I have a question about Cryptomeria japonica ‘Black Dragon.’ I would like to know the approximate diameter and depth of the root system at full growth. I am trying to convince the local cemetery to permit me to plant one there.
According to Richard Bitner’s Conifers for Gardens (Timber Press, 2007) Cryptomeria japonica ‘Black Dragon’ typically grows to about 6 feet tall. As a dwarf form of C. japonica, I would expect its roots not to be much of a problem. The (non-dwarf) species can grow to 160 feet, in which case, roots would extend a considerable distance. The local website, Great Plant Picks, features the variety ‘Black Dragon’ and says it grows to about 7’H x 8’W.
Here is general information on trees and their root systems, from Tree Roots in the Built Environment (Roberts et al., Centre for Ecology & Hydrology, 2006):
About root depth:
“A further misconception about tree roots is that they occur typically in significant quantities at substantial depths (i.e. greater than 3 m.) in the soil profile. […] from numerous studies involving comprehensive root excavations the indication is that typically as much as 90% of the tree root length occurs in the upper metre of soil.” According to this same source, conifers usually have about 70% of their roots in the upper 50 cm. of soil.”
About root extent:
“[…] large species differences exist but it is also the case that the horizontal extent of tree roots substantially exceeds the perimeter ‘dripline’ of the crown. […] there is a good relationship between crown spread and root radius but the relationship tends to be very species specific. Roots extending furthest from the tree trunk are usually found in the soil surface. […] the maximum extent ot the tree roots is reached before the canopy has completed expanding, suggesting that the ratio of root spread to crown spread may decrease as trees become older.”
This same source also says the root extent is highly dependent on soil environment (richness of soil, access to water and nutrients).
To summarize, what all this means is that the width of your tree’s crown will give you only some idea of the extent of the roots, and most of those roots will be shallow.