This is the second article about culture at the Orchid Project. The vital topic of temperatures.

We run our greenhouse at a range of temperatures to allow us the grow a diverse range of species and each section is based on one or two of the habitats we have experienced on our expeditions to South and Central America, the Himalayas, South East Asia and Africa.

Temperatures are related to both the altitude and latitude of the habitats we are recreating so in simple terms it gets cooler as you climb a mountain or travel away from the equator. At 3000m altitude at the top of Mount Bigugu in Rwanda (on the equator) it is cool but not freezing in the winter with Erica scrub and orchids such as Disa starsii and Polystchya spatella (below)

At 3000m in Sikkim in March 3000m is a cool place with the snow not much higher up the mountains and the only orchids are hidden away under the snow or like Pleione hookeriana, leafless and buried in deep moss on tree trunks.

 Lachung at 3000m Sikkim and snow at 3700m at Yumthang 

Of course we have also been to some very hot places with lowland Guatemala in July probably the hottest with the temperature at night falling to around 28C and the day time temperature in the high 30s.

 Hot work collecting orchid data in Guatemala near Yaxha.

The next key to temperatures in greenhouses is to consider both the minimum temperature and the temperature at which the vents open. This does not give a maximum temperature but does significantly affect the day temperature during much of the year.

We have five distinct section and I will give our minimum and vent open temperatures for these as well as the habitats we feel these sections replicate from our expeditions.

Temperate – Here we grow Cymbidiums, Coelogynes, Pleiones, Native species and Australian Species  – The vents are open most days even in the winter giving this section a very fresh feel. We shade above 400W/m2 and water heavily in the summer but a less in the winter. Several species here grow cooler than many books recommend but we find that the plants flourish in these cool airy conditions. In the summer we leave the door open in hot June/july weather but do not worry to much about high temperatures (regularly above 30C) as we keep plants very wet.

Cool Americas – A wide range of species from the mountains of Central and South Americas (as well as a few from Africa) Shady damp and cool is the target. Plants are packed in and help to maintain their own humidity and cool conditions. We find niches for plants that enjoy deeper shade or more light by hanging high or low in the greenhouse.

Warm Americas – This is not a very warm section as most Cattleya species do come from mountains but below 1000m. This section is brightest reflecting our experience of forests in Guatemala (shade above 500w/m2

Warm Asia – Our warmest section at 18C is not warm enough for some heat loving lowland species but matches the lowland forests we have seen in the Himalayas and south East Asia. The majority of tropical diversity is not found in very hot lowland forests and this is probably due to the impact of the ice ages when the global climate dries and tropical forests retreat to mountain rufugia.

Cool Asia – Is where we grow the majority of our Himalayan species (from 1000-2500m altitude), Coelogynes, dendrobiums and cool growing species from other areas such as the mountains of Madagascar that experience similar cool tropical conditions.

What about high summer temperatures?

The question we most often get is about how we cope with high temperatures in the summer (especially after this summer). A lot of books and references suggest that for cool growing orchids you should keep temperatures below 26C and people panic when their temperatures go over 30C in hot weather.

As shown in the table we have our vents automatically open to give a ‘maximum’ temperature in cool weather but of course in June when it is 28C outside the greenhouse temperature often goes over 30C and this year we recorded a record of 35C in Cool Americas. Our philosophy is that if plants are well watered wet at their roots they can cool themselves through transpiration so in hot weather we shade (additional internal shading gives areas where leaf temperatures are kept cool) and water well – that means every day and twice a day in extreme weather especially for mounted plants.

Some species do suffer in very hot weather particularly Dracula species and some Masdevallias where we get leaf damage (black spots and dropped leaves) and for these species we try to keep plants happy by hanging or moving them right down to floor level. If a plant really can’t cope with UK summer temperatures we don’t grow it. We certainly find that the cool growing Himalayan species such as Cymbidiums and Coelogynes don’t mind the hot weather as long as they are really wet.

Some of our warmer growing species really appreciate hot weather and put on a real growth spurt – again water is important to help this.

What about cold spells in the winter?

As I have already said there are some species that would like to grow warmer than our Warm Asia minimum temperature. If we have several days of sub-zero temperatures and snow the greenhouse will tend to sit at the minimum temperature and when this happens we try to avoid to much water on the leaves of these warm growing plants. We do need to water in cold spells though as the heating working hard tends to dry plants out. remember that the top of the greenhouse is significantly warmer than bench level.

As we have thermal screening that automatically deploys, at night and when the outside temperature is below zero, we do override the system to open the screening in the day to let the plants enjoy some light in a prolonged cold snap.

If the Temperate section has been at 6C for several days, and has not warmed with sunlight on the greenhouse, we do open the doors to Cool Americas to allow a day time rise in our coldest section