Shimmerleafs only appear around naturally-spawned silverwood trees. (Not when a silverwood is grown from a sapling.) The flowers drop normally but can be crafted into quicksilver. A shimmerleaf flower can be crucible-crafted into the Taint-purifying Ethereal Bloom.
Thuamonomicon Entry
The strange, glowing flowers only grow in the shadow of Silverwood Trees. Instead of nectar, these flowers produce quicksilver.
History
Before version 4.11, they did appear from sapling-grown trees, but dropped as quicksilver unless harvested with shears.