Our Spike Witchhazel has burnt leaves throughout the plant on the
tips. What could be the cause of this? This is its third year in the same spot with full sun.
Are you referring to Corylopsis (sometimes known as Spike winterhazel or Buttercup
winterhazel), or to a Hamamelis? (They are in the same plant family, but
look quite different). Here are some pictures for comparison:
Hamamelis family (Hamamelidaceae)
Corylopsis leaf edges do have a tendency to get sunburned in full sun and
windy sites. They prefer a partly shady spot, and organic, acid,
well-drained soil, although they will do well enough in full sun (except
for the burnt leaf edges). Again, though, I would be concerned about
fertilizer burn as well. I never use fertilizer of any kind, just the
occasional addition of organic compost, on my Corylopsis, and it does
nicely in a mostly sunny spot.