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301.3: What plants, shrubs and trees really want

“Plants prefer to keep their roots in the topsoil where most nutrients are located, IF the topsoil is constantly moist. Graywater gives us constant moisture, and with roots in the topsoil, plant growth to maturity is much faster than with traditional irrigation.”

During times of drought, water purveyors (and horticulturists) all tell us to water deeply, once or twice per week to encourage the plant / tree to send its roots down as far as possible.

This is great advice, IF (and only if) there is not a regular water source.

What they don't tell you is that if the plants / trees were watered everyday, 30% less water would be required.  This is because the plants cannot use lots of water immediately, and a good 1/3 of the water goes down into the subsoil (and keeps going down) unused by your landscape / turf. (Their concern is that most people would over water if they watered everyday - especially if using a garden hose).

So if you want to save water - water more frequently, with lesser amounts, every day. But how does this relate to what plants need to grow?

What plants need to grow:


  • 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 realize.

A diagram from Roots Demystified, by Robert Kourik
(Metamorphic Press)


Most nutrients are found in the top 3" 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 6".

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 3" 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.

Think we are crazy?

I and my staff have installed over 500 graywater systems in Australia and the US (and sold 6,000+ more), with many or our clients being horticulturists. All of our clients report garden health and growth far in excess of gardens being heavily watered (with a lot more water) once or twice per week.

But don't just take our word for it, visit Robert Kourik's website, he is the expert in obtaining better growth rates with less water. I strongly recommend his book, Roots Demystified (Metamorphic Press), it explains how plants want to grow.

You could also research commercial growing locations where they are moving away from deep watering to watering with a small amount every day (some cases every second day). It's why Netafim's (worldwide irrigation system manufacturer) motto is Grow More with Less.

Fertilizer and Soil Amendments:

If the existing soil is poor, and holds little organic matter in the first 3", soil amendment should be applied. However, it is vital that the amendment is spread evenly over the entire garden area.

This is the opposite of what most people do when they buy a new tree or shrub.

  • First they dig a hole, a little larger than the root ball of the plant.

  • Then they throw in some plant food / fertilizer in the hole, add lots of water and plop the plant in the hole.

  • If they have a potable irrigation system they then add an extra dripper right next to the plant.

This creates the following problems:

  • The roots of the plant quickly find an abundant source of food, therefore they do not need to grow any more to find food.

  • The roots also have a water source that is wetter below the plant than the surrounding topsoil.

Rather than seeking out water and food, this plant will now need regular feeding, instead of using the soil for food.

This diagram was taken from a nursery website - I won’t name them here!

Adding soil amendments to the hole creates a growing barrier for the tree roots.

I have added the red ellipses to show where the barrier will occur.
The roots are encouraged to stay in the nutrient rich environment (in the hole), rather than expanding into the natural soil.

If the soil quality is poor, consider broad area fertilization, with mulch and/or slow release fertilizer applied at the surface.

IrriGRAY University

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