It makes sense to try and reuse water, but cooking water isn't as benign as you'd think.
You might have considered recycling water from showers, tubs and the kitchen for your garden. The theory is that it’s free water and whatever was in the water will provide nutrition for the plants. In the most general sense, this tracks—it’s water; you should reuse it. But this advice ignores some basic garden wisdom: You should not reuse water in your garden if there is something in the water besides, well, water.
For instance, if you use leftover cooking water, there’s likely salt in that water. Salting your soil is a terrible idea. Salinity prevents your plants from uptaking H20, which means you are doing the opposite of what’s intended. You salt your garden when you want to kill things. If there’s citrus in the water, it’s going to throw off the pH, and deters earthworms. The acidity could be high enough to burn a plant. The same is true of onions in the water.
Your shower and tub water, meanwhile, is likely to have soap and shampoo in it. In some cases, this is okay—soap isn’t always a bad thing. In fact, soapy water, when applied to the surface of some plants, is a great pest deterrent, such as with aphids on roses. But the ratio of soap to water has to be specific—you don’t want to exceed 2% soap to water, or you’ll burn your plants. What’s in the soap is important, too—usually when used as a pest deterrent, you’re applying a very mild dish soap. If there’s something in your body wash or soap that is toxic to plants, you shouldn’t use the water. Even if it’s not toxic, it can throw off the pH of the soil, and plants usually want a balanced pH.
Consider the microplastics in some body scrubs and shampoos as well, which would be going right into your soil. If you have an outdoor shower that drains into the soil, be careful about what you're using. (By the way, you really don't want to reuse mop water in your garden—the cleaners used are way too harsh.
Since we don’t want to throw the baby out with the bathwater, here’s where I think it’s okay to use leftover H20 in your garden. Rainwater is fantastic, and whether you save it in open containers or a reservoir, it’s great to repurpose this free water. And if you use extremely basic cleaning products in the shower or bath that contain no salt, preservatives, or microplastics, you're clear to use that water in the garden.
