So can we grow edible bananas in Devon?

“Murs à pêches” in Montreuil near Paris

Even on a sheltered, south-facing slope, the UK's short growing season is unlikely to suit tropical crops. Further artificial modification to the site would be necessary to have any hope of growing bananas.

Increasing the temperature during the winter to provide an all-year round growing season may be possible with mesh cages or canopies to prevent loss of long-wave radiation during winter nights, in conjunction with passive heat storage in heat sinks made of stone, tyres or water reservoirs.

In Montreuil, near Paris, a system of small agricultural plots, aligned north to south and surrounded by 2.7-metre-high, plaster-coated stone walls called “murs à pêches” was created in the 17th Century to grow peaches for the French court. The south-facing walls heated during the day and emitted warmth during the night, allowing peaches, a Mediterranean plant, to be grown in large quantities in a temperate climate. The system was able to raise the local temperature by between 8 and 12°C.

Water and other reflective surface can be used to redirect low winter light. In Rjukan, Norway, and Viganella, Italy deep valleys have been illuminated in winter with the use of mirrors that track the sun and reflect light downwards.

And the results are in...

Example of maximum daytime temperatures recorded at contrasting slopes in the valley, compared to regional Met Office data from Exeter Airport, in summer 2014. 

Example of minimum night-time temperatures recorded at contrasting sites in the valley, compared to regional Met Office data from Exeter Airport, in Spring 2015.  

All the datalogger sites recorded average temperatures slightly warmer than those recorded at the Met Office weather station at Exeter Airport at Clyst Honiton over the study period.

Variations within the valley in which Easterbrooks sits were as predicted by the model: south-facing slopes were, on average, warmer than north-facing slopes, and the valley bottoms were cooler than the valley sides and tops. Air temperatures at the logger on the north-facing slope were, on average, 0.3°C warmer than Exeter Airport; the logger on the valley bottom was 0.7°C warmer; near the bottom of the south-facing slope it was 0.9°C warmer, 1.8°C warmer at mid-slope and 2.0°C warmer at the top of the slope.

The conclusion? The south-facing microclimates on slopes at Easterbrooks are likely to be suitable to produce a yield of some Mediterranean, warm-temperate and even subtropical crops, but the warm microclimate in itself is unlikely to be sufficient to produce a banana crop due to the long, cool winter season. Further artificial modification would be necessary in order to extend the growing season further.

Second step: fieldwork

Low-cost sensor shield improvised from two kitchen funnels and a clothes peg.

The purpose of the fieldwork was to test the computer predictions about microclimates where tender crops might be grown. We set out to do this by gathering actual temperature data from the chosen site on West Town Farm. We distributed nine temperature sensors across four points on the south-facing slope of Easterbrooks and at one point on a north-facing slope opposite. We used small, inexpensive iButton dataloggers, set to take measurements every 30 minutes, 24 hours a day.

Air temperature sensors were sheltered from direct sunlight and suspended to give a free flow of air around them. We improvised shields from plastic kitchen funnels (see above). Soil temperature sensors were wrapped in muslin and buried at a depth of approximately 15cms, marked with tent pegs.

Data was downloaded every four to six weeks and the loggers reset. Data collection commenced on 6th July 2014 and was continuous until 30th June 2015.

First step: computer predictions of microclimates at West Town Farm

Computer model of temperatures on West Town Farm (indicated by black border). Left: Frequency of days where the temperature exceeds 18˚C in West Town Farm area (darker orange indicates higher frequency). Right: Frequency of days where the temperature drops below 0˚C in West Town Farm area (darker blue indicates higher frequency)

When we began our quest to grow edible bananas in Southwest England, we identified temperature as the key obstacle to success. Frost can kill the plants and they also need a period of continuous high temperature to achieve full growth, flowering and fruiting. As a result, we decided to focus our fieldwork on air and soil temperature.

Our priority was to find a site which was unlikely to suffer severe frosts. As a tropical plant, bananas do not have a period during which they are naturally dormant and so their growing parts are not protected from low temperatures. In macroclimate terms, the Southwest is a good choice as it has relatively mild winters, particularly near the coast. However, it does not have the warmest growing season, so maintaining enough heat to produce a crop is a challenge. 

As a starting point, we generated a computer model which predicted the most banana-friendly areas on West Town Farm. The model uses the shape of the landscape to estimate the surface temperature. To do this it predicts how the shape of the land modifies the landscape; it calculates the movement of the sun across the sky, and the heating effect of the sun on different slopes; it estimates the cooling effect of wind in sheltered and exposed parts of the landscape; and it predicts the pooling of cold air at night in depressions and valley bottoms (frost hollows). 

From the maps of microclimate produced by the model we chose the slopes of a sheltered south-facing pasture, known as Easterbrooks, as our pilot site (top centre, just inside the farm boundary). This offered us the highest number of frost-free days and the highest number of days where the temperature exceeded 18˚C.

Dataloggers decommissioned

Today we collected the final batch of temperature data from our pilot banana-growing site at West Town Farm, and took down the posts and shields that held the dataloggers. Not all of the datalogger batteries lasted right to the end of the programme, but we do have almost a full year's data from the site, which we'll now evaluate and turn into graphs and charts so it's easier to see how this site compared with average temperatures for Exeter during the same period.

Overall there were seven sensors on the south-facing slope of Easterbrooks and two on a north-facing slope opposite. They were installed with the help of Dr Jonathan Bennie from Exeter University Environment & Sustainability Institute. The sensors took readings every 30 minutes, 24 hours a day from 6th July 2014 to 27th June 2015.  

Seeing changes

Looking towards Easterbrooks from the north-facing field opposite

We visited Easterbrooks today to collect the latest batch of data. We now have three rounds of data covering four months from July to October.

While south-facing Easterbrooks remains sunny and warm (if windy enough to blow a couple of the more exposed sensor housings off their posts), the north-facing slope is clearly now in deep shadow for much of the day.

In the autumn light it's easier to see the dip that our potential banana-growing hotspot sits in. It's in the upper right quadrant of the photo above. This sheltered position may also explain why the nearby trees (like those bordering the lane below) have retained their leaves.

First tests with thermal imaging camera

A normal digital image of the potential site for growing bananas at West Town Farm with thermal image alongside. White/red are hottest spots and blue/black are coolest spots.  Cattle can be seen in the distance at the top of the hill.