Only a guess, but the folding of the banana leaf would help to reduce transpiration (the water the leaf is giving off), as the surface area of the leaf that is fully exposed to moving air currents is reduced. PS. I've noticed this too--especially on really hot days.

Actually, further analysis of the leaf folding does show reduction in surface area exposed to the Sun. It's very simple. You can't get direct Sunlight on both halves of a folded leaf. In the shadows, the leaves unfold and are fully exposed.

I don't think transpiration is an issue as transpiration is part of the plants metabollic process, not like human sweat because of heat. Plants don't use water to cool themselves. Water transports nutrients and supports the basic physiology of the plant. You can make your bananas really hot in shade and the leaves are likely not going to fold. It's photonastic, not thermonastic behavior.

But if its just a question of sunlight (photonastic), that doesn't explain why they fold up more on a hot day than a cooler one. Sounds like an interesting experiment.

Interesting! I call it wilting-from-lack-of-water, when my potted Bananas fold. After watering, even if left in full sun, they perk right up.

Is it my imagination, or does the soil in the first photo appear dry?

Lisa

Of course, the surface of the soil is dry in the first photo. But deeper down, it must be wet.

I am not certain if temperature can cause the leaves to droop, but the temps on my patio are typically very hot because of the concrete and stay hot even after shade is introduced. So I tend to discount temperature as playing a role.

I'll need time to experiment to see if watering will perk up the leaves if they are left in sunlight.

My patio is dark gray stamped concrete. It gets so hot that we "dance" across it. The potted plants are on pot feet, but are still exposed to extreme sun and heat.

Well-watered plants of all kinds do fine on it. Including Cacao and Breadfruit.

I can tell by wilting/folding when the measly day's rain has not been enough. Flowering potted plants wilt horribly, but bounce right back within an hour of a fresh, cool water drink. Bananas seem a bit tougher, they can recover overnight, just using the overnight dew. Most of mine are in the ground now, but I've had up to 72 in containers at times.

Lisa

Wilting and folding behavior are likely two different things that can be confused in a Banana because they produce similar results.

Wilting can be seen as a decline in osmotic pressure of fluids used in plant matabolism. Banana I am told, do not have stems or a trunk. The central cane of a a Banana is constituted by leaves. So the metabolic processes can't be compared to what goes on under tree bark. It goes straight from rhizome to leaves. The rhizome must then, experience severe lack of watering that would cause leaf wilting.

Folding leaf behavior however, observedly, does not rely on reduction in osmotic pressure due to water reduction and this can be proved by simply moving the Banana back into the shade; the leaves will perk up without adding water. The actual mechanism therefore relies on cellular machinery or stimulated altering of the chemo contents of the plant cells in response to light or temperature. And make no mistake, it must be localized to where the sunlight hits the leaf. Otherwise, we have to postulate a nervous system for the Banana such that the fact that sunlight is hitting the leaves is communicated to the rhizome. I don't think that can be proved even if you can claim that plants grow better while music is playing. LOL.

There are different mechanisms at work between wilting and folding leaf behavior, producing similar results. I'm not a rocket scientist when it comes to botanical or horticultural science and more specifically, for Banana, but I think the average knucklehead layman ( myself ) can see this.

Can't you?

I think that the folding leaves are due to lack of water. But it doesn't necessarily mean that the plant needs watered. Let me explain:
The plant just can't move the water from the roots to the leaves fast enough in hot sun because the plant is transpiring water out of the thin leaves so fast.
It probably is a survival mechanism, to slam those leaves together so they don't transpire so much water, and the sun doesn't hit them so bad. The reason why the plants perk back up if you move them to shade even without watering them is because then the water system of the plant can catch up. I don't agree that it has to do with light because they will fold in the shade if it's hot enough. Try leaving one in a car in the shade with the windows rolled up when it's 100 degrees outdoors, it'll fold right up.
Bananas will fold leaves in full sun even if they are soppy wet. And they will unfold as soon as the sun goes off of them some and water can catch up and get back in the plant....
Another thing, did you ever notice when a banana gets extremely wilty, way too dry, the trunk feels spongy? My daughter missed some watering and I moved one before I watered it and it felt spongy. When it was watered and revived, it felt fine again.
Sandy @ Northern Tropics Greenhouse

Here in Southern California we have a large forest fire burning along the coast in Santa Barbara County. The smoke from this month old fire has been drifting over BKF for weeks now and creates days with wierd orange light.

The thing is that the temps remain in the mid to upper 90s, but my Super Dwarf obviously doesn't respond well in terms of folding leaf behavior to the scattered light. There are no shadows and light goes where normally there should be shadows. Some of the leaves fold and some do not.

Figure that one out?!!

What about: Leave plant in full sun until leaves fold.
Water it well.
Check on it in about an hour.

Weren't you going to try that too, as stated in an earlier post (August 12th)? Watering sure works for my folded Bananas. I'm curious if yours will react the same way.

Lisa

Watering while leaves are folded in sunlight does not perk them up. Already tried that and I think I stated that above. The folding is not due to wilting. If you see Banana leaves fold due to wilting, it is an entirely different mechanism than photonastic folding.

This has been touched on above, but here goes:

It may be root stres - it seems quite a small pot for such a lot of growth. The plant may be compensating for the small root run by folding its leaves when the sunis on them - saves transpiration and saves having to try and find moisture in such a small space.

I water my Banana generously. There's no sense to any postulation that transpiration or water needs play a hand here.

The thing to realize here is that solar radiation is fierce on life, even here on Earth where life abounds. To tolerate the radiation, life on Earth shows certain adaptations, everything from the time of year that seeds germinate and seedlings put on their most growth, to initial growth under forest canopy on the chance that a tree fall will let in sunlight after maturing, to having darker green leaves with thicker structure, to leaf folding.

A plant leaf is not just the plant. It contains microbes that are involved in the plants metabolic cycle, creating chhlorophyl, etc., just like we are not just human, but contain, stomach bacteria that help our digestive cycle. The microbes in plant leaves are extremely susceptible to harm from radiation. I'm sure we all know what sun burn is, when we expose our naked flesh to the Sun for too long. We inately understand that our best chance of getting the flu, is after a long series of cloudy days in winter when the flu virus can survive.

I think we under-estimate just how hostile an environment, a sunny day can be. We, ourselves, have protection from it, hair, clothes, sun glasses, tanning via melanin production in our skins and a tendency to spend time in shade or indoors. Plants too, have evolved protections and behavior strategies to cope with a Sunny day. Folding leaf behavior is obviously that.

Go to the Canna Forum and you'll see that I just recently posted a thread there, after observing that Canna do something similar. They roll their leaves on hot days. I have included photos that that show what happens, just 2 hours after photos were taken of the rolled leaf behavior, at twilight, after the sun has waned and temps fell.

I have this year, tried to grow a lot of plants from seed and most failed to germinate. Why? Because I was trying to germinate them on my back patio, which here in BKF was hellish without shade cloth. I noted when I tried growing Cactus from seed, starting them indoors as suggested on the seed packets ... that after placing them outdoors for just one hour, they baby cacti got bleached white and died. I speculated then and folks in the 'Growing From Seed' Forum confirmed that Cacti typically germinate during Winter months when the Sun's azimuth in the sky is low and days are typically clouded over. The Cacti put on their early growth and mature under those conditions and when Spring and Summer roll around again, some are mature enough to have hardened and darkened their skins and survive the on-slaught of clear sky, high azimuth solar radiation.

It is really fascinating to see how plants adapt to the Sun and developing a knowledge of the problems plants face with the Sun is certainly beneficial to someone serious about growing all manner of plants, especially from seed.

No my friends, water ( lack of water ) and transpiration is not the catalyst here. It is the main thing that every plant must deal with to preserve those microbes so innately and symbiotically involved in their life and survival, the Sun and its fierce radiation effects.

The bananas know what they are doing.Just enjoy them.LOL

The Banana might know what it's doing, but the Gardener may not always know what they are doing.

They are treasured as attractive plants to have around, but I'm an analytic persosnality and I treasure their secrets just as much.

yeah. my rajapuri bananas leaves fold up when it gets really hot. But I dont mean around 90 degrees. where i live it goes up to 120

So should I keep my bananas in the full sun or in shade?

I keep mine in full sun because I dont have any shade trees to keep it under. If you have trees then maybe you'd want to put yours under them. I have a Zebrina and even with the folded leaves she's beautiful with the deep purple.

My banana plant leaves fold up soon after putting it outdoors this is what I think going on. While the plant in the house I would water and drag it over to a window to get some sun it would suck up water the leaves would get so heavy they would break over and I would have to cut them off . I think the plant tries to protect itself by folding the leaves up when there's too much moisture in the ground or pot.

My banana in a large pot outdoors gets only a few hours of sun now - late PM, early AM.

Still the leaves folded over in AM, before the eventual heat of day (95 F).

This seems inconsistent with wilt or sunlight (photonastic).

Leaves are folded, while plant is in shade. Plant is well watered, but roots are probably cramped in pot.

Okay I have a theory. I’m not a botanist or anything, but I’ve noticed in my banana plant that a lot of the leaves overlap eachother and therefore some do not recieve sun. However, when I place it in the sun they fold and the leaves below recieve light. So therefore I think this is to allow the leaves below to get sunlight. Again, not a botanist, so I could very well be wrong, but let me know.