In the School Greenhouse our most diverse and crowded section is ‘Cool Americas’ where we grow around 400 species of orchids from the mountains of South and Central America.
Our culture is based on visits to the remarkable cool tropical forests of Brazil and Costa Rica where we have seen many of the species we grow flourishing in their natural habitat.
We have had the pleasure of six expeditions to the Americas with the Writhlington Orchid Project as shown on the map and had the opportunity to explore a wide range of forests including the warm lowland forests of Guatemala and Belize. In this article I will be focussing on orchids found at altitudes above 1200m.
Costa Rica is a remarkable country essentially consisting of a north West to south east row of volcanoes with oceans to the East and the West. The resulting tropical climate has a dryer period from December to April and a rainy season from May to November but altitude has a big impact on rainfall as well as temperatures and in the cool forests above 1200m there are mists and rainfall all year. The result of all this moisture is a lush forest with abundant epiphytes.
As can be seen from the photograph of forest on the Poaz Volcano, massed bromeliads, ferns, mosses and orchids cover every part of the taller trees. We found a number of charismatic orchids growing in this epiphytic mass although most were only identifiable through good binoculars or on fallen trees and branches.
In Brazil we have twice visited the Mata Atlantica cloud forests around Macae de Cima as the hosts of the Rio Atlantic Forest Trust. This incredibly rich forest has taught us a lot about orchid culture and in particular drew attention to the specific microhabitats enjoyed by so many orchid species. For example the species found on a windy ridge of elfin forest would be replaced by a different group of species in shady wet forest 100m away. This is a reminder to look for the microhabitats within your greenhouse.
Coastal cloud forests in Brazil have a more distinctly seasonal climate than that we found in the cloud forests of Costa Rica with a warmer wetter summer and a cooler dryer winter. It was however significant to note that in our dry season visits we have experienced weeks with no rain but every morning the forest was dripping from the heavy mists and dews.
Our Cool Americas section has a minimum temperature of 12C with the vents opening at 17C in an attempt to replicate the conditions of cool temperatures we have experienced in these forests. One of the most troubling challenges to many growers is the maximum temperatures advised for cool growing species from the Americas. We can confirm to climate records for these habitats that temperatures in the day time stay very comfortable in the low 20s Celcius and temperatures below 24C in the day time would be great to achieve in our greenhouse.
However, let’s be realistic the temperature in the UK is frequently well above 24C and in out greenhouse it is always warmer than outside. In the hottest days of May, June, and July the temperatures in our Cool Americas section frequently exceed 30C with an absolute maximum in 2018 of 34C.
In the past we have panicked at such high temperatures and have responded with under bench sprayers and several visits a day on weekends to open doors and try to cool things down. These days we are much more relaxed. Our observations have lead us to the conclusion that the issue for plant health in warm weather is not air temperature but leaf temperature.
Leaf temperature is cooled by shading and with watering as leaves are naturally cooled by transpiration. A cool growing masdevallia will suffer in the heat if it is dry and unshaded but if wet and shaded we find that most species show no ill effects at temperatures in the 30’s.
We water heavily in our Cool Americas Section. In most of the year this means watering daily and in hot summer days and cold winter days (when the heating is working hard) we water twice daily. We water with a hose supplied by an inline pump from our rainwater feeding tank in the greenhouse and include feed at around 300-400μS.
To cope with the heavy watering we grow use large bark and no moss although natural moss tends to develop on top of most pots and baskets. We grow plants in small pots, mounted and in baskets.
Automatic shading deploys when the light level exceeds 300W/m2 and so the plants are shaded in any sunny periods from late february to early October. The rest of the shading is managed through plants. We hang good light plants (Odontoglossums, Oncidiums, Laelias and Maxillarias) throughout the section and our shade loving Masdevallias and Pleurothallids live underneath. The hanging regime also affects our watering as plants higher up in the greenhouse tend to grow dryer (especially when watered by shorter children) than those low down.
Probably our most challenging plants are the ones that want it wet and bright (such as Cattleya coccinea) and we make a point of hanging these in very easy to reach spots so that they don’t miss out on their watering just because they are up in the light.
When it comes to mounted plants we find that an ideal location is a weld mesh tower in front of the benches so that plants are low in the greenhouse and very easy to water well. We also have a north facing wall that provides an extra cool and shady microhabitat for some of our tiny mounted miniatures such as Lepanthopsis astrophora.
We don’t damp down in any of our sections including Cool Americas for two reasons. Firstly damping down to maximise humidity will reduce the efficiency of transpiration which requires a higher humidity inside the leaf than outside. Reduced transpiration will reduce the leaf’s natural cooling and so increase heat stress. The second reason is safety as too much damping down creates a dangerously slippery floor, and thirdly damping down is very wasteful in water. We are 100% self sufficient in rain water collected from the greenhouse roof and do not use mains water at all in our greenhouse.
Admittedly, there are some cool growing Masdevallias and Draculas that do suffer from heat stress in our Cool America’s section. The signs of heat stress are black blotches on the leaves and leaf drop. We keep these sensitive species as low and wet as we possibly can but accept that there will be some damage in summers like 2018. In the autumn when temperatures drop the plants tend to pick up and grow a fresh crop of healthy leaves. If not they are species we no longer grow.
The wonderful thing about growing cool orchids from the Americas is the fantastic diversity of plants that can be grown together. Every time I push my way through the Laeia anceps roots to catch sight of an interesting Stelis species I find myself transported back to those magnificent cool mountain forests we have explored on our memorable expeditions.