How plants absorb water
Water is as vital to plants as it is to us. Here you can learn more about their amazing ability to absorb water, what happens when there is too much or too little, and how we as gardeners can help to quench their thirst.
Quick facts
- Plants need water to transport nutrients from the soil, make their own food by photosynthesis and stand up straight. Too much and too little water can hinder growth
- Roots take in water from the soil by the process of osmosis and it is drawn upwards through a plant inside pipe-like xylem vessels
- Different soil types have different moisture-holding capacities. Getting to know your soil helps you grow healthy, happy plants
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How water gets into plants
Plants absorb water from the soil by a process called osmosis – the natural movement of water molecules from an area of high concentration, across a semi-permeable, sieve-like membrane, to an area of low concentration. When the soil is moist, it contains a higher concentration of water molecules than the cells inside a root, so water moves from the soil, through the root’s outer membrane, and into root cells.
To maximise the amount of water they can drink, most plants have small, fibrous roots covered in thousands of tiny hairs, creating a huge surface area for absorbing water.
Thousands of tiny hairs give root tips a fuzzy appearance
Taking care of roots and getting them connected with moisture in the soil at planting time will help your plants establish. Here's how:
Keep an eye on new plants during their first spring and summer, when they need to get their roots into the surrounding soil to search for water. In prolonged spells of drought, look out for trees and shrubs in their second
To check moisture levels in the ground, scrape away a few inches of soil with a trowel and sense for moisture with your finger. If it feels dry, then water well.
How water travels up a plant
As water moves from the soil into root hair cells by osmosis, pressure inside these cells builds. Eventually, the water is squeezed out into the surrounding space and moves by osmosis into the next root cell along.
Once it has moved from cell-to-cell across the root tissue, it enters xylem vessels at the centre of the root. Xylem vessels are like a pipe network, delivering sap (water and diluted mineral nutrients) around a plant.
The movement of water up through a plant, against gravity, is mostly due to a drawing force known as transpirational pull, created by water evaporating from leaf pores. As water is cohesive (its molecules are attracted to each other and cling together) and adhesive (sticking to cell and vessel walls) it moves up through the plant as a continuous column.
Water moves as a continuous column up through a plant owing to the pull created by transpiration
How plants absorb water in different soil types
Different types of soil (such as heavy clay or sandy loam) have different water-holding capacities, depending on their structure and texture. The texture – the proportions of sand, clay and silt – as well as the amount of organic matter dictate the size and number of pores a soil contains.
Soil pores are the gaps between soil particles in which water or air is held. Their size affects how well a soil drains. If you have coarse sandy soil, it will contain large pores that let water drain away quickly. If you have fine silty soil, the pores are small, and water clings by surface tension to the soil particles, draining away slowly.
Take care not to compact your soil, damaging the valuable pores, so avoid walking on your veg beds and flower borders as much as possible. Fine particled clay soil especially can have its structure easily damaged if you walk on it in wet weather, effectively squashing out the air from its pores.
You can tell if you have a clay soil if it rolls into a ball or sausage. A soil with a high proportion of silt or sand particles will instead fall apart
To improve your soil's structure, add organic matter such as home-made compost or leafmould. Whether it's dug in as a soil improver or spread on the surface as mulch, it acts like a sponge, holding water in the soil for plants to use. Digging in organic matter also aerates the soil, aiding drainage. It's therefore useful in soils that are prone to drying out AND in soils prone to being excessively wet.
For more advice, see our handy guide on looking after your soil.
What happens when there is too much or too little water?
Dry spells interrupt the channel of water moving up through a plant, halting the delivery of vital nutrients and other molecules to cells. As a result, water stressed plants can show slow, stunted growth, poor or no flowers, undersized fruit, premature leaf drop and an increase in pest and disease problems.
However, wilting is usually the first symptom you’ll see when a plant is dehydrated. There are two different types of wilting to look out for:
Temporary wilting is commonly seen on warm, windy days when the rate of transpiration is high and stomata are fully open. There's moisture available in the soil, but plants are losing water from their leaves faster than they can take it up. Luckily overnight, with leaf pores closed and conditions cooler, plants are able to recover.
Permanent wilting occurs when there isn’t enough water in the soil for a plant’s roots to access. As cells dry out and shrink, they lose their firmness and go floppy as a result. In this case, recovery is only possible if water is added, either by rainfall or by you watering. Left too long without water and a plant will die.
Some plants are more vulnerable to damaging water stress. So, during dry spells, keep an eye on:
- Seedlings and young plants
- Newly planted specimens that aren’t yet well rooted-in
- Plants that are starting to flower or fruit
- Mature, ‘instant impact’ specimens that often have a small rootball compared to the size of their crown
- Plants in containers, as their roots are confined to a small amount of compost
Suprisingly, plants can also wilt in waterlogged soils. In these conditions, water has completely replaced oxygen in the soil’s pores, meaning roots no longer have the oxygen they need to turn sugars into energy (respire). With respiration hindered, other vital functions are restricted and water uptake into the plant is interrupted.
This has the same effect as permanent wilting due to a lack of soil moisture, and recovery is only possible if oxygen is returned to the rootzone. Other symptoms of waterlogging include yellowing, leaf drop, dead patches along the middle of leaves and rotten black roots when you dig them up.
In situations where the ground is regularly waterlogged by flooding or a high water table, it's best to choose plants that will cope with these conditions. Those that originate in boggy places are adapted to grow in the low oxygen levels of saturated soil, so are more likely to thrive. See our guide to moisture-loving plants and our guide to bog gardens.
What about fluctuating moisture levels?
Soils that fluctuate seasonally between wet and dry are particularly challenging for plants. A clay soil is often wet in winter but dries out to the point of cracking in summer. Very few plants are adapted to cope with these conditions, making it hard to choose plants that will thrive.
Improving the soil by digging in organic matter, installing irrigation and drainage systems and planting on a mound or in raised beds, can help widen the range of plants to choose from. For more ideas see our guide to plants for wet and dry soils.
Growing in a raised bed helps overcome the difficulties of soils that fluctuate between wet and dry
It's easy to think that poor flowering is the result of an immediate problem, but irregular dry spells, even in a previous season, can affect your plant's flowering display.
Spring-flowering shrubs like camellias and rhododendrons start their flower-making process the previous year, and poor flowering in spring can often be traced back to a dry summer the year before. Bulbs too can exhibit blindness if it's been dry after flowering the previous year.
A previous dry summer has led to poor flowering on this rhododendron
Fluctuating moisture levels during a growing season can affect fruiting. Tomatoes are especially sensitive to damage due to irregular watering. Their fruits swell suddenly when lots of water is applied, causing the skin to crack and split. This shortens storage life and increases the chance of mould.
Seasonal water shortage can also affect nutrient absorption. Although there may be enough nutrients in the soil, a lack of water means a plant can't take them up. Blossom end rot in tomatoes and bitter pit in apples are examples of water-stressed calcium shortage.
Irregular watering causes tomatoes to crack and split
Bitter pit in apples is due to water-stressed calcium deficiency
Your next steps
Now you know more about how plants absorb water, put this into practice to help your plants thrive:
- Check what type of soil you have before planting up a new garden. This helps you better understand how well it holds and drains water and how you can best manage it. See our guide to assessing your soil type and information on the RHS Soil analysis service
- Take care when planting to ensure good contact between roots and the soil, as this helps plants establish quickly. See our guide to planting perennials and our guide to planting trees and shrubs
- Water container plants regularly and thoroughly during dry spells, as their restricted root space makes them prone to water stress. See our guide to container gardening
- Improve your soil by digging in or mulching with organic matter like well-rotted manure or home-made compost. This helps retain moisture in dry soils and improve drainage in very wet ones. See our guide to using organic matter
- Try to water fruit crops consistently so they swell at an even rate, reducing the risk of splitting and ensuring soil nutrients remain readily available to roots. See our guide to watering
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