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different growth rates and habits. Planting pattern and density should be chosen with respect to the
distances of trees from streets and buildings defined by the national Traffic Law and local regulations
on urban green (when adopted).
2.2.2 Digging
Digging and preparing the holes where trees will be planted should be done with some basic
technical criteria.
Holes should be broad enough to favor root growth: in general, a diameter larger of 50-60 cm than
those of the root ball is recommended, so that bigger space and better aerated soil can be explored by
the roots. Small holes, on the contrary, can limit root growth due to soil compaction on the sides that
should be definitely avoided due to the consequent risk of forming circling roots.
Holes should be wider near the soil surface as this is where most root growth occurs and deep
according to the size of the tree to be planted (do not plant too deep). It is far more acceptable to plant
in a raised manner to avoid drowning or suffocating the roots (leaving free the collar area).
Considering natural lowering of soil during settling period it is appropriate that the collar is at ground
level or a little higher. Planting holes should be dug 2 to 3 times wider than the root ball and only as
deep as the root ball. Wide, shallow holes encourage horizontal root growth that trees and shrubs
naturally produce.
A correct drainage system should also be prepared so that water can flow throughout the soil and
roots rot can be avoided; a layer of expanded clay on the bottom of the hole can be a solution, for
example. Digging can be mechanized, in lowlands area for instance, or manual. When digging with a
machine, such as an auger or backhoe, score the walls of the hole to prevent glazing.
2.2.3 Bedding out
The best time to plant trees is during the dormant season, in early spring before budbreak or in
autumn after leaf drop (Figure 6). Trees planted during this period suffer less from planting. During
autumnal-winter period, especially in Mediterranean environment, tree can profit of more raining
water needed for their growth and their roots have the time to acclimatize before shoot growth (this
rule holds also for plants cultivated in pots, which in principle, can be planted all the year round).
Figure 6
- Bedding out of arboreal species during dormant season (Autumn, as shown by the
absence of leaves in the tree on the right)
New trees plantations will have to be realized according to best agronomic and silvicoltural
practices. Good quality nursery stock must be chosen with some criteria: root ball should be
compacted and wet, and handled with care during the transportation (see 1.5). Finally the interval
between the taking from the nursery and the bedding out should be as short as possible, otherwise it is
necessary to keep root balls wet and protect them from heat.
La gestione del paesaggio forestale urbano: l’esperienza del Parco Nord di Milano a 25 anni dai primi impianti
. Atti del Terzo Congresso
Nazionale di Selvicoltura per il miglioramento e la conservazione dei boschi italiani, 16-19 ottobre 2008, Taormina).