Today, when we grow trees in nurseries, they are often placed in a pot of loose ‘soil medium’ where both their water supply and nutrient source are constantly available to the emerging roots system. However, what is understated in importance is that air is also always available to those roots due to excess water draining away freely. These young tree root systems do not experience stress and as a result are generally small, compared to the volume of trunk and crown – healthy but imbalanced.
When we then take that nursery grown tree and place it in its final city home where the soils and subsoils have been subject to engineered design, the growing conditions are barely ‘friendly’ – thus city trees generally live only one quarter to one half of their normal life expectancy, if they manage to survive the first 3 to 5 years.
So, one of the first features of a Pocket Woodlands design is providing the loose/open soil that will give the air and drainage during those early years of establishment, and thus minimize ‘transplant shock’ which so often shortens a tree’s life.
It is essential that the roots be vigorous in these early years – this is fundamental to a healthy and long lived tree; it must be supported and enabled in its early life so it can adapt to the poor growing conditions of city life.
In a Pocket Woodlands® design this is done through berms, one meter or more in height, with a soil mix design that remains open and airy. In this familiar medium, the roots of the tree establish rapidly because the roots and mycorrhizae recognize what they need to flourish and so will propagate quickly, minimizing Transplant Shock.
Drainage freely occurs (as in the nursery) and so the roots/mycorrhizae occupy the soil mass rapidly. Vital access to water supply during dry summers is incorporated into the design – as would happen for other City trees – but in a Pocket Woodlands® design the water is able to be taken up by the vigorous root system and the tree grows throughout the full season, from May to August (in Eastern Canada). This is how the tree grew so well in the nursery.
The second feature of a Pocket Woodlands® design and one seen in nurseries as well, is that the trees and shrubs we plant on berms are no more than 3’ to 5’ apart. This is to provide the benefits of cooling of soils by the shared canopy.
Trees planted further apart than this find the new soil uninhabitable because it bakes during the heat of summer. By planting fast growing understory plants, along with the trees we want to shade and cool our cities, we replicate the naturally grown edge of a forest which has a similar mix. Examples of edgewood plants are Dogwoods, Redbud, Magnolias, Viburnums, Yews, etc.
A third important feature of a Pocket Woodlands design is succession. Part of succession planning is choosing to plant such species as Poplar, Cedar, and other shorter lived trees which we plan to remove after 20 to 30 years. Their rapid growth gives this new woodland both presence and shade whilst the slower growing but more long lived trees begin stretching into the already formed canopy.
This also means that our desired long-lived trees can be planted at a much younger age (and thus at less cost) because we know the first succession species will quickly nurture the developing woodland with its canopy and root support.
Another feature of the design is a berm covering of fabric and mulch to provide visual aesthetic enjoyment, physical access (for example, for children to play), and weed control. But most importantly for the tree is that it aids in cooling the soil mass in those early establishing years which are so primary for a tree’s longevity, thus helping to fulfill our aim in choosing to plant a tree at that site.