Q. Dear Umbra,

I usually collect all my laundry water to recycle into my garden. I know liquid soap is better environmentally, but a washing machine expert told me to use one tablespoon of detergent. I store the gray water in an old trash barrel and add used tea bags and compost, assuming the kitchen scraps will help break down pathogens in the water. Then I pour bucketfuls on the soil around my fruit trees. Do you think this method will not add too many phosphates to the soil?

Judy G.
Los Angeles, Calif.

A. Dearest Judy,

We call it gray, but the “gently used” water flowing from our washing machines, showers, tubs, and bathroom sinks is more like liquid gold, isn’t it? The stuff is pretty clean and perfectly suitable to do double duty flushing our toilets and irrigating many of our backyard plants. And of the 50 shades of gray water discharging from your home, laundry water tends to be the easiest to tap for recycling. (Thank you for allowing me a chance to work the phrase “50 shades of gray water” into my column. I was starting to fear that particular work of literature would fade from our collective consciousness before I could use it.)

As you no doubt already appreciate, Judy, gray water reuse is pretty darn smart for several reasons, which I’ve talked about before: It means we’re not tapping treated, potable, and in your case all-too-scarce-in-California water; we’re reducing the pressure on local water treatment plants; and we’re saving energy and money. As the practice is legally A-OK in the Golden State (not the case everywhere, so check your local regulations, people), why wouldn’t you take advantage? You do need to follow a few simple steps when recycling light gray water (as opposed to the dark gray water that comes from kitchen sinks and dishwashers, which requires a bit more care), but the payoff is well worth the effort.

Which brings us to the first part of your question: Which detergents work best in a household that recycles gray water in the garden? Smart move using it around the base of fruit trees, by the way: Because even light gray water may have traces of bacteria, oil, and grease, you should be careful not to get it on the edible parts of a plant.

Anyway, the water is meant to nourish living green things, so you want your washing machine’s effluent to be as pure and free of troublesome substances as possible — so avoid any soap products containing chlorine, sodium, or boron, all of which can damage plants and soil health. I can’t answer the liquid vs. powder question without knowing what kind of machine you have, but I can tell you a couple of things about laundry detergent in general: For those who are not collecting gray water, powder is typically the best choice for the planet. But in this case, Judy, you’re right that liquid soap is the way to go: Liquid formulas are much lower in sodium. You might doublecheck with your machine’s manufacturer, but my hunch is you could probably switch to liquid soap without any trouble. (Head here and here for a list of brands recommended by the gray water gurus.)

Believe it or not, phosphorus actually isn’t a problem when recycling water on a mulchy soil surface. Plants love it, and it doesn’t lead to harmful algal blooms when filtered through your backyard dirt the way it does when it hits wetland ecosystems straight on. It’s nice to cross something off the Worry List, isn’t it?

As to the second part of your question: Do you need to mix your gray water with kitchen scraps before using it? I posed this to Laura Allen, founder of the California-based advocacy group Greywater Action, and she advises dropping this step. It won’t do anything to break down germs in the gray water — in fact, it only provides a better environment for pathogens to flourish. “It’s best to use gray water as soon as possible so it doesn’t go anaerobic and breed bacteria,” Allen said via email. “Skip the barrel and make your compost tea with rainwater or tap water. Stored gray water is just yucky.” So use that laundry water within 24 hours to avoid any issues with odors or germs.

I have one other wild idea for you to consider: Some people rig up a “laundry-to-landscape” system that actually pumps gray water from the washing machine right out into the yard, via special tubing. More work up front, but it might save you a lot of headaches — and bucket-lugging — in the long run.

May your fruit trees be bountiful this year, Judy! I hope you appreciate how much water and power your thoughtful practices have saved with every juicy bite.

Slate, cloud, and heatherly,
Umbra