Trimming the Stem
The first cut should be a ½” or more from the bottom of the stem. Use a clean, sharp knife to facilitate renewed uptake of water and nutrients.
Thorns and Leaves
Rose leaves function as the lungs of a plant. On their underside are pores, called stomata, that facilitate gas exchange. They allow the rose stem to breathe. Breathing encourages the flow of water and nutrients up the stem, allowing the cut flower to open and color up.
Leaves that fall below the waterline favor the growth of bacteria in the water. These leaves should always be removed with a clean, sharp knife.
Roses do have thorns—but the quantity and quality of the thorns vary quite a bit. When thorns and leaves are roughly stripped from the stem, that process can tear at the skin of the rose stem and can create an open wound.
Any cutting that is done to a rose, including the peeling of guard petals and de-thorning, leaves wounded tissue exposed to floating bacteria. Above the water line, the wound becomes a place where air and bacteria can enter the stem.
It is ideal to remove only those thorns and leaves that fall below the water line.
The outermost petals, guard petals, play an important role in the way roses open up and support their form. These petals have a protective function. If the guard petals are truly damaged or appear to be already in a state of decay, they should be carefully removed. But if they are simply a different color or tougher in texture, consider leaving them in place. In the fully open rose, often the outer petals are pushed down so that from above, only the edges are visible.