Why a sharp knife?
A knife is recommended!
A sharp knife will allow you to cleanly cut just about any flower stem. A scissors or a clippers maybe more comfortable to use but can crush the tiny vessels that transport the water from the bottom of the stem to the bloom.
Scissors can be used to cut excess foliage from the stem (hydrangeas for example) and a soft cloth or rose stripper can be used to remove foliage and thorns from rose stems. Sharp clippers or pruner can be used to cut heavy wooded stems such as flowering branches, viburnum or curly willow.
See our Florist Tools section for knives and rose strippers.