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
on this article:
Click here to read them or to add your own.
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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20-01-21 - 12:27GMT
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