Whether you can replant a cut flower depends on how much of the stem is attached and whether there are nodes, or places where leaves attach, on the stem. Nodes are growing points, areas where roots can form if the stem is in contact with the soil. Without roots, the plant has no way to gather moisture or nutrients, so their formation is critical to the regrowth of the flower.
Preparing a Cutting
If you wish to root a flower stem, do so as quickly as possible after the stem has been cut for the best results. First, find the nodes on the stem of your cut flower. You need at least two of them: one to form roots and the other to sprout leaves. Many commercial cut flowers have no leaves attached or leaf nodes, and so they will not root. Cut the stem off about 1/4-inch below the bottom node and above the top node. Cut any leaves attached in half to reduce water loss. Dip the bottom end in rooting hormone powder, which is available at most nurseries.
The soil for rooting a cutting should be airy and well-drained but still moisture-retentive. You can use half sand and half peat moss, or half perlite and half peat moss. Don't add any soil to the mix to avoid introducing diseases. Fill a small pot with several inches of the soil mix.
Planting the Cutting
Push the stem of your flower about an inch into the mix and firm the soil around its base. Put the pot inside a light plastic bag, inflate the bag by blowing into it and tie it with a rubber band. This will keep the cutting moist until it has roots to absorb water from the soil. Place the pot in a warm place where it will get light but no direct sun.
Add water to the pot if the surface becomes dry and, after a few weeks, gently pull on the cutting to see if it has grown roots. Not all cuttings are successful, so don't be discouraged if your first attempt fails. If it has rooted, the cutting can be planted in a small pot as soon as new leaves begin to expand from the top.