Shelley calls the West Wind a destroyer because it strips all the leaves off the trees, tumbles them helter-skelter and piles them up all over the landscape. It is essential to dispersing them. But it also blows the seeds that will be sprouting when the weather turns warm again. And these seeds are scattered among the dead leaves as well as buried under them for protection. As the leaves decay they will provide compost to fertilize the seeds in the spring. The West Wind carries the winged seeds to their "dark wintry bed" where they are safe until "Thine azure sister of the Spring shall blow..." That is why the West Wind is both a destroyer and preserver--a destroyer of the old and preserver of the new.
O thou,Who chariotest to their dark wintry bed
The winged seeds, where they lie cold and low,
Each like a corpse within its grave, until
Thine azure sister of the Spring shall blow
Her clarion o'er the dreaming earth, and fill
(Driving sweet buds like flocks to feed in air)
With living hues and odours plain and hill:
The new seeds are an essential part of the debris the wind is blowing, and the dead leaves are essential to the germination of living trees, plants and flowers. This is why Shelley concludes his ode with the famous line:
If Winter comes, can Spring be far behind?