Fill a 3- to 4-inch container with a potting mixture of half sand and half commercial potting soil. Any container with bottom drainage will work.
Mist the potting mixture so that the potting mixture is damp clear through, but not soggy. Too much moisture can cause the gardenia stem to rot. Put the container aside while you take the gardenia cutting.
Cut a 3- to 4-inch stem from a healthy gardenia bush. The stem should be flexible, but should break with a snap when bent. If the stem bends but doesn't easily break, the stem is too immature. If the stem is so hard that it won't bend, it's too old. Make the cut using a sharp knife or pruners, with the cut just below a leaf node, which is where a leaf or bud grows from the stem. Each stem should have at least three or four leaves.
Remove the leaves from the bottom half of the stem, leaving two upper leaves remaining. Cut the upper leaves in half horizontally with clean kitchen shears, removing the tip of the leaves. Smaller leaves will require less water and will take up less room in the container.
Roll the cut end of the stem in powdered rooting hormone. Plant the stem in the container with the leaves above the soil. Several gardenia stems can be planted in the same container, as long as the leaves aren't touching. Mist the potting mixture lightly to settle the mixture around the stems.
Cut the bottom from a 1-liter plastic soda bottle. Put the plastic bottle over the container, but don't allow the plastic bottle to touch the leaves . Place the container in a sunny spot, but avoid direct sunlight and hot windows, as heat will build up in the plastic and can burn the stems.
Check the potting soil daily. Although the plastic bottle will keep the environment humid, the soil should be misted immediately if it dries out. Lift the bottle two to three times each week and mist the gardenia leaves. If condensation builds up inside the plastic, remove the lid from the plastic bottle for a few hours to allow air circulation.
Watch for roots to develop in two to three months. The appearance of new growth is a good indication that the gardenia stem has rooted. You can also test by tugging lightly on the stem. If the stem has rooted, you'll feel resistance to your tug.
Remove the plastic bottle from the container, and move the container to a sunny window. Keep the soil damp.
Allow the roots to develop for one to two additional weeks, then transplant the stem into a 6-inch container filled with commercial potting soil. Allow the gardenia to grow for at least three to four more months before planting the gardenia outdoors. Allowing the gardenia time to grow larger will increase the chances of survival once the gardenia is planted outdoors.