A windowbox with two halves

Nearly a year after its creation, the windowbox as two different sides. To the north (top photo) there are many plant species. These include the clover lawn with two kinds of cranesbill, a greater plantain, a rush with, just to its right, what I think is a small Himalayan honeysuckle. There are several sallows and, in the bottom left hand corner a tiny birch, the one that nearly died in the summer, then recovered. The different mosses grow better on this half of the window box too.

The southern half (lower picture) has very few plants and large bare areas. The Japanese maple seems alive and well but has lost its leaves for winter as has the small wild service in its bonsai pot. A dandelion to the left grows well and what I think is a petty spurge seems okay in the centre. There is also the only grass plant so far to have colonised hard against the sallow log. Groundsel which started well and spread well, now seems to have disappeared altogether.

Why there should be a difference between the two halves I cannot explain and it will be interesting to see if it is a long term problem.