The provides strength to those visible plants
The old things generally fade with time and become useless, but it is again not really true. There are many old things which do not fade with the passing time. People become experienced. The time teaches a lot of lessons which make things fade, but not necessarily lose their importance, in fact gain it.
Moreover, the poet says that it is the upper visible leaves and fruits on the tree and plants which are at the reach of the frost, but what matters are the roots of the tree which provides strength to those visible plants and leaves. Hence, one should not ignore the sacrifices made by the roots to nurture the leaves.
Next, the poet talks about the black ashes from the fire. The ashes are generally considered to be the symbol of destruction and failure, but the rising ashes here, denotes the revolutionary part. It means that every failure gives rise to that power which can revolutionize to achieve great heights and success.
The poet also talks about the light which shines from behind the shadows. The shadows are dark and dull which signifies the darkness, lost world, but one should not just look at the dark shadows but sometimes, the bright vibrant light emerges from behind the dark shadows provides with much more energy.
The poet, throughout the poem, highlights the importance of lost and deserted objects, which should not be ignored but one shall understand the relevance. Just like the broken blade is of no use, but this is not the case. A broken blade can be renewed and can be sharpened and can be used to cut many worries. Moreover, the king who lost his power, throw away from his crown, can also fight back and get his pride and power back by wearing the old crown which he lost during any war.
In the nutshell, the poet recognizes the worth of the so called destructed and lost weapons and objects of life, and how the useless considered things can be powerful again.