Evapotranspiration question

[rbarata]

Can it be compensated by watering, or by other words, can watering more frequently compensate a low air relative humidity?

Thanks

[estación seca]

Sometimes. It will depend on how much water the leaves need to stay alive in the ambient humidity. If the root system can provide that amount of water, things will be fine. If not, the plant will wilt.

[Roberta]

Choice of medium makes a big difference, too. Plants that need to stay damp should be potted in smaller bark. If they need to dry out between waterings, larger bark. As an example, if you put an Oncidium in a plastic pot in small bark and a Cattleya in a clay pot or basket with large bark, you can water them both every two days or so (in hot dry weather, even every day) and both will be happy... the Oncidium will stay damp and the Cattleya will dry out. You can make yourself crazy trying to give each one what it wants, or you can make the decision once every two or three years when you repot and give everybody the same regular watering (while the correct environment is maintained for each) Make it easy on yourself...

[AnonYMouse]

Originally Posted by rbarata

Can it be compensated by watering, or by other words, can watering more frequently compensate a low air relative humidity?

Thanks

You're talking about transpiration through the leaves? I don't know for fact but I would think that it would be a fixed rate, the process can only go so fast. You might be able to make the process more efficient though, don't know exactly how.

[Fernando]

With low humidity, the stomata close and the leaves cannot breath. The plant is forced then to change its metabolism, as most orchids can and breath exclusively during nights when humidity rises. As easily to understand, thick leaved, succulent orchid species have an advantage. Thin leaved species won't like it at all.
So, the answer is no.
But you can help your plant survive enhancing other parameters, like a bit more of shadow (for there still are water losses with closed stomata), a bit less of air movement (for the same reason). As these both parameters could help rise humidity as well (or extending the time with higher humidity after sunrise) you can modulate a bit.

Transpiration is a very dependable value. It depends on tº, on humidity, on leaf tº, on leaf extension, on number of stomata per given extension, etc. On one side transpiration is extremely necessary because it keeps the circuit of water moving and thus the transport of all nutrient the plant needs to take from outside; on the other side it is a tremendous water loss which can be a problem from the moment there is no new supply. As wilting must be avoided at any rate, plants close their stomata at a certain specific humidity level.

[AnonYMouse]

Interesting.

Does that mean the plant protects itself during the low humidity of day? (And can the plant be fooled with no light days?)

[Fernando]

No.
Without light, assimilation collapses. Assimilation is the process which provides the plant with energy. This process needs turgent cells to work at normal rates. The length of time a plant needs good light to 'work' well is specific. Shortening this time is a form of starving.

A plant can slow down. But the margin between water needs on one side and the necessity to evapotranspirate without having to prevent water losses (and thus closing stomata and reducing efficiency of the whole process) is very little, even more so when humidity falls.

[AnonYMouse]

Originally Posted by Fernando

No.
Without light, assimilation collapses. Assimilation is the process which provides the plant with energy. This process needs turgent cells to work at normal rates. The length of time a plant needs good light to 'work' well is specific. Shortening this time is a form of starving.

A plant can slow down. But the margin between water needs on one side and the necessity to evapotranspirate without having to prevent water losses (and thus closing stomata and reducing efficiency of the whole process) is very little, even more so when humidity falls.

Of course!

[Ray]

Extra watering to compensate for low humidity may, in some situations, be counter-productive.

If the plant has plenty of water at the root system, hormones signal the stomata to open, allowing residual oxygen to leave and more carbon dioxide to enter, favoring growth through the assimilation of carbon. That's how plentiful, frequent watering drives growth.

The downside is that the open stomata can allow moisture to leave the plant faster.

[rbarata]

The downside is that the open stomata can allow moisture to leave the plant faster.

This sugests that water absortion by the roots and lost by transpiration, for each plant have fixed rates.

Choice of medium makes a big difference, too. Plants that need to stay damp should be potted in smaller bark. If they need to dry out between waterings, larger bark. As an example, if you put an Oncidium in a plastic pot in small bark and a Cattleya in a clay pot or basket with large bark, you can water them both every two days or so (in hot dry weather, even every day) and both will be happy... the Oncidium will stay damp and the Cattleya will dry out. You can make yourself crazy trying to give each one what it wants, or you can make the decision once every two or three years when you repot and give everybody the same regular watering (while the correct environment is maintained for each) Make it easy on yourself...

This is aomewhat related with my previous question in this thread.