The following steps are provided to help greenhouse growers
utilize water properly with the minimum amount of waste, while maintaining
quality crops. The guidelines below are divided into three steps
for ease of implementation. Step 1 should be implemented wherever
feasible by all greenhouses. Step 2 is strongly recommended for implementation
whenever physically and financially possible. Step 3 illustrates the
ideal.
Irrigation
of greenhouse crops is a critical, yet often overlooked practice.
Waste or overuse of water is common, particularly when watering is
mistakenly treated as the least precise of all cultural inputs. When
relying on the "eyeball" or the clock when timing irrigation, crops
often receive excessive amounts of water, creating runoff.
Step
1 - Reduce Wasted Water/Runoff
A.
Group plants with similar water needs together to improve irrigation
efficiency. Adjust individual sections of the irrigation system
to avoid excess watering in some sections.
B.
How Much to Water? The general rule of thumb is to apply 10 -
15% more water than the container will hold. This will help leach
salts at each irrigation. Don't allow water to flow over the top
of the container. The rate of irrigation must be low enough to
allow the water to percolate through the growing media.
Make
sure media has adequate porosity (well drained) as well as good water
holding capacity. This will help minimize irrigation frequency.
Step
2 - Examine Efficiency of Irrigation System. Work towards adapting
new irrigation technologies to production systems to help lower costs
and reduce water waste or runoff. A well designed, efficient irrigation
system is a large part of the water use reduction equation.
There
are several means by which to supply a crop with irrigation water:
overhead sprinkler, hand watering, drip or trickle irrigation systems,
and subirrigation. Overhead irrigation and hand watering are typically
more wasteful delivery systems. These systems also wet the foliage,
increasing the potential for disease. Drip or trickle systems are
more efficient and provide the greatest control over the amount
of water applied.
Subirrigation
(ebb and flow, flood floors, troughs or capillary mats) systems
are extremely effective at reducing water waste. These systems also
require half the fertilizer of overhead irrigation and lead to less
disease as the foliage remains dry. However, they are expensive
to install and water may need to be treated before reuse can occur.
When
to water? There are several methods that can be used to measure
media moisture levels.
Appearance
or feel - many growers usually water when the media will crumble
easily when compressed in the hand. Examine the media at several
depths.
Tensiometers
- these devices are made of porous cups attached to a vacuum gauge.
The cup is inserted in the soil and filled with water. As the rooting
media dries, water leaves the cup and the resulting tension is recorded.
Weight
of media moisture - one pot plant on a bench is used as a control.
It rests on a scale that is adjusted to trip a switch when the moisture
level drops below a certain level. As the plant grows the setting
must be adjusted to account for added plant weight.
Light
Accumulators - these systems are based on the idea that increased
light causes increased evaporation. A photoelectric cell and counter
activate a solenoid valve when a predetermined level of light is
received.
Soil
Moisture Conductivity - several devices relate soil moisture to
electrical conductivity. When the soil dries to a preset level,
the electronic circuit activates the solenoid valve.
Step
3 - Collect and Reuse/Recycle Irrigation Water. Why capture runoff
and recycle irrigation water: The benefits.
Many
greenhouse operations across the country have already adopted capture
and recycling systems. Whether voluntary or mandated these capture
systems have environmental and monetary benefits. Many greenhouse
and other horticulture production facilities that have adopted these
practices state the most important benefit realized is savings on
the cost of water. For others the most compelling reason for adoption
has been to assure that an adequate supply of sufficiently high
quality water would be available when needed during production.
Implementation
of a new system means there will be an inevitable learning curve.
Potential problems that may occur with recycled water systems can
be easily avoided with careful planning and some monetary investment.
A
common method of collection and reuse of water is the installation
of retention basins, storage ponds, storage tanks and additional
pumping capacity. Concerns, or potential disadvantages of these
systems include build up of salts, chemicals, nutrients, and pH.
These may then be recycled back onto crops, ultimately decreasing
crop quality. Studies have also shown that water-borne pathogens
such as Pythium sp. may be present in runoff water at relatively
high concentrations. Sometimes these pathogens can be detected in
recycled irrigation water at the point of delivery to crops. Unfortunately,
there are no scientifically derived thresholds for levels of pathogens
in irrigation water. Because of the potential that these water "impurities"
may build up in recycled water over time, many growers err on the
side of caution by decontaminating recycled water before reuse.
These
potential disadvantages of using recycled water can be overcome by:
1.
Monitoring salts, chemicals, nutrients and pH. Test irrigation water
three times a year for salt levels, bicarbonates, and pH. Review
the results before any fertilizer is applied.
a.
If buildup of salts in recycled water becomes a problem dilute
with fresh water.
b.
Many growers use water treated through a process known as reverse
osmosis (RO) to remove potentially harmful salts. The systems
are relatively expensive but work well as a source of water for
back blending. RO water has almost no nutrient value and if used
over an extended period of time, growers may experience micronutrient
deficiencies
2.
Becoming proactive when dealing with water-borne pathogens such
as Pythium, which ultimately cause root rot:
a.
Increase the frequency of scouting of problem crops.
b. Remove diseased plants from the system quickly.
c.
Monitor pathogen levels of irrigation water. Water can be sampled
at different points to determine pathogen presence and levels.
Tests to determine which pathogens are present can be conducted
at some plant disease testing laboratories.
d.
Water can be treated for disease organisms by retention and dilution,
filtration, chlorination, ozonation, and/or UV light.
Costs
associated with installation of holding ponds, tanks, pumps, and possible
treatment systems eventually pay for themselves. However, the phasing
in of these capture systems, will help spread out capital outlay over
a number of years.
Obviously
there is no single water collection process that will work for all
greenhouse operations. Different greenhouses will have different water
quality problems, different irrigation demands, and different abilities
to deal with each situation. For some systems, complete decontamination
may not be economically feasible. Each system will have to accommodate
the unique requirements and conditions of the operation for which
it is designed.
