The first thing to do is ask your local government if they have rules about draining hot tubs and using gray water, to be sure it is allowed where you live.
The chemicals used to keep hot tub water clean and safe for users – chlorine, bromine and sodium carbonate (an alkaline used to raise the water’s pH level) – can damage vegetation if they are not “neutralized” before using the water on grass and plants.
Chlorine is Tough on Plants
If you are going to be draining your hot tub soon, do not add chlorine for a day or two prior to that time. Check the water’s chlorine levels with a test kit to be sure they are at zero. Chlorine usually dissipates quickly from very hot water, so leaving the hot tub cover off for a day may also do the trick – but test the chlorine level before using the water on plants.
Balance pH Levels
If your hot tub water is too alkaline, certain plants will suffer from it. Your hot tub water should test in the 7.2-7.4 range (neutral) before using it to water grass or plants. If it is not neutral, adjust the pH and run the jets to make sure any chemicals in the water are mixed well. In addition, let the chlorine or bromine level drop to under 1— and make sure the water is cooled down before you use it.
Keep Sodium Chloride (Salt) Level Low
A small amount of sodium is good for most plants, but too much can be toxic, especially in large doses in a short span of time. Salt absorbs a lot of water in the soil, leaving less available for plants – they can appear to be getting enough water, but actually be drying out instead. Soaking the soil with a garden hose between the times you drain the hot tub, can help flush out some of the salt in the water.
Monitor the Health of Your Grass and Plants
After a few waterings with treated hot tub water, watch for changes in your plants’ appearance that may indicate watering problems, such as:
Yellowing Leaves. When the salt level is too high in the plant’s soil or water, it can cause the leaves to turn yellow.
Slow Plant Growth. If your plants seem to be growing more slowly than usual, it could be a fertilization problem, or the result of having too much salt in its water. Test the soil and adjust as needed.
Delayed, or Lack of Blooms. Some plants are acid lovers, azaleas for example, and do not usually grow well near a hot tub. You can test the soil’s pH level and add limestone (per testing kit instructions), to raise the acidity level in the soil to the plant’s needs.
Fortunately, there are products available, with natural ingredients that will not harm grass or plants that will balance the chemicals in hot tub water. The treated water is even safe for pets to drink, and you will not have to wait around for the harmful chemicals to dissipate.