How Low Can You Go?

Jessica Gould welcomed staff from more than 15 gardens to this meeting of the PlantNetwork Glasshouse Forum on climate control in glasshouses. To what temperatures do you heat your glasshouses? What happens if you turn the heat down? Is your heating regime based on the perceived, but unchallenged, requirements of a few species? What’s the point of trying to conserve endangered plants in our glasshouses if, as a result, we affect their extinction in the wild by contributing to global warming? should we not all be examining our energy inputs, especially in the light of the Stearn report? These were some of the questions considered at this workshop.

A questionnaire (see here) had been sent out to those attending about temperatures maintained for various types of glasshouse collection and what had happened if the heat had been turned down or off. Jessica presented the responses and there was much interesting discussion as a result.

Among the 10 gardens that sent in data, the minimum temperature for display houses ranged from 12 to 18ºC for tropical houses, and from 5 to 15ºC for both warm temperate and cool temperate houses. For specialist collections, the minimum temperatures ranged, for example, from 8 to 16ºCfor warm temperate ferns and 12 to 20ºC for tropical orchids. Most of the gardens maintained these temperatures because they had done so for years, or as a compromise to suit a mixed collection. In some cases, heating systems were not adequate to maintain higher minimum temperatures. Occasionally, settings were changed to cope with extreme weather conditions. Plant growth is sometimes manipulated by adjusting temperatures in reserve or production houses. Temperatures had been reduced deliberately, as a result of total heating failure or inadequate boiler capacity in cold weather; or heating had been turned off in response to escalating fuel costs in recent years.. As a result, some species suffered or flowering was delayed, but others appeared to be little affected.

The discussion revealed that none of the horticultural staff present had been consulted when their glasshouses had been built or restored as to the growing conditions and controls they would like in them; but were doing their best with what had been provided. Some had computer-controlled systems, others manually controlled systems or a mixture of both, but few had been given full training on how to get the best out of their systems. Most did not use all the functions available and didn’t know the actual energy costs for their glasshouses. The importance of calibrating and checking monitoring equipment frequently was stressed.

Some gardens regularly turned off the heating in the summer, as had once been traditional. Every department in the University of Oxford had been asked to consider their carbon footprint and the use of ‘green’ energy supplies. When the heating was turned off in the glasshouses at the Botanic Garden from June to October (in response to rises in fuel costs), staff were surprised at how few plants were affected.

The use of thermal screens, vent control and monitoring the temperature and humidity in different parts of a glasshouse were discussed. The Royal Botanic Gardens Kew had 21 zones in their glasshouse complex. In general, botanic gardens are trying to grow as wide a range of plants as possible, to the best of their ability, for display to the public. Efficiency was not therefore the prime concern, as it would be for growing commercial crops. Most of those present used hot-water pipes rather than radiant heat in their glasshouses. The effects of high temperatures on light and nutrient requirements were considered.

The group enjoyed a tour of the Great Glasshouse: the displays of Mediterranean vegetation from different parts of the world were impressive, made up of beautifully grown plants. It was a clear bright day, and the views out to the landscape beyond were stunning. Over the past 6 years, minimum winter temperatures in the Glasshouse have been reduced by 4ºC, and yet the plants seem to be thriving. The reduction was forced originally by financial constraints, but has been maintained ever since. The new Tropical House to be erected (for monocots and basal dicots) in the Double Walled Garden would be constructed in polycarbonate. Other gardens are similarly reducing heating inputs. But at what point does this have to stop? Is there a real ‘minimum’, or are we all just doing as has always been done, without questioning received wisdom? A house with ‘tropical’ temperatures is being constructed at NBG Wales to grow tender plants that tie in to displays in the systematics garden, so this question has become pressing.

Launch of the glasshouse section of the PlantNetwork/BGCI Cultivation & Propagation Database; and a light lunch.

SEE the subsequent meeting at Oxford in June 2008, on Climate Control in Glasshouses