In a recent discussion on Kris Watson's blog at Simply Living, water conservation methods such as a gray water system to capture water from the kitchen, laundry and shower were being discussed. One of the comments left on the post said that you should not capture laundry water if you are making your own laundry soap using Borax because it "can be very toxic to plant life and not good as a gray water option." No offense to the person who posted the comment, but I feel it's important to clarify some things.
It is true that Borax can be used as a herbicide to kill weeds and stray grass. For this purpose, you need one-half cup of Borax mixed with one gallon plus one cup of water. Wear gloves and mix the Borax powder in one cup of water until well blended, then dilute in a garden sprayer containing one additional gallon of water. Spray it directly on the vegetation you want to kill. When mixed with water in this ratio and applied directly to a plant, Borax interferes with photosynthesis and the plant will turn yellow and die. Basically, it will starve due to the inability to process nutrients drawn up through the roots. A second and sometimes a third application are required to completely kill the vegetation.
When making laundry soap, the same one-half cup of Borax is combined with one-half cup Arm & Hammer Washing Powder, one-third bar of grated Zote soap. The final ingredient for making laundry soap is six gallons plus six cups of water. So the concentration of Borax required to kill vegetation is more than six times the concentration in homemade laundry soap. Consider also that the recipe for homemade laundry soap makes enough for at least 120 loads of laundry. So simplistically, each load of laundry contains 1/120th of one half cup of Borax. The concentration is then further diluted by the amount of water your washer uses to complete the wash and rinse cycles.
So let's think about this. If two and sometimes three applications are required to kill vegetation when Borax is diluted in a little more than one gallon of water, how likely is it that a dilution at least 126 times weaker is going to have an adverse affect on vegetation?
We have been making our own soap for over a year and a half now. When this house was built in 1985, the original owners set up a primitive gray water system to capture water from the kitchen and laundry only. I say 'primitive' because a state of the art system would include some kind of containment reservoir and some means of dispensing the captured gray water such as a sprinkler system or leach field. The system in place here is just a two-inch pvc pipe that runs underground about 40 feet out from the house. At the end of the run, about six inches of open-ended pvc protrudes above the ground to release gray water as it's produced. The outlet is situated at the edge of the backyard fence line, about fifteen feet uphill from six mature pecan trees. My husband trenched from the outlet pipe down toward the trees in an attempt to channel the water for a useful purpose. Any water generated from the kitchen or washing machine flows out the end of that pipe. In our current ongoing drought situation, the grass at the outlet and all the way down the trench is about the only green in the yard. We have nine mature pecan trees on this place but the six that benefit from the gray water flowing down the trench are the healthiest of the bunch.
Installing any gray water system is most affordable at the time of construction. After market systems can be installed but significant alterations to plumbing are always expensive. But if you are able to capture gray water and you happen to make your own laundry soap, please don't let concerns about plant toxicity of Borax stop you from including laundry water in the capture. You will be missing out on significant water conservation and irrigation opportunities.