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.

News

Irrigation With Air

Engineers at The University of Texas at Austin have created a growing medium containing special gels that absorb moisture from the air at night, when it's cooler and more humid. As the gels are warmed during the day, water is released and made available to plants. Some moisture will make it back into the air, humidifying it and contributing to ongoing water harvesting. In one rooftop experiment, all radishes planted in the gel mix survived 14 days without watering after an initial establishment period. In contrast, a sandy soil (collected from a dry region) could not support any radishes for more than 2 days after irrigation was stopped. This technology could make farming feasible in places where water availability and the cost of irrigation infrastructure currently prevents it. Gel-polymers could have other water harvesting and cooling applications, too. Source: Self-Watering Soil Could Transform Farming (November 2020)

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.

More Online Information

Older News

Avoid Urban Tree Thirst

Researchers in North Carolina USA looked at Quercus phellos to both in the landscape and in laboratory conditions study how urban trees could be affected by water stress. Higher temperatures could increase tree growth - provided the trees had adequate water. Also, scale insects had little effect. On the other hand, if trees were water stressed, growth rate was lowered, even more so if combined with heat and/or scale insects. Source: Lack of Water is Key Stressor for Urban Trees (March, 2018)

Water restrictions - think of the trees

A recent analysis of data collected in Los Angeles in 2010 shows that the city was losing approx 100 gallons per person per day to the atmosphere that summer, through lawns (70%) and trees (30%). The lack of watering restrictions at the time meant lawns were being overwatered and using as much water as they could. Trees used much less due to factors including leaf area and ability to regulate water loss. The amount needed to maintain tree cover may be far less than perceived and a more nuanced approach to urban water allocation, especially in droughts, could be worthwhile for the long-term benefit of the environment and community. Source: LA lawns lose lots of water: 70B gallons a year City's rich and famous lose twice as much as poor; trees relatively efficient (May, 2017)

Drought means dry air

A new study indicates that the effect of low air humidity been an underestimated cause of plant stress during drought. Closed stomates will consequently mean less photosynthesis and growth. Source: During drought, dry air can stress plants more than dry soil (September 2016) [Consider how much better plants respond to rainy weather compared to artificial watering - Editor]

Does greywater on the garden make you sick?

An Israeli study in which the health of users was compared to non-users over a one-year period has shown no greater risk of water-related diseases such as gastroenteritis by garden irrigation with graywater, at least in arid areas. Source: Graywater Reuse for Irrigation Deemed Safe (December, 2015)