Chlorinated wading pool water and drought-dried trees

(Question)

The current severe drought is making trees stressed all over the city. At Dufferin Grove Park, five younger trees have died already, and many others, even big trees, are starting to show brown leaves or bare branches. The short showers we get occasionally don’t help much.

Every day, over 40,000 liters of water go down the drain at the park’s wading pool, and it would be very good if some of that water could be put on the nearby trees instead. The problem is the large amount of chlorine that is put in the water. However, much of that chlorine is gradually inactivated by sunlight or by the sand/pee/leaves that get in the water (chlorine bonds to them).

Wading pool staff have been told that even end-of-the-day low-chlorine water might kill the trees. Researching the question on the web, it’s hard to tell what’s real and what’s hypothetical. But the trees are desperate for water.

Do any of you horticulturalists know more about the effect of chlorinated water on trees? Are there links about actual studies that you could send us? There seems to be a lot of disagreement among experts we’ve asked so far.

(Answer)

Thank you for contacting the Toronto Master Gardeners with your inquiry.

You pose a very interesting question and you are correct in that the research is not clear with regards to this problem. Reading through a number of articles the key to utilizing the chlorinated water is dependent on the amount (ppm) of chlorine in the water.

Plants are regularly irrigated with tap water which does have small amounts of chlorine. We know from observing houseplants that certain plants are quite susceptible to the amount of chlorine in the water as evidenced by browning tips. As a rule we recommend that people let the tap water stand for 24 hours before watering their houseplants so that the chlorine and other chemicals can dissipate from the water.

The article Impact of Watering Lawns and Gardens with Chlorinated Water from Plant Talk Colorado,states that since chlorine is added to pool and drinking water for the prevention of bacterial growth, could this chlorinated water have an impact on the beneficial microorganisms in the soil which in turn affect plant growth.  Their research shows that even though the chlorinated water may kill a number of microorganisms in the soil, the reproduction rate of the microorganisms rebounds in a short period of time. The study further states that “One reason chlorinate water has little impact is that chlorine binds to soil particle surfaces. This immobilizes chlorine and reduces its ability to kill microorganisms.”

An article entitled “Chlorine in Pool Water and Nearby Plants” from Agronomic Library states the following: “There should be no effect of pool water splashed on nearby plants from normal pool use. Larger plants (and animals) can tolerate the concentrations that are recommended for pool water. Dr. Alison Osinski (Aquatic Consulting Services, San Diego, CA) suggests, “If the pool or spa water has a high chlorine content, uncover it and allow the chlorine to dissipate before using the water for irrigating the landscape. www.spectrumanalytic.com/support/library/ff/Chlorine_in_Pool_Water_and_Nearby_Plants.htm

Here is an excellent article from HortScience on Response of Container Grown Nursery Stock to Chlorine. Finally, in an article by Aquatic Consulting Services, www.alisonosinski.com/?p=307  “Water drained from a pool or spa is safe to use for watering lawns or plants, or for any purpose “gray water” uses would be appropriate. It is environmentally correct to recycle water especially when drought restrictions are in effect. If the pool or spa water has a high chlorine content, uncover it and allow the chlorine to dissipate before using the water for irrigating the landscape.”

One of our earlier posts gives important information on the maintenance of poolside plants: “Evergreens and tender plants that are planted too close to the pool edge may suffer and struggle due to a variety of reasons. They need to be set back away from the edge of the bed closest to the pool. In fall and springtime, before opening and after closing the pool it is best to leech the soil by saturating the root zone of the plants and trees. Applying new compost to poolside plants can help them withstand the stress of being poolside.Washing the foliage with water to wash away any residual salt or chlorine from the pool spray and splashing is also good preventative maintenance.”

Here is an interesting article from the University of Maryland on Chlorine Toxicity – Trees and Shrubs

It is difficult for me to say with 100% confidence that it is safe to use the recycled pool water. It will depend upon the concentration of chlorine in your pool. However, if you would like to take a chance I would start by watering one plant. If you see any effects of chlorine toxicity such as curled scorched leaves I would stop using the chlorinated water  immediately and flush the area thoroughly with tap water.