Planting under Spruce Trees

The Scarboro Garden Scene

It is difficult to grow anything under spruce trees … apparently there is a “law” (an
alternative fact?) that spruce needles acidify the soil below the tree. There is some truth to
this, but not in Calgary where lack of moisture is the critical factor, and tests done by
several gardening experts have shown no increase with time in soil acidity. Spruce
foliage deflects rain and snow away from the drip line (the “umbrella” effect), and
moisture that penetrates the foliage may evaporate before it falls to the ground. Mature
Colorado Spruce (Picea pungens) roots are massive and grow at and just below the
surface of the soil, thereby reducing the available porous soil, available water, and
nutrient store, so planting and growing anything between the roots is very difficult.
A study in Denmark showed that soil under Norway Spruce contained about half of the
moisture of the soil found beneath a variety of broad-leafed trees and the moist Danish
climate did produce a slight increase in soil acidity under the conifers. We know that
Calgary soils are typically alkaline; i.e., they have a pH of greater than 7.0 - the alkalinity
is due to the abundant limestone (calcium carbonate) and dolomite (mainly magnesium
carbonate) in the catchment area of the Bow and Elbow rivers - so any slight increase in
acidity in our soils could allow us to grow a greater variety of healthy plants.
Lack of light also influences growing conditions beneath spruce: if you have a spruce
without low branches more light will be available, but branch removal is hard on the
tree’s health – and takes away some of the visual appeal. Disguising the lack of
vegetation by planting outside the drip line or adding pots of shade-tolerant annuals is a
solution – and don’t worry about gathering up the cones beneath the tree – there’s no
need.
Glynn Wright.