Why is Greywater better?

Re-using greywater in the garden is not only about saving water & saving money!

Gardens grow better (30-50% better) with greywater than fresh water out of a hose, or even an automatic freshwater watering system.

It's not just because the greywater has extra nutrients, but how the water is irrigated.

During droughts, Australians are advised by the water companies and garden nurseries to water deep, twice a week. This is an easy message to sell, and stops people from  over watering every day.

Because this method is deeply entrenched into our water saving brains, the following concepts might take a couple of reads to understand - but once you understand it, the whole concept oof greywater drip line irrigation will become clear.

Forget what you were taught years ago....

Watering twice a week actually wastes 30% of the water, and doesn't help the plants to grow (although it does help to keep them alive).

When a garden is watered twice per week, the top soil goes dry in between watering days. Within a couple of days, the soil is dry enough to repel the water. This stops the water from moving sideways through the soil, and the water is instead drawn down into the subsoil. This helps to promote tap root growth (assisting the plant to find moisture deep down during a drought), but doesn't help with lateral (sideways) plant root development.

Unfortunately the water keeps moving down, past the roots and a good 30% bypasses the plant completely.

Daily irrigation of Greywater keeps the water in the top 100 - 150mm of topsoil.

The soil / water interaction is very different if the soil is irrigated every day (or even second day). For most households, daily irrigation is exactly what happens with greywater - we make it every day, and have to irrigate with it every day (can't store it for more than 24 hours anyway).

Instead of the soil drying out, the moisture spreads sideways due to capillary action. We have a separate page about capillary action here. The end result is that a garden bed 2 metres wide only needs a single drip line straight up the middle. We don't care where the plants are, the daily irrigation from the single drip line ensures the entire garden bed has a constant moisture level, enough for thirsty plants, not so much that dry plants suffer from root rot.

What plants really want..

• Room to Grow, Temperature, Light, Air & Time
• Water & Nutrients

I can't do much for the room to grow, light, air or time requirements for your plants. But I can explain the basic interaction between water and nutrients.

The vast majority of plants and trees receive their water and nutrients via the root structure.

Roots can (and want to) travel much further than most people realise.

Nutrients:

Most nutrients are found in the top 75mm - 100mm of soil - the biologically active layer. The further down we go the less nutrients there are. In many soils there is little nutrient loading below 150mm.

It would make sense for a plant to grow most of its root in this layer - right?

Water (moisture):

When a garden is irrigated daily (with a small amount of water), the top 75mm - 100mm of soil acts like a moisture blanket (see capillary action), the roots and organic matter, together with bacteria in the soil trap the moisture here, right where the roots are also looking for nutrients.

However, if the garden is only watered once or twice per week, the roots are trained by you to go as deep as possible, away from the nutrient supply.

Therefore, the plant / tree is forced to spend more energy growing more roots than it ideally wants to.

There is a common misconception that the roots of a healthy tree extend out to the foliage line. The roots actually extend up to 3 times this distance if the ground has sufficient nutrients and water.