When a seed germinates, two types of roots may form: Taproots: One long root diving deep into the soil, looking for water, and providing stability for the plant. Many plants grow one of these roots in the early stages of live. The second type of roots are surface roots. These roots spread out wide around the tree. They grow just below the layer of decomposing organic matter that makes up the first layer of soil (consisting of old leaves, broken branches, treetrunks etc, the humus layer). With time and maturing of the tree, this second type of roots will typically replace the deep taproot. These are the roots with quickest acces to nutrients that are freed from the organic matter during decomposition and get a hold of rain water as if comes down. It is also these roots that form the basis for the nebari.
What is a good Nebari
A mature rootsystem will spread evenly around the tree. That means that the roots forming the nebari should roughly be of the same thickness all around, and be nicely distributed around the trunk. So a trunk with roots only on one side is in most cases not suitable for upright bonsai, unless a lot of work is done on developing a nice nebari (One common solution would be to graft roots from seedlings on the trunk). Considering how the roots also provide stability for the tree, the roots should give the impression of a strong connection to the trunk.
There are no clear guidelines on the ideal number of roots. This number naturally depends on the size of the trunk. When you have a bonsai with a thin trunk of say 1 cm (~1/2 inch) diameter it would look really odd to have 20 roots forming the nebari. However, having 3 roots would be too few to look pleasing and balanced. Although I have not been able to find any clear guidelines on this, one can come to a estimate of a nice number. For me personally, the most pleasing balance between thick stable roots, and a high number of roots comes in at about one root every 20 to 45 degrees of tree-trunk. (As a full circle has 360 degrees, this means you are looking at 8 to about 18 roots). With a thinner tree, use less roots than for a bigger tree.
As a tree develops, the roots develop too. As the roots get thicker, shallow of roots may push trough the layer of humus and become visible. As roots have a tendency to grow down, the shallowest part of the root system is close to the trunk, gradually dipping deeper below the soil surface. So a good Nebari will consist of roots that graduallly disappear from view. Furthermore, roots do not grow in a razor-straight line. They may encounter spots of nutrients, water, hard soil pockets, or soft pockets. All these factors affect the shape of the root. So it is very normal for roots to be as twisted as the branches in the crown. What the main roots typically do not do, is cross other main roots. Competition for nutrients and water in the soil will pretty much always result in the main roots spreading out evenly from the trunk, without crossing onanother. Finally, as roots grow, just like branches, side-roots will form, resulting in branched roots with taper. This should be reflected in the nebari, if the roots stay above-ground for a little distance beyond the immediate vicinity of the trunk.
So, in summary a good nebari has:
– Roughly 8 to 20 roots that are evenly distributed around the trunk
– Roots that twist and turn a little, but do not cross one another (Certainly not above-ground)
– Gradually disappearing roots
– Roots with a strong attachment to the trunk
– Ideally tapered roots
* Harris et.al. Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. 1999.