Have you ever practiced a vase and been disappointed that a stem looks too short? You hold your flower up against the vase, feel like you have made one confident cut, and when you put it in the vase you see that it was too short. Sometimes a flower even disappears entirely behind the rim! Stem length is very important because it is not just about the stem, it also is dependent on the opening of the vase, the length of the stem, the angle of the stem, the weight of the flower, and the height that you want the focal flower to sit when placed inside the vase.
When you practice in the vase, cut with extra length in mind. Having more than you need initially is good because you can shorten the flower at a later date but you can never make the flower longer. This is especially true for your focal flowers which may determine much of the height and the general angle of your vase arrangement. If you cut them too short and too early, the flower may be too low in the vase and the arrangement can look flat before you add in your secondary blooms, fillers, or greenery.
When practicing in the vase, it is best to try out a flower against the vase before cutting the stem. Hold the flower against the outside of the vase and see where the bloom would be at the end of the stem if you placed it in water all the way to the bottom. Then, you may want to move the flower up and down a little so that you get a better idea of how it will look when you add in other materials. In this way, you will not be cutting too short. When your vase arrangement is designed to be low and does not need tall flowers towering above the rim, you still need enough length so that the blooms are visible and do not just get hidden in the opening of the vase.
You can cut the stems in small steps, cutting first just a very small amount at a clean angle, adding the flowers to water, and checking it from all angles in front of you. If you feel that the bloom is still a little high in the vase, make another small cut. You may have the time and energy to cut in several steps like this; you are training your eye to see more accurately what needs to be cut and you are learning that stems act in different ways, that some are more pliable than others and can bend easily, that some heavier blooms can cause the flower to sink to the surface, and that a stem of greenery may spread wider when put against the mouth of the vase.
The difficulty is when you have already added in flowers and other greens to the vase and are cutting into the mix. You want to cut the stem short and be done, but you may not want all of your flowers at exactly the same height, which creates a line across the top that may not be the most visually appealing. Focal flowers may be the same height or they can vary a little bit with your secondary blooms and fillers sitting slightly lower, and the greenery providing a softer edge. Stem length is part of the design; not all stems need to be the exact same measurement for a pleasing shape.
Turn your vase slowly when you have already added in your materials to ensure that you do not have one side that is too low in the vase, you have one focal flower that has sunk down into the foliage, you have an imbalance in the vase design. This is also a time to make sure that you have allowed some negative space, which means that you do not need to fill in every space; you need to add a stem that is tall enough that the flowers have a little room to visually breathe.
Think about this, does each cut make the arrangement easier or more difficult to read? If you cut the flower and the vase design feels easier to read, the flower seems more apparent, the proportion of the flower is better, a flower is no longer leaning in an odd way, then that cut made a difference. If the only difference was that a flower was now more hidden deeper into the arrangement than it was before you cut it, you may want to think twice before you cut any more stems! These small choices when you practice in the vase really add up and make a big difference.