Ferns in the glasshouse
Ferns do very well in the glasshouse where conditions are normally more humid and brighter than in the house. The temperature, light and humidity can be controlled to suit the range of species that you want to keep. The glasshouse can also be used to overwinter tender ferns that will thrive in the garden over the summer months.
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Temperature
Glasshouses are either cold (unheated), cool (always above freezing), warm (never below 8–10 °C), or hothouses (never below 16–18 °C). Winter heating is best provided by electric glasshouse heaters and summer cooling by shading and ventilation control. Using automation is more reliable and easier. Insulation can make heating more economical.
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Light
Many exotic ferns come from places where there is little seasonal variation in light levels. Additional lighting in the winter months can maintain fernsʼ metabolism and encourage healthy growth. In summer, shading with blinds or greenhouse paint may be needed to reduce temperature and also light, which can burn delicate fern fronds.
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Humidity
The humidity of air outside the greenhouse can be very variable depending on rainfall and temperature. In general, having wet surfaces and damp pots in a greenhouse will produce a higher and more stable level. However, if ferns are showing signs of low humidity, this can be boosted by glasshouse humidifiers or by misting systems.
Choice Ferns
Most ferns do well in a glasshouse, but some demand the temperature and humidity that only a glasshouse can provide. Controlling the environment allows some very attractive and interesting ferns to be grown. In addition, delicate ferns do not get damaged by the weather. However, pests and diseases can thrive in a glasshouse, and vigilance is needed to deal with these.
Maidenhair Fern
Adiantum raddianum
Although not hardy outside in the winter, this fern is well suited to growing in glasshouse conditions. There are many cultivars available with beautiful, delicate fronds and pink new growth.
Toothed Brake Fern
Pteris dentata
Pteris dentata has very finely cut fronds that are held upright. It can reach 50 cm high and remains a pleasant lime green throughout the frondʼs life. It has a strong ferny smell when brushed against and it is easy to grow.
Downy Maiden Fern
Christella dentata
This tropical fern is very attractive with soft, downy fronds and purple stalks. In warm, humid conditions it spreads readily by spore, so it needs to be regularly kept under control. It needs a winter temperature of 10 °C.
Snail Fern
Tectaria gemmifera
An unusual feature of this plant is that it produces bulbils on the surface of its fronds, which drop off and root in the compost beneath. In the right environment, it can grow to 1 m but it is usually half that height.
Lacy Pine Fern
Goniophlebium subauriculatum
A hanging basket is the best place to grow this fern, which has long fronds of 1–1.5 m. It grows easily, but needs a minimum of 15 °C. As an epiphyte, it likes plenty of water with good drainage.
Whisk Fern
Psilotum nudum
This weird-looking fern has neither roots nor fronds. It comprises stalks which absorb water and nutrients from the atmosphere. It grows in misty places on coasts and in cloud forests and so needs regular spraying.
Caring for your ferns in your glasshouse.
Glasshouses that have the sole purpose of growing plants offer the most scope for growing humid and tropical ferns. Humidity levels can be raised with water trays, ‘damping down’ (where water is sprayed regularly onto a water-resistant floor to evaporate), ultrasonic humidifiers or pressurised misters, higher humidity levels being achievable as you work through this list. Humidity levels of 60 %RH is probably the minimum to achieve whilst some tropical ferns (and filmy ferns in cool houses) require levels of 80–95 %RH.
Heat can be provided by indirect methods such as hot water radiators or electric fan or convection heaters. What temperature is maintained depends on the requirements of the plants, and reference to books on fern culture and to flora, defining the characteristics and altitude of the ferns’ natural habitat, will help to determine minimum day and night temperatures. Some ferns (such as tree ferns) require that the temperature has a diurnal variation, with cooler nights than days.
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More of a problem can be cooling the glasshouse in the summer, when temperatures can rise to high levels very quickly if the glasshouse is in direct sun. Ideally, glasshouses should have some degree of shading from trees or buildings, and the resultant reduction in light levels will generally not harm ferns. Otherwise, shading will have to be provided by blinds, either external or internal. The level of light transmission should be 20–40%, but the shading should be capable of being removed in the winter months or on overcast days to allow extra light to enter. Heat-reflective greenhouse paint on the glass can also be used, but this will need to be removed in winter.
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Ventilation can also help control temperature but at the expense of humidity if simple evaporation systems are used. Automatic window-opening systems are more reliable than manual. Ultrasonic humidifiers and misting systems can be set to operate when humidity drops to a pre-set level, although a reliable supply of suitable water is needed for both. Hard mains water and ion-exchange soft water can leave deposits of calcium and sodium on the plants, respectively. Sodium salts can prove fatal to some ferns.
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Light can be controlled by lowering and raising blinds and can be topped up in winter months to extend the daylight hours to 12 or so. Intense LED bulbs give out virtually no heat and, although initially expensive, have very long lifespans.
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Finally, growing any plants in an artificial environment can lead to problems not encountered in the garden. The most obvious are pests and diseases, which can thrive in a closed environment, especially if it is crowded. Please see the section on ‘Problems’ for more information on this.
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Collections
Developing and maintaing a fern collection. How to safeguard your collection for the future.