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Humidity and Hungry Palms

We all know that many palms hate dry air but why? Much that Gary has written about was, I must admit, new to me, and I'm delighted to pass it on. Appropriate for the time of year, too.
Gary Parker, Surrey, UK
Chamaerops No.21, Winter Edition 95/96

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Most palms are sensitive to air humidity:
tropical palms suffer when humidity is low and desert palms can suffer when humidity is high. In fact, many tropical and sub-tropical palms cannot manufacture food if humidity is inadequate. If you have faint memories of fifth form biology lessons you may recall how all plants, including palms, photosynthesize sugars from water obtained via the roots and carbon dioxide obtained from air passing into the leaf through pores called stomata. Stomata are control gates, opening when air is needed for photosynthesis and closing when humidity is low to prevent excess water vapour loss. A balancing act is going on: stomata have to open long enough for the palm to feed itself adequately, yet close as much as is necessary to prevent wilting. When stomata open in the dry air of a centrally-heated room, water evaporates out of the leaf very rapidly. This presents an insoluble problem for a palm: it can produce all the food it needs, or it can retain water, but it cannot do both. Faced with the choice of wilting or going hungry, palms seem to choose going hungry. This is why tropical palms placed in a centrally-heated room often appear fine for a few months and then suddenly drop dead: they have starved.

Recent research has shown that certain rainforest plants such as alocasias are unable to close their stomata at all. They live in an environment where the surrounding air is so intensely saturated with water that even though the stomata are open water vapour is not drawn out. I wonder if certain tropical palms are also unable to close their stomata? That would explain why some tropical palms fare worse than others in imperfect conditions.

Of course there are palms from dry regions that are adapted to low humidity. The relative humidity (RH) in rainforests is constantly close to 100% (any higher and the vapour would fall out of the air as rain) . Summer days in northern Europe have an RH of about 50-70%, whereas arid and semi-arid regions have an RH of perhaps 30-50%. Palms from arid regions (Brahea, Chamaerops. Phoenix and the like) have special adaptations for photosynthesizing in dry air, such as water-retentive leaf surfaces and hair-fringed stomata that minimise vapour loss. Agaves and yuccas go even further and have highly specialised internal processes that allow them to photosynthesize despite keeping their stomata shut during the day, storing air and waste gases in the form of solid chemicals and waiting until the humidity rises at night to open their stomata. Perhaps some desert palms have similar processes? However, while tropical palms are untroubled by high humidity, desert palms can rot easily in such conditions. It's horses for courses in the palm world.

What does this tell us about our palms? For one thing, it should be clear that sensitive tropical palms are never going to survive low humidity for extended periods, no matter how carefully we attend to light levels, temperature or watering (it may be tempting to increase watering, but this is the worst thing to do since the palm is drawing very little water from the soil because its stomata are shut). So what can be done? Lowering the temperature is one way to improve humidity, because humidity is related to temperature. If you take a roomful of cold, humid air and heat it, the humidity plummets. Last winter I visited an office in New York where the RH fell to 15% (lower than any naturally-occurring humidity on earth) when it was -20°C outside. The only office plants were dead ones, and the people weren't faring too well, either.

Achieving both humidity and warmth is not quite so straightforward. Trays of water can be positioned near plants, or plants grouped so that they share their water vapour, but these methods only go a small way towards remedying the situation. Electric humidifiers do the trick. There are various types, but perhaps the best ones simply warm a tub of water so that vapour is produced. A humidistat (which controls humidity in the way a thermostat controls temperature) is also useful. The investment may seem excessive - you could buy a palm instead! - but the air feels better to humans, too, and heating bills can be reduced because humid air feels warmer than dry air (which is why a winter room can feel chilly at 20°C, whereas summer days of 20° seem warm).

Does humidity affect outdoor palms? Low humidity does not seem to be a problem outdoors in northern Europe. However, high humidity can cause difficulties for desert palms in winter. Many of the hardier palms - Brahea, Phoenix, Butia, Jubaea, Nannorrhops- take lowhumidity desert freezes in their stride but are upset by damp northern winters. As far as I can see, the only guaranteed cure is a move to Morocco...

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