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Gardener's Botany - How Plants Use Water

Water is essential to plant life. Even plants that live in arid deserts need moisture and have developed sophisticated techniques to retain moisture within the plant.

Plants pull water from the soil into the plant; the water is in the form of a 'soup' that contains the nutrients that the plants use to grow.

The plant draws the water from the roots and, in a process called 'mass flow', it moves up through the plant to the leaves.

The water in the plant is lost through a process called transpiration. Transpiration occurs when the water reaches the leaves and escapes through tiny holes called stomata.

The amount of water each plant loses varies; some plants have developed ways to reduce water loss while others, such as tropical plants, typically lose great quantities of water. Most plants, however, lose over 95% of the water the take up through transpiration.

Dry soil, hot air, low humidity, and wind can all cause stomata to close. Plants that have adapted to hot dry climates open their stomata in the cool of the evening.

Transpiration is essential for plants, not only does it allow the plant to pull nutrients from the soil through the plant, but the evaporating water enables the plant to cool its leaves. Think, it always feels cool and damp when you are in the bush.

Water in the Vegetable Garden
Plants take up their food in solution, dissolved in water, and unless there is enough water in the soil then they suffer from nutrient deficiencies, however good and fertile the soil.

Regular moisture allows the plant to take up the nutrients that it needs. If soil moisture levels are low then watering increases the yield in the vegetable garden.

Balance is the key
Watering must be even and regular. A good watering regime can prevent bolting or running to seed, and problems such as blossom-end rot on tomatoes result from erratic watering.

Some plants, such as melons and courgettes, plenty of water is a must for the fruits to swell to their full size. Tomatoes and peppers, on the other hand, can suffer nutrient deficiency and develop problems such as blossom end rot.

Erratic watering, from the hose or the heavens, can cause some crops (e.g. cabbages) to split, a result of a sudden spurt of growth when a dry period is followed by moisture.

Excessive moisture has its own set of problems and does not necessarily led to increased yields. Damage to the root system can easily follow over-watering.

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Plants in arid conditions have developed to retain moisture
Plants in arid conditions have developed to retain moisture

Transpiration moves water and nutrients up through the plant
Transpiration moves water and nutrients up through the plant

It feels cool and damp in the bush
It feels cool and damp in the bush

Erratic watering can cause problems
Erratic watering causes problems

Watering must be even and regular

Watering must be even and regular

Tomatoes are ThirstyTomatoes are Thirsty
The average tomato plant will transpire about 136 litres of water in a growing season

More Garden botanyMore Garden Botany

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Last revised 21 Jan '02