips, whiteflies and aphids are some of the most common insect pests of greenhouse cucumbers. One of the most distinctive signs of an infestation is the mottled, chlorotic cucumber leaves resulting from spider mites. At first glance, it could appear like high-light damage or a nutrient deficiency, but this occurs when spider mites feed on leaves and cause stippling all over them.

Diseases: Cucumber mosaic virus can have detrimental impacts on crops. Since aphids can transmit this disease, keep aphids under control and your plants virus-free to avoid outbreaks. Powdery mildew is the most common disease in cucumber production. Look for those varieties that have some resistance to powdery mildew and start control measures early to avoid the development of hot spots.

Physiological disorders: Fruit abortion occurs when there are too many fruits on the plant at any one time and there is insufficient vegetative growth to maintain the crop load. Fruits may also abort when there is a sudden change in crop culture or the environment.

Harvesting: Growers can harvest from cucumber plants for approximately 12 weeks, though this will vary with cultivar, environment and culture. For standard-sized European cucumbers, fruits are usually harvested when they are between 12 and 14 inches, with the USDA requiring a minimum fruit length of 11 inches. Frequent harvests up to three to four times per week are necessary to keep plants balanced between vegetative and reproductive growth, as well as to maintain fruit quality.

Postharvest care: While the thin skin of greenhouse cucumbers is a positive attribute with respect to flavor and convenience, it creates challenges for postharvest; the thin skins allows more water loss from fruits compared to thicker-skinned field cultivars. To minimize water loss and extend postharvest quality, individual fruits should be shrink wrapped. For Beit Alpha cucumbers with the slightly thicker skin, multiple fruits may be placed on a polystyrene tray that is then shrink-wrapped, or they can be placed inside a plastic bag with ventilation holes. While fruits can be stored at room temperature, they can also be cooled to extend product life, but temperature should not get below 50 to 55° F to avoid chilling injury. Cucumbers may yellow in response to ethylene, so exposure should be minimized throughout the postharvest chain.

Christopher (ccurrey@iastate.edu) is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.

It’s an interesting time to work in horticulture, and many young people are taking notice. Fruit and vegetable production is a task that’s now being carried out in warehouses and greenhouses as well as in fields. Production technology is developing at an incredible pace, reducing labor needs, improving food safety and changing horticulture as we know it. It’s also attracting a different group of potential produce growers.

According to a 2017 National Young Farmers Coalition survey of “3,517 past, current and aspiring farmers under 40 years across the U.S.,” 75 percent of respondents said that they did not grow up on a farm — wow! When you look at the controlled environment agriculture industry, I wouldn’t be surprised if many of those younger folks also fall into this category and are new to growing commercially. While one group may be known more as “farmers” and the other group “growers,” there are many similarities, and they share one common goal — growing high-quality fruits and vegetables efficiently.

These new-to-the-industry folks, along with experienced growers who are following in their family’s footsteps, are evolving along with the market and creating a more forward-thinking, innovative produce industry. In this month’s cover story, we delve into the family-focused history and future of Big Marble Farms, a year-round Canadian greenhouse cucumber operation. Although CEO Ryan Cramer says he “grew up in a cucumber crop,” he did more than step into an existing role in the family business — he, alongside his father and uncle, created a new, high-tech greenhouse operation in 2009 with the future in mind. “You’ve got to be always moving forward,” Cramer says. “That’s a common thing. You always have to be constantly innovating and adapting to the changes — constantly becoming more efficient and looking for ways to produce better.” Read more about Big Marble Farms here.

kvarga@gie.net | (216) 393-0290

Twitter: @Karen_GIE

“I grew up in a cucumber crop,” Ryan Cramer says, recalling his formative years helping around his father Albert’s greenhouse. Now, Ryan has a cucumber greenhouse of his own, called Big Marble Farms.

In 2009, Ryan started Big Marble as a four-acre greenhouse near Medicine Hat in Alberta, Canada, with his father and his uncle, Rick Wagenaar. Ryan sticks close to his family — he also works with his wife Brianne, who is the company’s executive assistant, and they are raising their daughter, Eden, and son, Rhys, around the greenhouse.

Family was the foundation for Big Marble Farms, and it is now a huge operation. In less than a decade in the role of CEO, Ryan has expanded the business to its present 35 acres of glasshouses. This city of cucumber production, with 190 employees, is the largest produce greenhouse in Alberta, Ryan says. And fitted with high-pressure sodium (HPS) grow lights throughout its entire production area, “It is one of the biggest lit operations in Canada,” he says.

Several key practices have contributed to Big Marble’s success: marketing geared toward healthy diets, crop selection based on market demand, year-round production enabled by the use of supplemental lighting, near-total adoption of beneficial insects for the benefit of both crops and customers, and packing onsite and selling local to ensure freshness.

“You’ve got to be always moving forward,” Ryan says. “That’s a common thing. You always have to be constantly innovating and adapting to the changes — constantly becoming more efficient and looking for ways to produce better.”

From one generation to the next

Raising Ryan on a farm was a natural follow-up to Albert’s own childhood. Albert was raised on a dairy farm where he tended to crops such as alfalfa, timothy and wheat. In the mid-1980s, Albert opened Rolling Acres Greenhouses, an operation that has, at various points, produced cucumbers, peppers, specialty crops and tomatoes. Although Albert brought his experience to Big Marble as founder, he still operates Rolling Acres Greenhouses, which is currently situated on 9.5 acres in Medicine Hat. Albert splits his production between cucumbers and peppers.

“My dad was always close to home,” Ryan recalls. “He worked on the acreage [where we lived] — that’s where the farm was. And he was able to be around. But we were also in the greenhouse a lot. We learned how to work from a young age, from just small jobs like dumping the leaves out in the field to actually having to pick cucumbers when we got a little bit older.”

Albert bought a quarter section (160 acres) of land in the early 2000s with the intention of growing on a larger-scale. He noted how Ryan, who had developed a serious passion for growing, needed an opportunity to advance in the business. “When we had started talking as a group about doing something, I said to my brother-in-law, Rick, and Ryan, ‘Let’s just start our own,’” Albert says. “There was nothing around here with year-round production, and I said, ‘Let’s do something completely different. We’ll put glasshouses up and put lights in it.’ We just had to pull the trigger, and we did, and there we go, and now we’re at 35 acres. It grew fast.”

With 20 years as a grower under his belt, Wagenaar wanted to try something new by 2007, when he sold his own produce greenhouse, Sunquest Growers. Originally, he and Albert had more of a hands-on role at Big Marble. But now Wagenaar, who performs public relations and marketing, and Albert leave most of the day-to-day decisions to Ryan. “[Albert and I] are not scared to take a leap of faith, and we’re not scared of debt — we’re not scared of risk and reward,” Wagenaar says. “Both of us being in that same position, but at an arm’s length to the day-to-day, is perfect. And Ryan, being youthful and spending his whole life within the greenhouse industry, was the perfect third partner.”

Cuke choices

With Ryan at the helm, Big Marble Farms grows Long English Cucumbers and Mini Cucumbers. The Long English Cucumbers fill about 21 acres of production space, and its Mini Cucumbers fill about 14 acres. The offerings and how the greenhouse space is divided are based on market demand, he says.

The operation uses rockwool starter cubes for both types of cucumbers. It then grows them in high-wire systems using bags containing coconut fiber, growing off the main stem of each crop and pruning off the side shoots. “We keep on lowering the plant down so that we keep the head in the same spot so that the cucumbers are always being harvested in the same spot,” Ryan says. “It’s much better for labor, it’s better for quality of the product and it also has potential to yield higher. But it’s a premium system. It’s more labor, so it’s a bit of a commitment, but it also gives you better product quality.”

Growing Mini Cucumbers and Long English Cucumbers are similar practices, Ryan says, but they differ in some ways. For instance, growers need to pick Mini Cucumbers every day, but they can get away without picking Long English Cucumbers for a day. “Mini Cucumbers will grow too fast in two days — they’ll just get too fat,” he says. “They’ll get too big, and then they’ll go to waste.”

Mini Cucumbers are ideal snacks for children’s and adults’ lunches, Ryan says, and Long English Cucumbers are well-suited for slicing up and including in larger dishes.

Growing year-round

Big Marble Farms takes advantage of the sun’s rays at its location near Medicine Hat in Cypress County, Alberta. This part of the country receives the most annual hours of sunlight — it’s what Ryan calls the “sweet spot of Canada’s Sun Belt.”

In Medicine Hat on the Summer Solstice, the sun shines 16 hours a day, but on the Winter Solstice, it shines only eight hours, according to The Old Farmer’s Almanac. Big Marble Farms maximizes light transmission by using glass structures, Ryan says. And when the days get shorter, the farm increases the amount of light its crops receive by using HPS lighting throughout its 35 acres.

“Our tagline is ‘Always Growing,’ which is describing the fact that we produce all 365 days a year,” Ryan says. “It’s a summer-like product, even in the middle of winter. We’re able to turn on the grow lights once the fall light starts to set in, and we can actually push right through November, December, January, February, when most growers are changing crops and cleaning their greenhouses.”

On the marketing and sales side, Wagenaar sees major value in informing purchasers, produce managers and end consumers about the processes that go into a quality cucumber crop, such as supplemental lighting. He notes that not all customers have time to talk to food producers and learn about where the crop came from, but if he could tell all of them about the production systems at Big Marble, he’d be able to secure and maintain their business.

“When you explain it to them — the science of what it takes to grow a good cucumber and the effort of atmospheric computers and the lights and the waking up of the plant — they can’t help but grin,” Wagenaar says. “[They say] ‘You’re kidding? It’s not just a cucumber?’ ‘No, we wake them up in the morning, we put them to bed at night. We have to tickle their feet — tickle the roots — we call them the feet. We tickle them with the little sensations in the morning to get them active, to get them out of their sleepy slumber. And then okay, it’s worktime, let’s go.’”

A biological approach

To fight pests and diseases, Big Marble Farms uses approximately 99 percent beneficial insects, Ryan says. The farm, along with other operations, takes the biological approach rather than using pesticides to assure customers that their food is safe. This is one of the main assurances about produce that is important to customers, he says, along with local and fresh.

In addition, growers are beginning to find more sense in using biological programs than using pesticides, Ryan says. “It might be more expensive, but it’s just easier on your crop,” he says. “If you’re always spraying pesticides on your crop, you’re damaging it, to a degree. Plants don’t like being sprayed. In the long run, people have realized that this is the better option.”

Big Marble Farms’ biological program manager scouts the entire farm for pests and diseases, Ryan says. “He’s got a really good eye for it, a real passion for it,” he says. “What he does is he orders the beneficial insects, and they come from Holland. He monitors on a weekly basis how many good guys and bad guys we have in the greenhouse, and then he makes decisions based on that for how many more good bugs he’s going to need for the next week.”

When it comes to educating end consumers on organics, Wagenaar says he tells them that it is far better that they are eating a vegetable rather than a processed food such as Pizza Pops or Crispy Crunch — Canadian junk food. He never says the product is organic, but he tells them, “Spraying pesticides is always, always, always the very, very last chink in the armor.”

Freshly packed

Once harvested, Big Marble Farms packs its own product to avoid bruising and guarantee freshness and proper labeling. The Mini Cucumbers can be packed in different ways, Ryan says, including six or eight packed together on a foam tray with a labeled plastic-wrap on top; or placed in one, one and a half or two-pound bags. The English Cucumbers are plastic-wrapped either individually or shrink-wrapped with others, placed in a box and sold in bulk.

Big Marble Farms uses grading lines, which Ryan says requires a lot of labor. Employees fill boxes with cucumbers and place the vegetables on trays by hand.

“All the big operations have been packing their product for a long, long time,” Ryan says. “But as far as Alberta goes, we are one of the only ones that do it. So [it’s] somewhat new to Alberta, because we never had big enough operations. We always co-packed.”

Big Marble Farms sells its cucumbers it through the RedHat Co-operative, a co-operative of about 30 growers near Redcliff, Alberta, “The Greenhouse Capital of the Prairies.” Ryan is a board member of the co-op, as is Albert, who chaired it for several years due to his roles at both Rolling Acres and Big Marble. The co-op sells Big Marble’s product throughout Canada — mainly in the Prairie Provinces of Alberta, Saskatchewan and Manitoba.

The importance of family

Family still plays an integral part at Big Marble Farms. As executive assistant, Brianne helps with tasks such as marketing, branding and office work. And Ryan sees a benefit to having his children grow up around the operation.

“One of the things I think that I find very important from running a family business like this family farm, for my kids, is that I hope that they learn the same work ethic values that I learned and they have the same family life that I had,” Ryan says.

The business’ marketing emphasizes the importance of nutrition for children. Images of children — including Eden and Rhys — playing and eating cucumbers are prominent on the company’s website. There are also old photos of Ryan as a child — one of him on a tricycle holding a cucumber and one of him standing beside Albert, who is kneeling.

Albert says he enjoys seeing his grandchildren growing up in Ryan’s greenhouse like Ryan grew up in his. “It’s pretty exciting to see them being involved with that, and the opportunity for them is pretty cool, if they want to go with it,” he says.

The farm’s name also refers to children. “The name Big Marble harkens back to a simpler time when kids would spend hours playing with their friends and family,” according to the company’s site, which offers instructions for playing the game marbles, as well as Jacks and Conkers, two other games. (Ryan says the name is twofold — it also refers to Earth. “The Blue Marble” is a nickname for the planet, and an image the Apollo 17 crew took of it in 1972.)

More developments are on the horizon for Big Marble Farms, which has an expansion plan for 2020. Consumer demand will determine the crop type, and acreage will increase by another 10 or 20 acres, enlarging the greenhouse space to more than 10 times its original four acres.

Big Marble completed its 2016 expansion — a whole 20 acres — within a year of breaking ground, and Ryan has the same plans for this next expansion. “It will be complete in 2020,” he states, confidently. “We would break ground [in] early 2020, and we would have plants in it by the fall of 2020.”

Rooftop farming is intensive agriculture using engineered soil and irrigation on building roofs. Commercial rooftop farming is an emerging practice at the intersection of agriculture and urban planning, where economic and environmental returns are equally important. Rooftop farms can take many forms, including gardens, high tunnels or climate-controlled greenhouses. Rooftop farming enables production of hyper-local food and its associated social and educational benefits in urban areas where land is unavailable or prohibitively expensive for farming.

The basics of rooftop farms

Municipal zoning laws and building height restrictions are barriers to rooftop farms, but some cities are beginning to revise their codes to accommodate urban agriculture and rooftop farms. In New York City, for example, it became easier for property owners to obtain the city’s approvals for constructing rooftop farms in 2012 when a citywide planning initiative known as “Zone Green” removed the zoning impediments for incorporating ornamental and agricultural uses on rooftops. In this new context, the Brooklyn Grange, a 1.5-acre commercial rooftop farm, was constructed atop an 11-story building of the former Brooklyn Navy Yard. For this construction, $592,730 was funded by the Community-Based Green Infrastructure Program of the NYC Department of Environmental Protection based on the expectation that the farm could reduce drainage volume and nutrient load to the East River while producing fresh vegetables for local consumption. While commercial rooftop farming is still in its early stages, New York City alone has 0.9 billion square feet (about 21,000 acres) of flat roof surface, 14 percent of which is considered suitable for large-scale (>10,000 ft2) commercial farms (Ackerman et al. 2013; Acks 2006).

Rooftop farms can increase revenue through diverse social and cultural programs, not just vegetable production. For example, the Brooklyn Grange became a popular destination for environmental and agricultural tourism, and is used for exercise classes, weddings, photo shoots, music events and organic food tasting. Also, the Grange offers farming internships as well as environmental education programs that have engaged approximately 40,000 K-1 students. These programs emphasize participation of immigrants, refugees and other under-represented groups, which are subsidized by the municipal programs for green-job training and diversity.

Growing media in rooftop farms

Load-bearing capacity of the building is among the constraints specific to the design of rooftop farms. In comparison to using field soil, it is relatively easy to specify engineered soil within the weight limitation. These engineered soils can be categorized into the following two types: expanded shale, clay and slate (ESCS); or potting soil (PS). ESCS are common base materials for green roof soil products, which meets the industrial standards for rooftop landscape construction, including ASTM (American Society for Testing and Materials) standards (Ampim et al. 2010). ESCS soils can be useful for rooftop farming if they have sufficient water-holding capacity and nutrients (Whittinghill et al. 2013).

The second type is potting soil, i.e. container media. If peat, coconut coir and other organic materials are used as base material, potting soils can be lighter and have greater water holding capacity than ESCS soils (Harada et al. 2017). Whether ESCS or potting soils are used, the precise management of organic amendments becomes challenging because organic matter lost to decomposition must be replenished. Also, lightweight materials must be self-knitting (ex. peat, coconut coir) because lightweight granules (ex. vermiculate, perlite) can easily be lost by wind erosion.

Production in rooftop farms

Precise management of irrigation and fertilizer inputs can reduce the drainage loss of water and nutrients, which enhances both environmental and economic returns of rooftop farming. Unlike in-ground agriculture, for example, rooftops do not have immediate access to water in the ground level. Thus far, irrigation has used municipal water, which is both expensive and competes directly with human consumption.

The economics of rooftop farms

The economic viability of urban agriculture relies on the sales of quick-turn/high-yielding leafy greens and fruit-bearing vegetables with high market value. These crops also have high demands of water and nutrients, which can make rooftop farming prone to drainage loss of water and nutrients. While the yield of rooftop farms can exceed in-ground agriculture, efficiency of water and nutrients use can be lower. Optimizing these inputs is an exciting area for research at the junction of science and practice (Harada et al. 2017; Sanyé-Mengual et al. 2015). The development of soil mixes with enhanced water-holding capacity combined with drainage recycling systems are central to this effort.

Rooftop greenhouse considerations

The use of climate-controlled greenhouses allows for more intensive year-round vegetable production on rooftops. Design of such facilities involves a complex process of navigating zoning laws, load-bearing capacity of buildings and rooftop access. Rooftop greenhouse design also takes into account the potential for beneficial heat from the underlying building as well as greater wind speed than similarly sited ground-based greenhouses. Because of the challenges in site selection and design, some rooftop greenhouses have found it easier to incorporate rooftop farms into new construction projects rather than retrofit existing buildings. Once constructed, operating a rooftop greenhouse is remarkably similar to traditional greenhouses other than getting loads up and down through the building.

Because greenhouses allow for control of temperature, light and relative humidity, rooftop greenhouse operations usually focus on producing one particular class of vegetables to optimize production efficiency and output. Two examples of rooftop greenhouses include Sky Vegetables based in the Bronx and Gotham Greens with four locations including: Queens, two in Brooklyn, and Chicago. Both Sky Vegetables and Gotham Greens focus on leafy greens and herbs using NFT (nutrient film technique). In a rooftop setting NFT is favored over deep water culture (DWC, raft/pond hydroponics) because DWC uses a large volume/weight of water. Both operations sell to local restaurants and supermarkets.

While rooftop agriculture has its challenges, we may be seeing a lot more of it as the human population becomes increasingly urban. By 2050, 70 percent of the global population is projected to live in urban areas.

Acknowledgements Financial support was received from the USDA National Institute of Food and Agriculture, Multistate Research Project NE-1335: Resource Management in Commercial Greenhouse Production. Yoshiki Harada (yh535@cornell.edu) is a recent PhD graduate, Tom Whitlow (thw2@cornell.edu) is an associate professor, and Neil Mattson (nsm47@cornell.edu) is an associate professor and extension specialist within the School of Integrative Plant Science at Cornell University.

References Ackerman K, Dahlgren E, Xu X (2013) Sustainable Urban Agriculture: Confirming Viable Scenarios for Production, Final Report No.13-07 vol NYSERDA No. 13-07 [Online] (Accessed on April 01, 2016).

Acks K (2006) A framework for cost-benefit analysis of green roofs: initial estimates Green Roofs in the Metropolitan Region: Research Report Columbia University Center for Climate Systems Research

Ampim PA, Sloan JJ, Cabrera RI, Harp DA, Jaber FH (2010) Green roof growing substrates: types, ingredients, composition and properties Journal of Environmental Horticulture 28:244 Harada Y, Whitlow TH, Bassuk NL, Russell-Anelli J (2017) Biogeochemistry of Rooftop Farm Soils. In: Lal R, Stewart BA (eds) Urban Soils. Advances in Soil Science Series. Taylor & Francis Group, Portland, United States, Sanyé-Mengual E, Orsini F, Oliver-Solà J, Rieradevall J, Montero JI, Gianquinto G (2015) Techniques and crops for efficient rooftop gardens in Bologna, Italy Agronomy for Sustainable Development 35:1477-1488 doi:10.1007/s13593-015-0331-0 Whittinghill LJ, Rowe DB, Cregg BM (2013) Evaluation of vegetable production on extensive green roofs Agroecology and Sustainable Food Systems 37:465-484