Drought-tolerant turf

Australia is the driest inhabited continent on Earth. It is about 80% of the USA´s land area, but supports only 21 million people compared with just over 300 million in the USA.

Australia’s population is concentrated in the limited areas with higher rainfall along the eastern, southern and south-western coasts. Water, or lack thereof, is the major reason for Australia´s low population.

Australia has:

• 1% of the world´s surface fresh water resource
• only a few snow-fed rivers
• less than 300 mm average annual rainfall across 60% of the country, while another 20% receives 300-600 mm
• highly variable rainfall from year to year
• experienced 12 major droughts in the past 150 years.

Australia´s water crisis

In a dry continent currently in the grip of one of the worst droughts on record and with unprecedented water restrictions, making better use of our water must be a priority.

The theme of water use runs through much of the Department of Employment, Economic Development and Innovation’s (DEEDI’s) turf research program on warm-season grasses at the Redlands Reseach Station. It is critical long-term research be conducted into better and more water-effective solutions to apply during the ongoing cycle of droughts in Australia.

No ‘silver bullet’ solution

Homeowners and the media want a quick fix: ‘Just give us your most drought-tolerant grass’ is the usual request. But even the most drought-tolerant grass will not survive in a few centimetres of soil over rock, which is not an unusual situation in new housing developments.

One major building company promises customers 300 square metres of lawn laid on just 5 cm of topsoil around their new house. This amount of topsoil is grossly inadequate and is actually being promoted as a positive marketing tool.

The plant itself sets the potential in terms of the level of drought tolerance that is possible. The soil profile in which it is grown and the management practices applied determine how much of that potential can actually be achieved.

Drought-tolerant turf is built from the ground up by making a series of incremental improvements, not through one simple solution.

The basic steps are:

• to put a good soil profile in place
• to ensure that water can enter the soil and be stored there for the plants to use
• to plant a well adapted turfgrass, bearing in mind other site restrictions such as shade, wear or salinity
• to check water quality, particularly in the case of alternative irrigation supplies.

Water use efficiency or drought tolerance?

Firstly, we need to be clear about our objective: Are we looking for better water use efficiency in our turf, or the ability to survive for longer periods while losing water through evapotranspiration? The answer depends on climatic conditions and the chances of rainfall.

In a desert climate where there is very little chance of rainfall, the turf is reliant on total irrigation. In this context, water use efficiency is important, even though recent research in Arizona by Kopec and others (2006) showed only small differences among the various species and cultivars used.

In a humid, subtropical climate like Brisbane, where there is much higher probability of rain in the near future, irrigation is generally used to supplement rainfall on turfed areas. This means that drought-tolerant turf that can go longer between rainfall can lead to substantial savings in irrigation water use.

Start with the soil profile

When grown on a properly-constructed soil profile in South East Queensland, most warm-season, drought-tolerant species will survive long periods of drought without any irrigation, such as green couch (Cynodon dactylon and hybrids) and blue couch (Digitaria didactyla). The less drought-resistant ones need no more than an occasional strategic watering to survive.

For turf to cope with extended dry periods, the soil profile should be a minimum of 10 cm (and preferably 15 cm or more) deep to provide adequate soil water storage. Where the profile depth varies, shallow patches will dry out more rapidly and the turf on these may even appear dead by the next fall of rain. But with moisture in the profile once again, many such apparently ´dead´ patches of blue and green couch can stage a rapid and complete recovery.

Not only is the depth of topsoil under the turf important, so is the quality of that topsoil. For example, second-rate soil stripped from a building site will not give the desired result.

Increasingly, soil suppliers are mixing components to create artificial soils, as sources of good natural topsoil become scarcer. Products with raw compost that is still decomposing should be avoided. Additionally, soil mixes with high organic matter (greater than 25%) will eventually slump to lower levels as the organic matter decomposes. This is an area where more research and more regulation is required to improve the quality of topsoil used under new turf plantings.

Soil water entry and storage

At low moisture levels, many soils will become water repellent, a problem caused by organic acids coating the sand or soil particles. Rainfall and irrigation are then much less effective, as water tends to run off or through the soil and it does not easily wet up again. While this is a common problem seen on golf greens, it is not widely recognised that soil water repellency is also a common condition on the extensive areas of infertile forest soils found in urban areas around Brisbane.

The normal treatment for soil water repellency in high-quality turf areas is to make regular applications of surfactants, which improve water entry by reducing surface tension. Research on new generation surfactants has demonstrated their effectiveness in improving infiltration. By maximising the amount of water captured in the soil during short, high intensity storms, improved infiltration in areas treated with surfactant translated into visibly better turf quality.

Newly-laid sod of all turfgrasses has only a limited root system and is vulnerable to drying out. Regular irrigation is needed until deeper roots have grown through the turf underlay. A number of soil amendment products (e.g. cross-linked polyacrylamides and water-absorbent foam) have been developed to improve soil water-holding capacity. The role of these products in turf establishment was assessed by the Redlands turf team through a research grant from the International Turf Producers Foundation.

Plant a well-adapted turfgrass

There is no such thing as a perfect turfgrass, or one that will grow everywhere and under all conditions. Drought tolerance is not the only attribute to be considered when selecting a turfgrass.

Some 25% of turfgrass sites, for example, are affected by shade, where the most drought-tolerant species – green and blue couch – do not perform well. Buffalo grass (Stenotaphrum secundatum), manila grass (Zoysia matrella) and sweet smothergrass (Dactyloctenium australe) grow much better than green and blue couch under shade, and maintain green healthy turf much longer than they would in full sunlight.

While larger differences in drought tolerance are found among species, differences within species also occur, which will help maximise water savings. The Redlands turf team in collaboration with University of Queensland (UQ) scientists received a government grant to develop more drought-tolerant turfgrass cultivars for a range of uses. The project focuses on collecting and evaluating Australian Cynodon genotypes for turf quality and drought tolerance.

Water quality

Using poor quality alternative water sources, including greywater, invariably means that salinity will be an issue. Research at Redlands has been directed towards growing turf on salt-affected soils, and has identified salt-tolerant turfgrasses that can also be used with poor quality water.

In the first project, 41 turfgrass cultivars from 9 different species were screened hydroponically to assess their tolerance to salt levels up to 40 deciSiemens per metre, which is 74% of the salt level in seawater. In addition to confirming the high levels of salt tolerance in seashore paspalum (Paspalum vaginatum) and manila grass shown in US work, there was considerable variation in salt tolerance among buffalo grass and green couch cultivars, enabling the more tolerant cultivars to be specified for future use on moderately saline sites.

Water use of turf versus landscape plants

Garden commentators promoting shrubs and trees in the media often describe turf as a high water user. This could not be further from the truth.

Savings in water use on community-level sports fields are still possible without compromising turf quality or playability. Researchers at the DEEDI’s Redlands Reseach Station looked at year-round water use across a number of soil-based community sports fields in Brisbane. Under normal frequent irrigation scheduling, the average field uses around 5 ML/ha. Strategic weekly irrigation (applied only when no rain had fallen in the previous week and surface soil moisture was rapidly declining) still maintained good turf quality and a safe playing surface, but on average required less than half the irrigation water (about 2.4 ML/ha). By comparison, tree crops such as citrus typically require 5.0 to 7.5 ML/ha.

These and other facts about water use should be publicised by the turf industry, in addition to commissioning further research to compare reticulated water use on turf and on shrubs and trees. In Florida, Park and Cisar (2006) showed that, after the first year when more water was used to establish the turf, their shrub landscape used more water than the turfed landscape, and water use by the shrubs continued to increase as they grew larger, while turf water use stabilised.

We need studies of this kind in Australia to help get the message across to the public that turf is not a high water user, but rather a sustainable and environmentally-friendly option as the pressure on urban water supplies increases.

Acknowledgement

This information was developed from an article that was first published in TurfCraft International, Issue No. 117, November-December 2007, pages 30, 32.

References

Kopec, D.M., Nolan, S., Brown, P.W. and Pessarakli, M. (2006). Water and turfgrass in the arid southwest. USGA Green Section Record 44(6), 12-14. Also available electronically.

Park, D.M. and Cisar, J.L. (2006). Documenting water use from contrasting urban landscapes: Turf versus ornamentals. TPI Turf News 29(3), 38, 40-42.

Seminar information

This information also featured in a seminar entitled ‘Turf for tough times: Keeping grass cover with less water’. The seminar was conducted by the then DPIF’s (now DEEDI’s) Redlands turf team in Brisbane on 6 June 2007 and was repeated in Toowoomba on 30 October 2007. Other topics covered by the six presenters included:

• irrigation management on sports fields
• using recycled water for irrigation and its implications for soil health
• how good soil management can improve outcomes in dry times
• wear tolerance of different grasses
• progress results from soil amendment trials.

Copies of the 40-page seminar proceedings can be purchased for $18.50 (including postage and GST) by writing to:

Redlands Research Station,
Department of Employment, Economic Development and Innovation,
PO Box 327,
Cleveland Queensland 4163

Ph: +61 7 3286 1488, Fax: +61 7 3286 3094, email: cynthia.carson@deedi.qld.gov.au

Cheques should be made payable to the Department of Employment, Economic Development and Innovation.