Treated greywater irrigates a garden best when it flows fresh to the soil beneath mulch or through a buried pipe, to fruit trees and other plants whose edible parts stay clear of it, never sprayed and never poured over leaves or root crops. At a crop need of 5-8 mm a day, 100-200 L of greywater a day waters only about 12.5 to 40 m², so it supplements a garden rather than carrying it.
Part of Greywater Recycling Systems
Greywater irrigation is the use of treated household wash water to water plants through the soil. It saves carrying and pumping fresh water, but only when the water goes to the right plants by the right route.
Three Rules Before Any Method
- Fresh, not stored. Greywater is used within about a day of being made. A store turns septic (Contamination Risks & Limits).
- Under the surface or under mulch. No sprinklers, no puddles, no pouring over plants.
- Soil, not leaves. Water goes to the soil at the base of the plant. Keep it off leaves, fruit and anything eaten raw.
The Two Methods On The Parent Page
| Method | Best for | Page |
|---|---|---|
| Mulch basin | Fruit trees, large shrubs, other big plants | Mulch Basin Distribution |
| Subsurface pipe or drip | Beds of established plants | Subsurface Drip Systems |
Pick plants that suit it (Suitable & Unsuitable Plants), and have a diverter so the flow can go to a dry well when the ground is frozen, saturated or someone is ill.
How Much Garden Can It Water?
Inputs: crops need about 5-8 mm of water a day, which is 500-800 L per 100 m² (Irrigation); a family of four makes 100-200 L of greywater a day.
- Largest area = 200 L ÷ 5 L/m² = 40 m² (high flow, low demand).
- Smallest area = 100 L ÷ 8 L/m² = 12.5 m² (low flow, high demand).
So greywater waters 12.5 to 40 m², about one or two garden beds, in the growing season. The parent page says greywater is “enough to irrigate a significant garden area”, which is generous at these figures. The gravity-plumbing page gives garden irrigation as 50-200+ L per 100 m², far below the 500-800 L the irrigation page uses; the two pages disagree by roughly four to ten times, and I have used the irrigation page’s figure because it is the more cautious one for planning. In wet seasons and winter, the crop needs less and the greywater should go to a dry well.
What You Need
- Treated greywater from the stages in Filtration Systems
- Distribution: mulch basins or buried perforated pipe
- A diverter valve to a dry well or drain field for rainy or frozen weather
- Mulch: wood chips or straw, 10-15 cm deep
Method: Start Simple
Step 1: Choose the plants first, then the route.
Step 2: Lay the pipe so water runs by gravity with a steady fall to the garden.
Step 3: Water the soil under mulch only.
Step 4: Rotate the beds you water so salts and soap do not build up in one place.
Step 5: Check the plants and the soil each month.
Common Mistakes
- Using sprinklers. They throw organisms onto leaves and into the air.
- Watering root crops and raw greens. Contact with contaminated soil and water.
- Watering every day in the same spot. Soap and salt accumulate.
- Using greywater in rain. Saturated soil cannot absorb it.
Safety
Greywater is not drinking water and carries disease organisms
Never drink it, never use it for washing food, and keep children and animals from wet beds and basins. Keep every pipe away from the drinking supply (Plumbing Separation). Wash hands after working in a greywater bed.
What’s Next
- Learn next: Mulch Basin Distribution — the simplest way to spread it
- Combine with: Suitable & Unsuitable Plants — what to water