Vampires, Tillandsias... Things That Go Suck In The NightBy Andrew Flower (Anwyl Bromeliads)
[This description of bromeliad biology (and in particular Crassulacean Acid Metabolism - CAM) resulted from a discussion of the pros and cons of watering Tillandsias at night. --ED]
One school of thought suggests that it does no harm to water Tillandsias at night. This gives them many hours to absorb the water before the demands of the hot sunny days begin.
Ostensibly, this "suggestion" is contrary to the oft-quoted advice to water Tillandsias in the early morning, or late afternoon provided they are dry by nightfall. So I think it is worthwhile commenting on this a bit further, since taken at face value such contrary advice can be confusing. As it happens, both pieces of advice are correct in particular circumstances.
The question of watering xeric Tillandsias involves considering two of the primary requirements of plant life: carbon dioxide and water. The CAM business relates to the different way Tillandsias (and some other plants) actively take up carbon dioxide. A "standard" plant model, like most of your terrestrial types, takes in carbon dioxide through its leaves during the daylight and uses energy from sunlight to convert the carbon dioxide into a solid form of stored energy, mainly starches. The huge problem with this process is the fact that as soon as a plant opens up the pores (stomata) in its leaves to pull in carbon dioxide from the air, during the day, water vapour rushes out through the pores. This is bad news if you are a little Tillandsia sitting on a twig with no way to replenish the lost water through your roots, and on a hot day you would lose so much moisture you would just burn up and die. So "air" Tillandsias were only able to evolve because of a different carbon dioxide-absorbing system, namely "CAM" respiration. The CAM plants do not take up carbon dioxide during the day, they wait until night. At night they lose FAR less moisture when they open their stomata, because the ability of the air to suck moisture out is much less (this is a function of the lower temperature and higher humidity - expressed as "VPD" or the vapour pressure deficit of the air - which I won't go into here... (sighs of relief...)
So think of your Tillandsia as a little night vampire, sitting there sucking carbon dioxide out of the night air. Throwing water on it at night has a similar affect to throwing sunlight on old Dracula. Because when the water-absorbing scales on the Tillandsia's leaves get wet, they flatten down and cover their stomata and the little chap is effectively suffocated.
Some people have heard (or even read) that bromeliads cannot stay wet for more than 24 hours. I think the real point is that Tillandsias cannot stay wet for 24 hours of the day on a continuing basis, ad-infinitum. Why not? Because they die of carbon dioxide starvation, for one thing. I have an airtight plastic box into which I put Tillandsias, and measure the carbon dioxide content of the air with an analyser. When you put dry CAM Tillandsias in the box, the carbon dioxide content of the air decreases during the night, as you would expect from the theory that they are absorbing carbon dioxide during the night. And during the day, the carbon dioxide content of the air in the container INCREASES, which you may not expect. What this indicates to me is that the Tillandsia is continually LOSING carbon dioxide, day and night, at a small rate (consistent from what you would expect from osmosis as the concentration of carbon dioxide inside the plant's cells is greater than it is in the atmosphere). So the Tillandsia as a living form is continually "leaking" carbon dioxide, but during the night while it is actively taking up carbon dioxide, considerably more carbon dioxide comes in than goes out. In one experiment, I soaked a number of Tillandsias in water for a couple of hours so they were well saturated, then put them in the plastic airtight box. During the night, the carbon dioxide content of the air INCREASED, during the day the carbon dioxide content in the air again increased, and so on for several days. So apparently the plants were just losing carbon dioxide continuously, and if left in this state they would presumably reach a point when the carbon dioxide concentration inside the plant cells was the same as the concentration in the air - not enough to sustain the plant.
Getting back to cultivation. The standard advice I give is to water your Tillandsias in the early morning, allowing them a couple of hours at least to absorb water before the air temperatures start rising and drying the plants. This avoids the problems you will get if the plants are wet during the night when there are lower temperatures and higher humidity, hence little drying effect. There are going to be times when you have hot night temperatures and lower relative humidity (hence higher VPD) and then you can water the plants at night knowing they will have some time to stay wet, but there is still sufficient drying capacity in the air to get them dry.
This story also tends to accommodate the situation others observed in nature where xeric Tillandsias receive water from a night mist that apparently comes down mid-way through the night. This early-morning wetting is going to still leave a period during the night hours when the plants are dry, thereby able to entrap carbon dioxide.
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