Can plants 'drink' through their leaves?

August 4, 2008 7:16 AM

I always assumed that plants draw all their water/nutrients from the soil, getting only CO2, etc. through their leaves. My girlfriend is adamant that 'leaves need watering because they absorb moisture too'.

When I arrived at my her place this evening, she was just starting her evening watering in the garden. She put the hose on 'spray' and proceeded to give a good soaking to the upper parts of a number of the plants, and it seemed to me that not much water was getting into the soil around the plant. When I questioned this she said that it was OK, and that this water she was spraying on them would be absorbed by the leaves.

I'm no gardener at all, and she's good at it, so I held my tongue, but is she right? Do leaves drink? (The specific plant we were discussing was a large cucumber vine, draped over a trellis thing ...)

posted by woodblock100 to home & garden (18 comments total)

Related: Foliar feeding
posted by OmieWise at 7:21 AM on August 4, 2008 [1 favorite has favorites]

I've always understood that you shouldn't water the leaves of garden plants like cucumbers because they can develop diseases like powdery mildew. I've heard the same about tomatoes, although I think it was a different disease. Also, watering at night makes these problems more likely because the leaves stay wet longer and there's no sun to discourage the fungus or mold or whatever.

So, I don't know if that's just an old wives tale or not, but it's certainly a commonly accepted thing to only water vegetable plants at the soil.
posted by cabingirl at 7:34 AM on August 4, 2008

Not really, but some water could be absorbed through the stomata (pores in the leaf for gas exchange), this would be a tiny amount compared to what is taken up through the roots.

There could be a cooling effect of 'watering the leaves' could lower the leaf temperature to a level that is more conducive to photosynthesis.

On the other hand, moisture on the leaves could promote fungal growth.

-Chris
posted by buttercup at 7:34 AM on August 4, 2008

I always assumed that people sprayed indoor plants to get the dust off them.

If plants liked having wet leaves, wouldn't they have evolved to keep more of the rain on them? Yet leaves are mostly flat and smooth. Some plants, for example Ficus elastica, seem to have evolved a waxy, glossy leaf cuticle and a leaf shape that help evacuate the rain from the leaf towards the roots quickly and easily.
posted by stereo at 7:56 AM on August 4, 2008

Leaves don't really drink very much. You can feed/fertilize leaves and they will take it in, and that's recommended by some schools of thought (the Dirt Doctor, for example, is big on foliar feeding), but actual hydration is taken by the roots of most plants. Underwatered roots stay close to the surface where predatory critters live, don't get access to the nutrition available deeper down in the soil, and the plants topple more easily.

In my part of the world (TX), wet leaves get fungal, mildewy, and/or cook in the heat. Other parts of the country can get away with it more, and obviously it rains sometimes and leaves get wet, but it's definitely current conventional wisdom to water the roots deeply and leave the leaves alone as much as possible. Cucumbers aren't going to pollinate as well if their flowers are getting wet all the time, also, and the fruits tend to deform if they get too wet as they grow.

Spray watering is kind of a water waste, too. For what little the plants take in through the leaves, more is going to evaporate.
posted by Lyn Never at 8:03 AM on August 4, 2008

I was wrong about waxy leaves.
posted by stereo at 8:06 AM on August 4, 2008

I've only ever sprayed the leaves on really scorching days to keep them from drying up in the sun. The volume of water on the surface of the plant is pretty small compared to what you can give it through the soil.
posted by stopgap at 8:26 AM on August 4, 2008

Leaves don't drink much, but they do dry out, so watering them on hot days is advisable.

In any case, nothing is lost because the water that isn't absorbed by the leaves falls to the ground and is available to the roots anyway.
posted by crickets at 9:31 AM on August 4, 2008

In my college plant phys class, we were taught that plants absorb water through root hairs not leaves.

From Plant Physiology (Taiz and Zeiger): “Although uptake through the stomatal pore could provide a pathway into the leaf, the architecture of the pore…largely prevents liquid penetration.”

(As an aside - foliar applied nutrients are thought to be absorbed through the leaf cuticle. This is most effective when nutrients are applied with a surfactant chemical.)

Finally justification for holding on to my plant phys book!
posted by fiore at 10:06 AM on August 4, 2008 [1 favorite has favorites]

Leaves don't drink much, but they do dry out, so watering them on hot days is advisable.

Don't water the leaves and then have them stand in the scorching sun though. Those drops that are left are nature's own little lenses.
posted by bjrn at 10:22 AM on August 4, 2008

OmieWise's link has a particularly fascinating discussion of absorption of nutrients by the leaves of tomatoes (objects of worship in this household):

But in some cases, with tomatoes, for example, it is believed that foliar feeding during flower set causes a dramatic increase in fruit production...

Dr. H.B. Tukey, renowned plant researcher and head of the Michigan State University (MSU) Department of Horticulture in the 1950s, working with research colleague S.H. Wittwer at MSU, first proved conclusively that foliar feeding of plant nutrients really works. Researching possible peaceful uses of atomic energy in agriculture, they used radioactive phosphorus and radiopotassium to spray plants, then measured with a Geiger counter the absorption, movement, and utilization of these and other nutrients within plants.

Potassium and phosphorous are among the most important nutrients found in bird guano.

Birds are considered the primary modes of dispersal of wild tomatoes-- the red color attracts the birds, they gobble the tomatoes whole ("essentially all the wild species of South America have fruit no bigger than small cherry tomatoes.") and poop out some of the seeds undamaged.

So it looks as if tomatoes are monitoring their leaves at the time of fruit set for the presence of bird droppings, and if they detect them, the plants respond by dramatically increasing the amount of fruit they produce, thereby enhancing their reproductive success.

How clever of them! I wonder how many other plants which have evolved to take advantage of bird dispersal would respond to these nutrients the way tomatoes do.
posted by jamjam at 10:48 AM on August 4, 2008 [1 favorite has favorites]

People absorb water through their skin when they stay in the bath too long (prune-y skin). It doesn't mean that it's the best way to take in water.

Roots are designed to take water up. Each root is covered in billions of tiny hairs that are designed to aid water uptake. The water travels up through the plant, carrying minerals, etc, to the growing parts. So that the leaves don't swell up, the water evaporates from them.

For proof, chop the top off a plant, and water just the leaves. See how long it lasts in hot sunshine.
posted by Solomon at 11:02 AM on August 4, 2008

The stomata are on the underside of the leaves. They stay closed during the heat of the day. It's true that you can "water" a plant partially by spraying water on the underside of the leaves, but those stomata function by absorbing moisture from the air. The majority of water absorption takes place in the tiny feeder roots on the periphery of the root system.

It's better, in most climates, to water in the early morning rather than in the evening, not only for more effective watering, but also to help prevent fungal growth.

There are large greenhouses that water like your gf does, but they also use products to prevent fungus. It's true that a plant that lives in a moist environment needs less water in the soil. That level of moisture isn't usually easy to achieve in a home gardening situation.
posted by reflecked at 11:02 AM on August 4, 2008

Not only do plants take in their water via roots, but they take up nutrition from the soil through the water in it. No water in the soil, no nutrition to the plant. Additionally, water uptake from roots is required for the plant to function properly. One example: Calcium moves through plants via transpiration of water as a gas through the stomata in the leaves. Insufficient soil water, insufficient movement of calcium through the plant.

Stomata are lined with a waxy substance to prevent water intake. Foliar feeding is not the same as water intake. Nutrients applied through foliar feed are absorbed through leaf pores, and water needs are actually increased by foliar feeding as it stimulates chemical processes in the leaf.

In short, your girlfriend is not helping her plants. Shallow watering makes plants surface root, increases the chances of drought stress, and limits activity of soil microorganisms.
posted by oneirodynia at 5:55 PM on August 4, 2008

Thanks for all the various inputs and links, people.

I'm certainly not going to go back there and wave some of these opinions at her - "You see, it's not doing any good!" - that wasn't the point of asking. And her garden is doing so well, it would be foolish to criticize. I was simply trying to get this straight for myself ...

(We're in Tokyo, by the way, extremely damp all through the summer months, but she seems to be having no trouble with the fungus, etc. stuff that people have mentioned.)

Now as to the scrawny lawn grass common here in Japan ... that's a different story ... (maybe another question later!)
posted by woodblock100 at 7:23 PM on August 4, 2008

You could drink milk through your nose but I wouldn't advise it...
The underside of leaves is for breathing, I do periodically rinse them to make sure the pores aren't all clogged up with dust. The topside of leaves is for making water fall to the roots. (I imagine lilies work in reverse?) Would they not be cup shaped otherwise?? (Or in cone shape like bromeiliads? Ya-know?)

If she's watering leaves - whatever, the plant doesn't care. It has ways of making water from the sky work. But no, wet leaves aren't generally all that helpful. Roots = water, nutrients and staying upright. Leaves = sunlight and breathing. It doesn't really matter but your plants are definitely happier the more you understand them...
posted by mu~ha~ha~ha~har at 7:57 PM on August 4, 2008

I could be wrong, but I think the major function of stomata on the underside of a leaf is to release water. Nutrients move through a plant by the movement of water. Since a plant has no system of arteries, lymphnodes and veins, this movement happens cell by cell. A vacuum is created in a cell further away from the root system of the plant and the water in the cell next to it moves in through osmosis. This vacuum is ultimately created by evaporation from the stomata. The waxy leaves of plants in hot, dry climates are designed to slow this process down. This system of water movement due to evaporation also creates water tension in the plant, holding the plant rigid.

It's been a long time since I took biology but this is how I remember it goin' down.
posted by Foam Pants at 12:10 AM on August 5, 2008

You're right, Foam Pants, the stomata do "release" water, or take it in, if needed. Look at the plants that live in the high canopy of a rainforest, you'll see major stomata action. In AND out.

I very much like the point that oneirodynia made differentiating between foliar feeding and watering a plant's leaves. Well said.
posted by reflecked at 1:52 PM on August 5, 2008