Q: I’m getting ready to pressure wash the outside of my home. I’m concerned that the plants near my house will be harmed by the chemicals I use to do the initial cleaning. I’m afraid that the bleach mixture will hurt the foliage or get into the root system. Do you know of a good ratio of bleach to water that will kill the mildew but not harm plants?
A: If you are using one of the common homeowner-type pressure washers which has a suction tube to deliver cleaning chemicals, it is typically designed to deliver a chemical:water concentration of 1:10 or greater. The ratio varies widely among different machines. To be scientific about it, put the suction tube in a quart of water and capture the discharge from the washer over a five minute period. Compare the amount of water sucked from the quart jar to the amount shot out by the machine to get an idea of the dilution ratio.
In my opinion anything more dilute than 1:10 would not harm your plant foliage. Simply wet the leaves beforehand and rinse them occasionally as you work. Bleach quickly changes to chlorine gas and evaporates. I also doubt that you will pollute the soil below your plants. Consider visiting a paint store to buy housewashing concentrate that both kills mildew and loosens dirt effectively.