Can I use bath water on the garden?

For established ornamental plants in the ground, occasional bath water is usually the lower-risk option. Use it soon, spread it around rather than soaking the same spot repeatedly, and skip it for vegetables, seedlings, and pots.

Check what went into the bath, not just whether the product says biodegradable. Avoid water with bleach, disinfectants, strong cleaners, lots of oil, or a heavy dose of salts. Biodegradable soap may break down more readily, but that does not make every ingredient harmless to soil or plants.

Keep the setup simple. Collect only what you can use that day, carry it out in a bucket or watering can, and apply it to the soil rather than splashing leaves. Don’t leave grey water sitting in a storage tank, since it can become unpleasant and is best used fresh.

I’d be cautious with bath salts. Epsom salt and table salt aren’t the same thing, and neither is a reason to assume the bath water is good for every plant. If you can’t tell what kind of salt or how much is in there, use that water somewhere other than the garden.