Can I use bath water or washing-up water on the garden?

Which household water is safe on which plants, what in detergents and soaps actually matters to soil, and the rules that keep greywater useful rather than a health risk.

Difficulty: beginner
Time: 15 min
Read: 5 min
Safety: caution

Short answer

Bath, shower and hand-basin water is the best greywater and is fine on established ornamentals, shrubs, trees and lawns. Washing-up water is usable if it is not greasy and not heavily salted. Use it the same day, never store it, keep it off anything eaten raw, keep it off seedlings and acid-loving plants, and alternate with clean water so salts do not build up in the soil. Never use water from a washing machine's whites cycle or anything containing bleach or softener.

In a dry summer a household throws away a great deal of perfectly usable water. Using it is genuinely worthwhile and genuinely has limits, and the limits are about bacteria, salts and detergent chemistry rather than about squeamishness.

Safety

Greywater is not drinking water and grows bacteria quickly once stored. Never use it on anything you eat raw, never store it for more than about a day, never let it pool where children or pets play, and never use water that has been in contact with nappies, soiled clothing or raw meat. Wash your hands after handling it. Do not use it in a pond, which it will foul.

Step by step

Use it the same day.Greywater carries organic material and bacteria multiply in it quickly, especially in warm weather. Within a day of collection it starts to smell, and after that it is a nuisance and a hygiene problem rather than a resource.
Keep it off edibles.Nothing you eat raw, and nothing where the water would touch the part you eat. Established fruit trees watered at the base are a reasonable use; salad, strawberries and root crops are not.
Choose the right plants.Established ornamental shrubs, trees, hedges and lawns tolerate greywater well. Seedlings, young plants, containers and acid-loving plants such as rhododendrons, camellias and blueberries do not — greywater is alkaline and salty by comparison with rain.
Apply it to the soil, at the base.Never over foliage, both for hygiene and because soapy residue on leaves does them no good. Under a mulch is better still, since it goes straight into the soil and does not sit on the surface.
Spread it around rather than repeating on the same spot.Salts and sodium from soaps accumulate where greywater is applied repeatedly, and sodium damages soil structure on clay in particular. Rotate the areas you use it on and alternate with clean water or rain.
Check what is in your products.Choose detergents and soaps low in sodium and phosphorus if you plan to use greywater regularly. Avoid anything containing bleach, disinfectant, fabric softener or boron on the garden entirely.
Move it simply.A bucket, a siphon from the bath with a hose, or a jug into a can. Permanent greywater plumbing is a different proposition and is regulated — a diverted or stored system needs to comply with building regulations and water supply rules.

Tips

A bath holds a very large volume by garden standards. Siphoning one out to the border is one of the most effective single water-saving actions available in a dry summer.
Cool it first. Hot water straight from the bath damages roots and soil organisms.
Strain washing-up water through an old sieve or a piece of net. Food solids attract vermin and smell within hours.
Greywater use is unaffected by a mains hosepipe ban, which is exactly when it is most worth the effort.
If you are collecting regularly, keep a dedicated bucket and label it so it is never used for anything else in the kitchen.

Common mistakes

Storing greywater in a butt for later — It goes anaerobic and foul within a day or two, smells strongly, and becomes a genuine hygiene problem. Greywater is a same-day resource.
Using salted cooking water on plants — Sodium chloride is directly toxic to most plants and degrades clay soil structure. Salted pasta water is not a garden resource.
Putting greywater on containers — A pot has a small soil volume and no way of flushing salts through, so they concentrate quickly. Containers should get clean water or rainwater.
Using greywater on a vegetable bed — The risk is not theoretical — greywater carries gut bacteria, and root and leaf crops are eaten with soil contact. Use it on ornamentals and keep clean water for food.

Alternatives

Bath and shower water: The best greywater there is: large volume, low contamination, mildly soapy. Fine on established ornamental planting, shrubs, trees, hedges and lawns. Avoid it where bath oils, heavy salts or medicated products have been used.
Hand-basin water: Small volume but clean. Suitable for the same uses as bath water and easy to collect in a bowl in the basin.
Washing-up water: Usable if it is not greasy and not loaded with salt. Let it cool, strain out food solids, and put it on established ornamentals. Not on edibles, and not repeatedly on the same plants.
Cooking water: Water from boiling vegetables is fine once cooled, and mildly nutritious. Salted pasta or vegetable water is not — the salt does real damage to soil structure and to plants.
Washing machine water: Only from a rinse cycle, only from ordinary loads, and only if the detergent is low in phosphorus and contains no bleach or fabric softener. Collecting it is awkward on most machines and it is the least worthwhile of the sources.
Water from a dehumidifier or condensing dryer: Effectively distilled and free of salts, so it is good for containers and for ericaceous plants. Do not drink it and do not use it on edibles you eat raw, since it can carry bacteria from the machine.
Aquarium water: Excellent on the garden — mildly fertile and free of detergent. Freshwater only; never use water from a marine tank, which is salt.

Questions people ask

Will soap harm my plants?

Ordinary quantities of ordinary soap in bath water do not harm established plants. What accumulates over repeated use is sodium and, in some products, boron, both of which do damage — which is why rotating the area and alternating with clean water matters more than the soap itself.

Is it legal?

Using household water on your own garden by bucket is not restricted. Installing a plumbed greywater system that diverts or stores it is subject to building regulations and water regulations, and is a job for someone who knows those requirements.

Can I put greywater in a pond?

No. It will foul the water, feed algae and harm fish and invertebrates. Ponds top up with rainwater or, at a pinch, tap water left to stand.

What about water from a hot tub or paddling pool?

Chlorinated or brominated water should not go on planting. If the pool is unchlorinated and has been standing a day or two, it is fine on established plants — and a paddling pool holds a very useful volume.