Many of the runes have an association with a specific tree, and there is certainly a connection between the Sowilo rune and the Willow tree (or more fitting is the trees alternative though now rarely used name – Sallow). Recently I have been looking into the overlap between the Sieg-Sowilo rune and the Hail-Haegl rune – and there is more evidence within the etymology of the word willow /sallow to continue with this idea.
Sal and Hal – Salvation and Health. From the root Sal we get Salt (a product of the sun [evaporation]) Salute (to greet the sun) and Salvation, from Hal we get to Hale (to breath) Hael (to Hail or Heil) and Health. Sal, from the Proto Indo European sal(i)k gives us the trees Latin name ‘Salix’. If we look at Celtic words for willow we see the name rooted in hal, as in the Welsh ‘helygen’ and the Breton ‘halegen’ – Holy or Healing Tree. The Irish word for willow is Sail, which we can compare with the Sieg rune ‘Sigel’ meaning to Sail (on the sea). It is from the Salix acid in the bark of this tree that we get aspirin – a word that itself has its roots in spir, again meaning to breath (respire).
The Old English word for Willow is ‘welig’ – from the root *wel which means to twist or turn. From this root we get words like withy, widder, wicker etc. In Old Norse this word is Vidir – maybe the tree is sacred to Vidar? If we look at the rune shape the Sowilo rune can be seen as a ‘twist’ – like that of the long twisting leaves of this tree.