Since stem length is a primary objective, flowers that grow on long, unbranched stems are cut as close to the ground as possible. Examples are sunflowers (many varieties do not branch) and gladiolas and other bulbs. Sunflowers and glads are typically cut 30" to 36" long.
When cutting bulb flowers with multiple flowers or florets, cut stems as soon as the first florets on the stem begin to open. If possible, cut the day before the first bud opens (learned by observation) so that the unopened blooms will get less battered when bunched together. Push the leaves away from the stem and cut deep down into the foliage, trying not to cut the leaves of the plant. Bulb foliage is the energy factory for the plant and is required for next year’s flowers. For this same reason, only remove 2/3 of the lily stem so that some of the leaves remain to nourish the bulb. If growing bulbs as an annual crop which will be pulled out and discarded, ignore the foliage rules and cut down to the ground. When cutting daffodils, give them their own bucket. They exude a sap so thick that it will plug the stems of other flowers in the bucket if they are all placed together right after cutting.
Branching plants are cut with an eye towards keeping the plant branching and producing additional flower shoots. Celosia is cut low, but not all the way to the ground, as it will produce additional branches after the main stem is cut. Larkspurs also produce secondary stems. Marigolds and zinnias frequently produce a large central flower on an unsaleable short stem. Remove this first flower to hasten flower development and stem lengthening of the lower branches. When cutting branching plants always cut in such a way as to leave at least one leaf node remaining on the branch attached to the main stem. A new shoot will originate at the node and produce additional branches and flowers. If branches with leaf nodes are not left, the entire plant will be cut down after harvesting only a few flowers. If a node is left for new shoot development, the plant can stay in production for as long as 6 - 8 weeks. Zinnias are typically cut 15" to 24" long, marigolds 15" to 20" and celosias 24" to36".