Garden & Greenhouse


Chlorine Disinfection in Greenhouse and Nursery Applications

Posted August 24th, 2016 by Robin Nichols in ,

Chlorine is the most widely used water disinfectant/sanitizer in the US and most of the world. Chlorine is a powerful oxidizer. When added to water it steals electrons from other substances altering the chemical makeup of unwanted organisms and combines with dangerous inorganic compounds to render them harmless.

Pathogens in the greenhouse and nursery are easy to find on root substrates, containers, under benches and on the floor. They can be easily introduced to plugs or transplants and can make their way into a facility on shoes or other materials brought in by guests.

Irrigation water is quite often overlooked as a source of infection. Water sources considered clean such as municipal supplies may carry dangerous pathogens and should be tested.  Other sources such as surface water, irrigation wells and especially re-circulated feed water pose a substantial threat.

If an effective sanitizer, controlled and injected properly is not regularly used, crops (i.e. Geraniums, Perennials, Bedding Plants, etc.) can suffer significant infection and other detrimental conditions such as Root Rots, Mildews, Chrysanthemum White Rust, Viruses, Foliar Diseases, and more.  This can devastate an entire year’s crop costing substantial investment.

Plant DiseaseIntegrated Pest Management

As a result, thorough Sanitation Practices should be implemented as follows:

  • Prevention
  • Inspection
  • Environmental Control
  • Eradication

Chlorine is a powerful tool when used as part of a sound Sanitation plan.

  • Chlorine is typically introduced to water in the form of hypochlorites (tablets, solutions, or powder) and gas.
  • Chlorine (Cl2) combines with water to form “free chlorine” (hypochlorous acid) which is a powerful bactericide/sanitizer.

Cl2 + H2O <-> HOCl + HCl

Chlorine + Water <-> Hypochlorous acid + Hydrochloric acid

  • HOCl (Hypochlorous Acid) is the fast acting free chlorine. If is the most effective form of free chlorine for disinfection. It is a weak acid and dissociates as the pH rises to form…
  • OCl- (Hypochlorite ion) is the slow acting form of free chlorine. OCl- is 80 to 300 times less effective than HOCl.

HOCl <-> H+ + OCl-

NOTE: The concentration of HOCl decreases (OCl- increases) very rapidly with an increase in pH, changing

the oxidizing potential of the water.

Forms of Chlorine

There are solid (calcium hypochlorite), liquid (sodium hypochlorite) and gas forms of chlorine. All three forms deliver hypochlorous acid (HOCl) upon dissolution in water, which is the sanitizing form of chlorine.

Below, the focus is on the liquid sodium hypochlorite (i.e., bleach) and solid calcium hypochlorite (i.e., tablets) forms.

Calcium Hypochlorite (Ca(OCl)2)

Ca(OCl)2 + 2H2O <-> 2HOCl + Na(OH)

Calcium Hypochlorite + Water <-> 2 Hypochlorous Acid + Calcium Hydroxide

Sodium Hypochlorite (NaOCl)

NaOCl + H2O <-> HOCl + Na(OH)

Sodium Hypochlorite + Water <-> Hypochlorous Acid + Sodium Hydroxide





NaOCl Liquid 12% 13-14
Ca(OCl)2 Tablet or Granular 65% 12-13
Cl2 Gas Gas 100% 0.2

Dosing Chlorine

Chemicals containing chlorine can simply be dosed directly into the irrigation water using volumetric proportion. A specific initial dose (e.g., 5 parts per million) is provided at the well head, to ensure adequate residual (typically 0.5 to 2 ppm) at the outlet. The difference between the initial and residual concentration is termed chlorine demand. The treatment system is most effective if the irrigation water quality (chlorine demand) remains constant. If the pH, biological load or temperature never varies then the dosing ratio can be determined using a simple chlorine meter. This however, is unrealistic. Simple dosing is less effective when water has a fluctuating chlorine demand, which applies to a majority of greenhouse watering systems.

Factors that increase chlorine demand include:

  • Warm weather.
  • Use of recycled or pond water.
  • Accumulation of fertilizers, peat and plant debris in the irrigation water.
  • Biofilm and algae.

The mode of action of hypochlorous acid is through oxidation and chlorination of many types of organic material, and not just pathogens or algae. To make the addition of hypochlorous acid more effective for pathogen control, water should be pre-filter to remove excess organic material.

Fluctuating chlorine demand of the water system affects the amount of chlorine that needs to be initially dosed to provide a consistent residual level. Research at the University of Guelph showed that maintaining 2 ppm of free chlorine for five minutes can control most common plant pathogens. However, biological load varies with temperature and tends to increase during spring and summer, especially with surface or recycled water sources. Higher temperatures require more chlorine injection to combat algae and pathogen growth. Seasonal fluctuations in water alkalinity or pH can also change the chlorine demand because the concentration of hypochlorous acid decreases as pH increases. Consistent and simple chlorine dosing without adequate control of residual chlorine concentration can be harmful to plants. If chlorine demand suddenly drops (for example, because of cold weather) then residual chlorine can rise to phytotoxic levels (typically above 4 ppm hypochlorous acid for short term exposure and 2 ppm for long-term exposure). If chlorine demand increases, hypochlorous acid level may be inadequate to control pathogens. Overdosing or release of gas into the air (off-gassing) at very low pH are potential worker safety hazards.

  • Chlorine Demand

The point during the addition of chlorine where the reaction with organic and inorganic materials stops.  At this point “Chlorine Demand” is satisfied.

  • Chlorine Residual

The total of all the compounds with disinfecting properties PLUS any remaining uncombined (free) chlorine.

Chlorine Residual (mg/L) = Combined Chlorine Forms (mg/L) + Free Chlorine (mg/L)

  • Available (free) Residual Chlorine

Free uncombined chlorine remaining after chlorine demand is met.

  • Chlorine Dose

The amount of chlorine added to meet chlorine demand plus the amount of chlorine needed for further disinfecting.

Chlorine Dose (mg/L) = Combined Demand (mg/L) + Chlorine Residual (mg/L)

  • Breakpoint Chlorination

The process of adding chlorine to water until the chlorine demand has been satisfied.  Public water supplies usually chlorinate past the breakpoint.

A Typical Chlorination Process

  1.  Adding a small amount of chlorine (first) reacts with (oxidizes) manganese, iron, and nitrite…assuming those elements are present.  No other reactions occur.
  2. Increasing the chlorine dose will facilitate reaction with organics and ammonia; Chloramines and chlororganics will form.
  3. Adding a bit more chlorine destroys the chloramines and some or most of the chlororganics. Breakpoint chlorination is achieved.
  4. Adding more chlorine creates a free residual concentration for future disinfecting.

Oxidation-Reduction Potential (ORP)

Controlled injection and measurement of chlorine concentration can be combined with the measurement of oxidation-reduction potential (ORP). Oxidation-reduction potential is read in milli-volts (mV) and measures the oxidative power of the treated water. Basically, the ORP number is a gauge of the sanitizing effectiveness of your water; the higher the value, the more powerful the sanitizing effect. ORP is a proven technology for municipal water treatment and food safety and is becoming more prominent in the greenhouse and agricultural industries. An oxidation-reduction potential level of 650 to 750 milli-volts is typically used to indicate adequate sanitation based on killing of human pathogens. Research is ongoing to refine oxidation-reduction potential levels suited for plant production but is has been shown that levels as low as 550 have proven effective in destroying many common pathogens destructive to plants.

Control using chlorine and oxidation-reduction potential is ideally undertaken with inline sensors and dosage systems. This method of control can be applied to dosage of all chlorine forms (gas, liquid and solid), as well as some other oxidizing chemicals. An additional controller for water pH is also needed where water is acidified. In a low-tech installation without inline controls, water pH and free chlorine can be measured weekly using calibrated handheld meters. Handheld oxidation-reduction potential meters are also available.

Tips for Using Sodium and Calcium Hypochlorite

  1. Chlorine should be combined with other water treatment components such as filtration and aeration to increase efficacy.
  2. Store liquid sodium hypochlorite protected from ultraviolet light, either at low temperatures (60 degree (F) – 70 degree (F)) or in smaller volumes so that it is turned over in 15 days or less (degradation rate doubles for each 10 degree (F) temperature increase).
  3. Store solid calcium hypochlorite in a dry location.
  4. At the low chlorination rates (below 2 ppm) used for constant treatment, sodium, calcium or chlorine will not significantly affect plant nutrition, water electrical conductivity, calcium deposits or pipe corrosion.
  5. However, when handling concentrated chlorine, use injectors and piping designed for caustic chemicals. Because chlorine can react with some metals and plastics, check with the manufacturer of your irrigation system components to make sure that problems won’t occur if chlorine is injected.
  6. Follow guidelines for operator safety and handling. Never mix concentrated chlorine with other chemicals.

This article was submitted by Hanna Instruments

Want more information? Read these articles:

Fall Cleaning in the Greenhouse

Plant Disease Control with Biofungicide

Proper Garden Hygiene to Prevent Pest and Disease Problems

Summer Cleaning in the Greenhouse

Winterizing the Greenhouse

Subscribe to Garden & Greenhouse Magazine

Subscribe to Garden & Greenhouse Email Newsletter