Robert W. Walton Jr.


roots growing up a slope

The roots of the eucalyptus in these photos started to grow up the bank, then grow out & down the slope. This is the way roots grow on steep slopes. This is why large trees on steep slopes fail.

roots growing up a slope
Fallen Pine on Bank .

Roots grow sideways and then down banks.
They do not grow straight into the bank because of nutritional requirements (oxygen, water, etc.). They also do not grow very far up a bank. This is due to geotropism, which is the gravitional effect on growth regulators within the roots. That is why roots grow down.

An unhealthy root system increases tree stress making the tree more susceptible to disease & insects.

Grass & trees do not thrive together in nature. Turf can reduce the amount of tree roots in topsoil by 90%. A tree with an underdeveloped root system will;
  · be slower growing,
  · predisposed to certain insect
  · and disease problems as seen
  · in the picture (right),
  · have a shorter life span.

Fallen Oak on Lawn
This live oak had been here for over 150 years until a lawn was put around it. The increased irrigation necessary for the grass was not compatible with this species. It led to Armallaria mellea, oak root fungus, which decayed the roots and led to complete tree failure. The decayed root stubs can be seen on the stump. Fortunately, no one was injured.
Installing lawn around an old tree lead to root fungus and tree failure.
Mulch around trees


Organic mulch is possibly the best and least expensive soil enhancer around. This is Decomposing mulch may be the only nutrients your tree will ever need. the way nature does it, as a tree mulches its own root system, returning nutrients to the soil.

A mulch covering should be 2 to 4 inches deep and extend outside the dripline.

Decomposing mulch may be the only nutrients your tree will ever need.



Mycorrhizae are fungi that form a symbiotic relationship with the root system of a plant. Mycorrhizae:
  • increase the absorbing capacity of roots up to 700%,
  • mycorrhizae are also able to access water in micropores in soil that are too small for roots. This is a survival strategy for many California native oaks growing on decomposed granite slopes.
  • increase a roots tolerance to drought, high soil temperatures, and a variety of soil toxins, high salinity, & heavy metals
  • It is more efficient for a tree to support mycorrihzae than to produce the equivalent amount of roots.

Deadman's foot,
Pisolithus tinctorius

This is a common mycorrihzae fungus found in hard, compacted or rocky soils in southern California. Very beneficial in reforestation, I commonly find it on hard rocky slopes around eucalyptus.


  • The top 8-12 inches of soil should be kept evenly moist within and just outside the dripline.
  • Water should be applied slowly (without run‑off) within and just outside the dripline.
  • Established trees only need to be watered every two to four weeks depending on the species and assuming watering was done as noted above.

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