Garden Watering Issues

This page will address the subject of soil moisture, plant water use, water quality and irrigation techniques in general. See the column at right for related topics on other pages.

Be sure to consult your local government authority for current information about local watering restrictions, irrigation regulations and related issues, as these can change at short notice. Links to many Qld councils are provided on this website's Regional Pages.

Special article

Stomata - an overview

As plants colonised drier terrestrial habitats, they developed water-resistant surfaces to conserve water, but these impede the movement of gasses - essential for respiration and photosynthesis - in and out of tissues.

A stomate is a microscopic pore on the surface of a leaf (or stem, petal or developing fruit) which facilitates the exchange of gases between the atmosphere and the inner tissues, but it's far more than a simple fixed opening. Because they regulate the movement of carbon dioxide, oxygen and water vapour, they're involved in several critical plant processes and therefore survival, growth and yield.

A "guard cell" lies on each side of the stomatal opening. The specialised cell wall architecture means that when the guard cells swell with water their shape alters. They pull apart, creating an aperture. When these cells lose turgidity, the pore closes.

Water loss through the stomata is a compromise the plant must make in order to photosynthesise (although this transpiration can serve other purposes). Unfortunately, pathogenic fungi and bacteria can sometimes take advantage of these openings to invade the plant, too.

The mechanism of stomatal function and how it's regulated is complex and the subject of ongoing research. Flux of potassium ions plays an important part. Stomata can respond to water supply, carbon dioxide concentration, light, hormones and the presence of pathogens. Stomata also exhibit circadian rhythm.

Typically, they open in the day and close at night, although some plants adapted to arid environments have a special way of collecting CO2 at night so that they can keep stomata closed during the day.

Another water-conserving adaptation seen in some species is the sinking of stomata below the main leaf surface and/or protection of the opening with hairs, to slow the rate of diffusion of water vapour.

Because the underside of the leaf is less susceptible to desiccation, it's not surprising that they are usually most concentrated on that surface on broadleaf plants. However, the number and placement of stomates typical of a species varies as an adaptation to its habitat. The floating leaves of some aquatic plants, for example, have stomates on the upper side. The environment in which an individual plant is grown (e.g amount of sun/shade) will also affect development of stomata.

Plants can undergo a certain amount of wilting and recover, but it should be realised that once the stomates close, there will be little photosynthesis occurring. This might be tolerable in the case of perennial landscape plants (and some desert-adapted species have special photosynthetic mechanisms to cope with this problem). However, it will be an issue when maximum yield and quality of vegetables, fruit and flowers is the goal.

Further reading:
15.2 Stomatal physiology Plants in Action, a plant physiology textbook available online at the UQ website
Gas Exchange Florida State University
Transpiration University of Hawai`i at Manoa
Stomata and pathogens A 2009 article from Plant Signal Behaviour

More Online Information

Watering your garden Queensland Department of Natural Resources and Water

Rest of Australia
Getting Started 7: Watering Association of Societies for Growing Australian Plants
Effective Watering in the Garden Includes discussion on managing irrigation with saline bore water. Association of Societies for Growing Australian Plants
Water Conservation in the Garden - Greywater general information and comments on Australian native plants. Association of Societies for Growing Australian Plants

Water and Plants Simple article discusses the various roles water plays in the plant. Arizona Cooperative Extension, Yavapai County
Water Deficits Lead to Stressed Plants Arizona Cooperative Extension, Yavapai County
The Myth of Wilting Leaves Is leaf wilt the best indicator of insufficient soil moisture? Washington State University (PDF)
Record Temps Require Rapid Response to Watering Arid-Southwestern Gardening Information, University of Arizona
Does watering burn plants? New Mexico State University
The Myth of Hot-Weather Watering Does watering plants in the hot sun scorch their leaves? Washington State University (PDF)
Different Watering Methods University of Illinois Extension
Wrong way to water trees New Mexico State University
Caring for Trees in a Dry Climate Colorado State University
Take a Hard Look at Water-Holding Compounds University of Hawaii at Manoa, USA
Hydrogels: Are They Safe? - January 14, 2009 Discusses polyacrylamide (PAM) hydrogels Arizona Cooperative Extension, Yavapai County

Waterlogging and flooding Royal Horticultural Society, UK
The Myth of Antitranspirants Do antitranspirants prevent drought stress? Washington State University (PDF)

If your tree appears dangerous, or if you are especially worried about losing a valuable tree, consider consulting an Arborist.
Caring for Trees in a Dry Climate Colorado State University
Current Drought Threatens Many Urban Trees Arid-Southwestern Gardening Information, University of Arizona
Moisture Stress and Trees: The Hidden Problems Virginia Cooperative Extension
Drought Stress, What does it mean to wooody ornamentals? in: "What's Happening" September 23, 2005. University of Tennessee Extension Service
Long-term Drought Effects on Trees and Shrubs University of Massachusetts Amherst
Irrigating tree with pipe in the ground (this technique can cause problems) New Mexico State University
Wrong way to water trees New Mexico State University
Despite Usually Wet July, Some Texas Trees Parched (Ideas for managing urban trees during times of drought and other stress) Texas A&M University



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