Why do plants need water?

PimlicoDan
Published on May 25th 2019
17
A close up of a green plant
Benjamin Franklin once famously said that there are only two certainties in life – death and taxes. He clearly wasn’t a gardener, otherwise he would have identified a third: watering.
Novice or pro, there’s no escaping it, watering is essential and probably the most (in)famous aspect of gardening, particularly indoors.
So why do plants put us through it? Why exactly do they need water?
A close up of a green plant
Healthy growth, such as that shown on this cucumber plant, requires a lot of water during the hot summer months.

Why do plants need water?

It’s an easy fact to forget, but life on earth needs water to survive, whether it’s you, me, your pet dachshund or the fiddle-leaf fig in your hallway. The difference is that plants are about 90% water and therefore need more of it compared to most animals.
When a plant is well-watered, its leaves and stems are in a state called turgor, where the cells are at full pressure. The cucumber, pictured above, is in this state and this is how we would normally see most plants when they’re growing in good conditions.
A green plant in a garden
This plant is showing the signs of wilting, limp leaves hanging down and a dull colour. Some pests and diseases can also cause wilting.
During times of drought, when there is insufficient water, then the cells and overall ‘skeleton’ of the plant are unable to support themselves. This is when wilting occurs.
In most cases, once the plant receives a drink it pumps back up to its full structure and strength again. If a plant repeatedly dries-out it will 'shrink' a little, perhaps dropping leaves and slowing growth to conserve moisture.
A fountain in front of a fire hydrant
Plants lose water at a rate 200 times greater than the intake of CO2, meaning it takes around 200 litres of water for 1kg of dry matter.

Fed and watered

Unlike animals, plants make their own food through a process called photosynthesis.
Water is essential to this, along with sunlight and carbon dioxide which together combine to form glucose for plant growth. Oxygen is also created as a by-product – albeit very useful one for us!
As the plant must open it pores (stomata) to obtain carbon dioxide, then the vast majority of the water a plant takes in is actually lost through its leaves, rather than used in the photosynthetic process.
A close up of a logo
A diagram showing photosynthesis. Plants transpire for 3 reasons: to take in CO2, to cool down in hot weather and to create a flow of nutrients up through the plant from root to growing tip.

From roots to shoots

As water transpires through the leaves, then the plant must draw up yet more from the ground and this is where the roots come in.
Deep down in the earth, the roots probe for moisture. As plants are passive, they obtain the water they need from the growing medium through a process called osmosis.
Without getting too scientific, this is where the water moves from an area of high concentration (the soil) to an area of lower concentration (the plant’s roots).
It may seem a little odd as, of course, plants are 90% water already, however, due to the action of transpiration up in the leaves this means that water continues to be drawn into the plant.
A close up of a plant
The fine hairs on roots actually create a much larger surface area, allowing greater absorption without having to take up more space.

Heat and humidity

Plants that grow in a humid environment will lose water less rapidly through transpiration as the air is already saturated with moisture. This is why jungle plants, like the Swiss cheese plant and many figs carry large leaves.
Aquatic and marginal plants, like giant rhubarb and lotus, also have large leaves as water intake is rarely an issue in their habitats.
In dry ecosystems, like deserts, leaves tend to be much smaller or, as in cacti, non-existent. This reduces rapid water loss, useful when you don’t know when your next drink is coming.
You’ll also see other plants, often Mediterranean or South African, with furry, glaucous (blueish in colour) or waxy leaves, which is another way of preventing water loss and the effects of continuous burning sunlight.
A close up of a green plant in a garden
The leave of this mullein (Verbascum spp.) are covered in silvery hairs which reflect sunlight, keep the plant cooler and reduce water loss.

Overwatering vs. underwatering

Generally, we associate watering most closely with houseplants as in this environment they are completely at our mercy.
Plants in hot, dry rooms, particularly with central-heating, will dry out far quicker than those in a cooler room or one with higher humidity levels.
Terracotta pots dry out quicker than plastic ones and a large plant in a small pot will need more water than if it were the other way around. It’s difficult to overwater a root-bound plant, yet a small plant swamped in giant container can be overwatered very easily.
A vase filled with flowers sitting on a table
Most plants will recover from a day or two of dryness, it often depends on the species. This one looks like their owner went on holiday!
A really cruel twist with houseplants is that the effects of overwatering and underwatering are very similar. In both cases the plant is not getting enough water, in the first case due to drought and in the second because the roots have rotted off and the plant can no longer drink.
Always err on the side of caution and keep a plant slightly drier rather than too wet as it’s easier to remedy – saving an overwatered plant is like getting toothpaste back in the tube.

Different water types

Plants can be fussy about the type of water they receive.
Acid-loving plants, like rhododendrons, heathers and gardenias will not tolerate ‘hard’ water with high mineral content and pH. Similarly, carnivorous plants only like rain water or special reverse-osmosis water if they are to thrive.
In winter caution should be used with some houseplants not to use icy cold water – Poinsettias I’m talking about you!
A close up of a plant
The Venus fly trap may not be a fussy eater, but it is a picky drinker. This is because it's wild habitat is nutrient poor acidic marshland and it finds the chemicals and higher pH in tap water disagreeable.

Weird ways to obtain water

Some plants don’t have roots in the conventional sense, such as many tree-growing epiphytic orchids, whose roots act, instead, like a sponge after each rainfall.
Air plants absorb through tiny scales on their leaves, which require misting in the home rather than a watering can and other bromeliads keep a well or ‘urn’ in their centre topped up with rain water.
Mangroves tree have adapted to grow in saltwater, sending up roots called pneumatophores to help breath in the difficult conditions.
The Welwitschia, of the Namib desert, is so drought tolerant that it can exist without rainfall at all, living off the tiny droplets of moisture from morning mists and living up to a millennia in the process!
Tillandsia, airplant, needs water like all house plants
The tiny silver scales on this air plant help it to absorb moisture directly without the need for roots.

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