I’m an indoor hydroponic farmer (just culinary herbs and leafy greens), and I recently had a bout with some spider mites. Being that we are a 100 percent pesticide-free operation, I’m trying to source some other methods to deal with the problem. I have tried using some predatory mites, and while they certainly helped, I’m still noticing signs of new problems. I stumbled upon ozone generators and found a Maximum Yield article regarding the use of them to kill pests. I also did some research and found that the O3 (Ozone) generator may also cause some damage to the plants that are still growing. Can you recommend a method for using the generator, and the levels / duration needed to kill the pests while doing the least amount of damage to the plants?

Although there is not a lot of information on the subject, some indoor growers claim to have successfully used ozone to treat pest insects. For ozone to be an effective insect killer, the concentration of ozone in the air must be extremely high. Again, there is not a great deal of information on the subject, but it appears that a concentration of 1,000–2,000 ppm or even higher (some people say as high as 10,000 ppm) is required to kill spider mites. In an environment with a concentration of 2,000 ppm of ozone, pest insects will most likely perish in anywhere from a half hour to two and a half hours. However, ozone at this high concentration can be extremely dangerous for people and pets. In fact, ozone levels of 25 ppm and higher are considered immediately hazardous to human life. It is also very difficult to achieve this high of a concentration of ozone without a special ozone generator (an oxygen-fed ozone generator). High concentrations of ozone can also cause damage to plants, which is why high levels of ozone are usually reserved for treating or sterilizing an empty room. Horticulturists who decide to implement high concentrations of ozone as a method of pest control need to do so with the utmost caution.

I do not know how your specific plants will react to ozone treatments, so I do not feel comfortable giving you specific recommendations for treatment. I can tell you that sensitive, leafy greens will probably be more adversely affected by ozone treatments than robust plants. If you want to experiment with ozone as a pest insect treatment, you could increase the level of ozone for half an hour to an hour and then wait 24–48 hours to see if it is effective or if there are any adverse reactions. If no noticeable plant damage occurs after 48 hours, you could slowly increase the concentration of ozone and/or the duration of the treatment time until you have achieved the results you desire. Just be sure to completely evacuate the ozone from the garden before any people or pets enter the area. I hope this helps.