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Food for thought: making more of green roofs

by Matt Downs

In his latest Academic Corner column exclusively for Greenscape Magazine, Dr Tom Young, Blue-Green Infrastructure Associate at The Environmental Partnership (TEP) and GRO Board Member, focuses on food growing opportunities for green roofs…

Above: Dr Tom Young, article author.

I think readers of this magazine will know that roofs are underused spaces and offer fantastic opportunities to green-up cities. However, they also present a great opportunity to grow super local, fresh food. 

There are loads of examples of business and community-led urban farms utilising roof space to grow in cities. Many of these provide so much more than just fresh produce, with the spaces allowing community engagement and the chance for people to get involved in growing crops for the first time. The mental and physical benefits of this cannot be shouted about enough. 

However, in order for these rooftop farms to be successful, crops need to grow! In this article I will explore the scientific literature on this topic and give a few pointers to improve rooftop farming.

Substrate
As with normal green roofs, depth of substrate is key to growing vegetables in this extreme environment. In a Greek study, tomato and lettuce grown in standard green roof substrate with an additional fertiliser showed increased yield and improved nutrient content of fruit and leaves when substrate depth was increased from 7.5 cm to
15 cm. This is likely due to the increased water availability for crops in a deeper substrate. 

The makeup of substrate is also very important, with conventional extensive substrate unlikely to contain enough nutrients and retain sufficient water for high quality vegetable and herb production. Generally, increasing the level of compost increases vegetable yield, with the optimum level between 60-80% by volume (Eksi et al. 2015). 

Care must be taken when adding substrates with these types of properties onto roofs as they can become very heavy and slowly draining. However, this may be mitigated against by the high intensity of maintenance any type of vegetable plots would receive.

Generally, deeper substrate equals better crop yield. However, the greatest amount of roof space opportunity exists on extensive roofs which can only take less than 15 cm of substrate depth (Walters & Midden 2018). These shallow systems are more suited to crops which require less inputs or depth, such as lettuce, kale and radish. Deeper rooted species such as tomatoes can produce good yields, but require additional inputs such as water and nutrients (Walters & Midden 2018).

Above: Vegetable plots on the Stavros Niarchos Foundation Cultural Centre green roof, Athens. Note drip irrigation required. 

Inputs
The main inputs that can help productivity on green roof farms are irrigation and nutrients. Multiple studies have assessed the optimal frequency and volume of these inputs. Too many inputs can be wasteful, leading to wasted water runoff containing high levels of nutrients causing pollution, as well as making any enterprise uneconomic (Walters & Midden 2018). A study of the Brooklyn Grange 0.61 hectare rooftop farm in New York showed that the site produced more water than landed on it through rainfall, showing that irrigation practices could be improved to reduce water loss. However, net use of water per crop was actually lower than comparable systems on the ground, showing that the system was already quite water efficient (Harada et al. 2018).

In some situations, the addition of high nutrient fertilisers in a readily available liquid form can increase salad leaf concentration beyond EU safety limits, and so care is needed when applying these types of fertilisers (Nektarios et al. 2022).

A study looking at 4 x culinary herbs on a green roof in Southern Illinois, USA, found that herbs needed supplementary irrigation of around 13-25 mm a week to produce commercially viable yields in amended green roof substrate (Walters et al. 2022). The type of irrigation can also have a large impact, with subsurface passive wicking irrigation more effective at maintaining substrate moisture content than conventional drip and flood irrigation (Cho et al. 2010). The use of mulches can also help reduce water loss, and can range from conventional bark chips and gravel through to living Sedum mulches (Whittinghill et al. 2016).

Above: Terra Green Roof at CSU Spur Colorado State University. Photo credit: Jen Bousselot.

Other benefits
As with conventional roofs, roof top gardens provide multiple green roof services to the surrounding area. This includes urban cooling, water retention, carbon sequestration, building insulation and social benefits. Roof garden-style roofs have been shown to provide the same, or sometimes better, water retention and microclimate improvement compared to conventional green roofs (Almaaitah & Joksimovic 2022). However, due to the harvesting cycle, the services provided by roof top farms can alter throughout the season, for example less runoff attenuation and microclimate cooling immediately post-harvest when there is no crop cover.

Above: Hydroponic growing system.

Alternative systems
Alternative systems exist that can remove some of the issues and constraints of roof top farming highlighted. Hydroponic growing systems use close cycle irrigation systems which provide constant supplies of water and nutrients to crops. This can lead to very rapid growth and harvesting schedules, and also reduce the amount of water runoff and nutrient loss from these roof systems (Rapisarda et al. 2022).

Vegetable plots can also be integrated with solar panels. These provide shade to the crops, collect water, and provide electricity, whilst the vegetable plots help to reduce the air temperature around the solar panel and increase electricity production (Bousselot et al. 2017).

To conclude, rooftop food production is possible and can be very efficient. It will never replace conventional food production but can be a very important supplement to local food sources, whilst also providing multiple social and educational opportunities for urban areas, as well as additional green roof environmental benefits.

For more information, I can highly recommend this blog post from Pritchard and Pritchard which has some fantastic rooftop farm examples and some great videos of each site.

Contact the author, email: tomyoung@tep.uk.com

Further references available on request.

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