Water is absorbed by plants throught the root system and taken up through the stem into the leaves where it plays a vital role in photosynthesis - the process by which plants collect energy from the sun. It's a little like our own circulatory system and, in that respect, water is the life-blood of every plant. Just like us, plants can afford to loose some of that life-blood but too much and it means certain death.
Water is not just important for photosynthesis, it also carries water-soluble nutrients from the soil and distributes them throughout the plant. So plants can't just circulate the same water around and around like we do with our blood. They need to constantly pull fresh supplies up from the ground, taking in the fresh water and getting rid of the old stuff. They get rid of water by 'sweating' through stomata in the skin of the plant. In the same way that we sweat through our pores, plants sweat, or transpire through the stomata. Humans perspire - plants transpire.
But plants have been around for a long time. They're pretty wise to the fact that they'll have to go through periods of drought and they've evolved ways of coping with this. Learn to recognise how a plant signals it's in trouble and you can make sure you give it a helping hand when it needs it.
When a plant is deprived of water
Much of the water loss in plants happens through the leaves - this is where they have the biggest surface area. During periods of drought, the plant will first of all close the stomata to reduce water loss through transipration. Often its leaves will curl up, creating a smaller surface area and sometimes catching any water that is lost on the roof of the curled up leaves. Without water to keep them upright, the stems will begin to collapse and the whole plant will droop.After a while, it will start shutting down so that energy is conserved. In other words, it will stop growing for a time. If that doesn't work and no more rain falls, the plant will begin the process of self-sacrifice.
The plant will start by sacrificing any energy-intensive operations so flowers and fruits will be the first to go. Next with the stomata closed and less energy to be had from photosynthesis, the plant can no longer maintain its green leaves and, starting from the tip of the leaves, they will gradually turn brown. The plant can afford to sacrifice some of its leaves if it means the whole plant can survive so the leaves may suddenly fall off. Eventually, the plant begins to die. Starting from the top down, its structure breaks down and, finally the roots are too dry and weak to take up any water at all.
This all sounds very dramatic but the truth is, in temperate climates like ours, plants growing outside rarely suffer long enough periods of drought such that all its survival mechanisms fail. When the soil dries out on the surface, there is often stored water to be had deeper down and plant roots can draw on this for a while.
For container plants however, its a different story. Confined to their plastic or clay cages, container plants rely entirely on us (their jailers) to supply them with adequate water. Beware though, symptoms of water loss in container plants are very similar to symptoms of over-watering.
In the garden watering regularly is the key
We know our garden plants can probably survive even quite long periods of drought, albeit under considerable stress. But why should they suffer any stress at all when they have us? You can give your plants a stress-free summer by adopting a regular watering regime. You'll be rewarded with better flowers and fruit and a much more attractive garden all round. Pay special attention to fruits - tomatoes, peppers, etc. and also vegetables like courgettes, peas, beans, etc. In periods of drought, fruits and seedpods will dry out and shrink. When you water them again, they will swell and the skin will split so its important to water these kinds of plant regularly.
Mulching in summer with compost, bark chips, etc. will also reduce water loss but only mulch over soil that is well-watered as you can't lock in moisture if it's not there in the first place.
Don't waste water
Water at the best time of day and don't over-water and you'll be doing the environment a favour as well as your plants. Here's some great tips from Nick Coumbe of Grown for You
Water before the hottest part of the day and again in the evening if necessary.
The temperature of the soil around a plantís roots is vital for good growth. Water stops the temperature rising too quickly, and also cools the plant as it evaporates. So watering early helps keep the roots moist during the hottest part of the day, and watering late ensures they have plenty of at night when they actually use most of their water.
Donít let them get too dry, but donít saturate them every day.
Compost, and to some extent soil, is like a sponge. If it dries out too much it becomes harder to make it wet again without soaking it. If it gets too wet, the excess drains off leaving the ground damp. So the aim is to water efficiently, filling up the top level of soil or compost so it drains down into the lower levels, but not over watering so it simply runs off and is wasted.
With pots use 10 per cent of the pot or root-ball volume each time you water.
A simple rule, but effective. A 25 litre pot would need 2.5 litres of water and so on. As with plants in the ground, donít wait too long to water pots, and do so when theyíve begun to dry slightly, but before the compost starts to shrink back from the edge of the pot.
Negelected fern photo by Dano via Flickr under Creative Commons.