Efficient water use starts with a well-designed and well-managed irrigation system. But it doesn’t end there because nurseries are under increasing pressure to manage the amount and quality of the water discharged from the site.
Matching irrigation volumes to plant needs will obviously reduce run-off. But factor in the prospect of restricted supply, once the regulations stemming from the Water Framework Directive begin to take effect, and many nurseries may need to consider collecting and recycling irrigation run-off.
Growers in other parts of the world, including Australia, the US and Europe, have already had to face the challenges of supply restrictions or discharge legislation, so there is considerable international experience the UK industry can draw on.
Algal blooms can be a particular problem in summer in irrigation recycling systems — excess algal growth is stimulated by warm temperatures and high levels of plant nutrients that can leach from growing media.
"The biggest challenge for any recycling system is how to clean the water to make it suitable for irrigating," says Debbie Deckys, production manager at Alstonville Palms in New South Wales, Australia. "Algal blooms are the biggest enemy, blocking irrigation, upsetting pH and making it impossible to kill pathogens."
In 1994, this 4ha container-shrub and -foliage plant nursery was the first in the -region to install a reservoir and capture all run-off, in anticipation of legislation. Its bravery certainly paid off when, within a decade, the whole of eastern Australia found itself in the grip of a major drought.
Initially, the company used chelated copper to combat algae, but this caused decomposing organic matter to accumulate at the bottom of the reservoir, which periodically bubbled up and was impossible to filter out. The solution was to install aerators in the reservoirs.
"Aerators combine with beneficial microbes that eat algae and keep it to a minimum," says Deckys. "Aerating the water improves conditions for the microbes, while the algae seem to dislike it." Layers of decomposing algae in the bottom of the reservoir are no longer a problem.
Water treatment systems not only need to deal with algae, but also solid particles that can block filters and nozzles and plant pathogens such as Fusarium and Phytophthora.
Alstonville Palms has installed filters with automatic backwash, which helps prevent blockages, to remove sediment. The heaviest sediments can be removed by traps at the ends of the drains that conduct water away from the beds. These small tanks of still water slow the flow, causing sediments to sink to the bottom where they can be cleaned away. Finer particles, such as clay, are harder to deal with and may require separate treatment with a flocculating agent such as alum (aluminium sulphate), which causes them to stick together and sink.
Organic materials can be filtered out through coarse sand. Slow sand filtration is proving a particularly cost-effective method of treating water against plant pathogens. The sand in the filter tank provides a medium for the growth of beneficial microbes, and it is these that kill the pathogens and break down organic matter in the water. Recent HDC research has shown that sand filters are effective enough to kill some of the most damaging organisms, including Phytophthora species, which might otherwise spread in recycling systems. Water with a high algal or silt content should be pre-filtered before slow sand filtration to prevent the filter itself becoming clogged.
Once Alstonville Palms had found a way to remove algae, it turned to UV treatment to kill plant pathogens. UV is another way to control fungal pathogens such as Phytophthora, Fusarium and Alternaria, but the water has to be free from suspended particles and minerals such as iron and manganese that absorb UV light because solids can protect fungal spores.
Few nursery water sources have the clarity for effective UV treatment, even after filtering. An alternative method of treatment is filtration itself. Micro-filtration, using very fine filters, can remove most pathogens, but because the pores are so fine they tend to clog quickly, making regular maintenance essential.
Chemical treatments, such as chlorine, may also be an option for some growers. However, accurate dose control is essential while pH also needs to be controlled to below 7.5 when using chlorination or chlorobromination treatments.
Another possible disadvantage of chlorination is that it can kill both pathogenic and beneficial organisms — with residual effects on beneficials in, for example, capillary matting that could then enable pathogens to take hold.
Some UK?growers use reedbeds to treat nursery run-off before it is discharged, rather than for recycling back on to the crop. But in the Netherlands, where the industry is forced to recycle to meet strict water discharge regulations, biological treatment is increasingly seen as a cost-effective alternative to chemical disinfection, or filtration, to treat water that will be returned to the crop.
"Water is not just a fluid to dissolve chemicals in," says René Jochems, whose company, GroeiBalans, installs biological water treatment systems at nurseries in the Zundert area of the Netherlands. "It is important that recycled water is treated in a way that maintains or improves its biological quality.
"Integrated pest and disease management is more successful when you have healthy, biologically active water because it aids the introduction of beneficial micro--organisms, including mycorrhizas."
In Jochems’ system, water draining from the nursery beds flows into a holding tank — which regulates flow and prevents the risk of the system flooding in heavy rain — and then through a treatment or filtration ditch before arriving at the main storage reservoir.
Ideally, the filtration ditch is built at the same time as the water recycling and storage is installed. The active components of the ditch are aeration, water movement, sunlight (as a natural source of UV), Iris pseudacorus, waterweed (Elodea densa), predatory fish and water snails.
The ditch is divided into compartments to contain the different active species. Arranging the compartments on a series of interconnected levels and allowing the water to flow from higher to lower levels over a weir increases aeration and exposure to sunlight. Aeration maintains biological -activity and reduces calcium bicarbonate levels in hard-water areas.
The water plants absorb excess nitrogen and phosphates, which helps prevent algal blooms. They help aerate the water too. "The treatments reduce EC levels, making it easier to manipulate liquid feeding in the resulting irrigation water," says Jochems. "The water plants maintain a good habitat for highly diverse populations of beneficial microbes, which is important for disease suppression — active bacteria growing on their root systems produce natural antibiotics that suppress pathogens.
"Phytophthora is present in every water storage system but biologically active water treatment prevents it from building up enough to become a problem."
The waterweed is important for oxygenating the water, while the predatory fish are there to pick off water-borne plant pests. The snails remove other sources of harmful organic matter.
International Plant Propagators’ Society members recently saw Jochems’ system in action on a tree seedling nursery in Zundert, the Netherlands. Since using it, the nursery has not needed to use routine applications of fungicides against damping off; the roots of the seedling trees become naturally colonised by mycorrhizas. Perhaps even more impressive is the grower’s claim not to clean or sterilise propagation trays during their six- to seven-year life, in order to maintain their healthy beneficial microflora. In fact, he says, he can’t even grow Cedrus deodara in a new tray.
Making beds for recycling
It is cheaper and easier to install -recycling systems as part of a new container bed than to retro-fit.
A typical installation sees the bed lined with heavy-gauge plastic sheet, which is then covered with gravel, on to which textile mat is laid to prevent too much organic matter getting into the system.
Each bed drains into a gutter or gully. These feed into main drainage gullies or pipes, usually running along the roadways between the beds. The layout is designed to make maximum use of any natural slopes to minimise the need to pump. In René Jochem’s treatment system, he aims to site the treatment ditch as close as possible to the final storage reservoir.
The Belgian Nursery Research Station at Destelbergen compared a number of nursery bed ground-cover systems using plastic base sheets and drainage and capillary mat coverings to find which gave the best combinations of drainage and water retention as part of a water recycling system. The results were presented at a International Plant Propagtors’ Society event in Belgium.
The main difference is between those in which water runs off horizontally across the bed surface, and those where water drains through an upper layer first, and then away, says researcher Els Pauwels. She labels these horizontal and vertical drainage systems and says that depending on the covering it is possible to recover up to 60 per cent of the water and 30 per cent of the fertiliser applied to the crop.
Plastic sheet overlain by a ground-cover material is the basic system most commonly used on Belgian container beds, and has the horizontal drainage pattern. During dry seasons, the disadvantage of having no capillary matting is that more irrigation is needed, says Pauwels. Growers also find that the moisture content of the growing media varies significantly between plants at the top of the bed and those at the bottom. "In drier seasons or climates, use of capillary matting as part of the ground-cover system leads to a faster and more even distribution of the water and improved water retention.
But for wet conditions, you can end up with too much moisture around the roots for long periods."
In vertical drainage systems, as for horizontal, the bed is also first covered by plastic sheet. This can be overlain by sand, traditionally used in nursery stock where vertical drainage characteristics are required, or alternatives such as gravel, crushed rock or Bubbledrain, which resembles a tougher, thicker version of plastic bubble-wrap and has to be laid bubble side up and covered by a permeable ground-cover material on which the containers stand.
In the Belgian trials, the average water recovery was 30 per cent from the horizontal systems. Drainage was faster in the vertical systems where the average water recovery was 60 per cent. "The capacity of a water recovery system and disinfection installation will depend on the drainage percentage and therefore on the kind of bed material," says Pauwels.
The moisture content of the growing medium also depends on the drainage characteristics of the bed. With vertical draining systems, Pauwels recorded remarkably small differences in moisture content between plants at the top and bottom of the bed. Root growth was always better in crops grown on vertical drainage. Pauwels suggests this is the best system for wetter conditions because of the faster drainage.