Part 3: Watering Trees

Good tree root health

We’ve discussed oxygen and microbial life as essentials to the soil food web. Obviously, water is the other essential. The reason for its lastly placement in the discussion is quite deliberate, though. Although water is absolutely essential to plant life, in the case of trees in the built environment it may not be very helpful to provide water to the surface without preparing the soil to receive it. Let’s discuss this.

Most of us irrigate our properties in the conventional way of simulating rainfall with some kind of spray system. This has a wonderful effect upon the shallow-rooted plants. Turf will thrive when given a moderate amount of water from a spray head irrigation system. The water that is delivered soaks in just enough for the grass roots to get what they need. However, beyond the grass roots level there will be little to no saturation.

In fact, repeated soaking of a lawn in this way will in many cases compact the deeper soil in the depth zone that would be optimum for tree roots. The weight of the added water just continues to compact the deeper soil, thus worsening the absorption barrier at the line between the A and O soil horizons. As we know, compacted soil prevents root progress because of its inability to exchange gas and water.

So what we see in lawns that are irrigated conventionally is tree roots growing at or near the surface in the only space that they can live. This creates problems for both the turf and the trees. The competition between grass and trees is an unnatural situation. After all, in nature we see meadows and forests, but not in the same space. In landscapes and built environments the soil must be manipulated in order to support both in the same space.

The way to accomplish this is to break the absorption barrier so that oxygen and water can migrate deeper and promote a soil food web throughout the different soil horizons. In sophisticated (and expensive) systems this is accomplished by adding tree root zones to the irrigation system. Water is delivered at depth in chambers that aerate the water and allow it to soak into the tree root zone.

There are ways to achieve this on a more modest budget, though. Spray irrigation water, ground water, and precipitation can be channeled through the absorption barrier with rootwells or with vertical mulching holes.

The key to a lush, thriving landscape is always in the soil. A healthy soil food web will support the plants, grass and trees in harmony. Consult an arborist to help you to achieve this in your landscape.