As a gardener I know how important roots are. Without a strong and well-developed root system a plant will not thrive. It will not reach it full potential in it own right or as a valued contributor to the garden as a whole. We can prune and fertilise, water and protect, but without strong roots, the plant withers and dies. The Celts were right to worship the Oak.
Oaks have a very strong root system.
They need it for their large dark green leaves to offer shade and protection in the Summer, to provide a vibrant display of Autumn foliage and to survive harsh Winters standing bare against the cold. Oaks naturally grow in groves, larger stronger trees offering protection to seedlings and each tree relying on the strength of its neighbours to survive. If an Oak is weakened and crashes to the ground it can take down a lot of other Oaks with it unless they in turn are strong enough to withstand the assault. So it is for people. We too need a strong root system.