what type of fertilizer is best for salix integra flamingo

Not particularly. Just use Growmore for feed, bit of organic compost in the planting hole, or garden compost if you've got it, with a bit of Blood, Fish and Bone mixed in at the bottom of the hole.

I wouldn't particulary fertilize a Salix. It will like a humus rich soil that will retain a bit of moisture and keep its roots cool. Add some garden compost or bonemeal if you like.