How can I cut back an indoor Norfolk pine?
You might consider propagating a new plant from a cutting of the original plant. Here is some information I found from several sources on how to do it:
Cutting for Quick Results
1 Fill a 4-inch plastic pot with sanitary, fast-draining medium such as peat moss or coarse sand. Pour water onto the medium and stir it around until it feels moderately moist throughout. Let the excess water drain from the holes at the bottom of the pot for 10-to-15 minutes before potting the Norfolk pine cutting.
2 Gather a 6- to 8-inch cutting from the terminal leader of the Norfolk pine, which is the vertical stem located at the top of the plant. Sever the cutting where it emerges from the symmetrically arranged cluster of side branches using a utility knife. Do not use a side branch as a cutting because it will create an irregularly shaped tree.
3 Strip off the needle-like foliage from along the lower half of the stem. Dust the stem with rooting hormone powder, then tap it to knock loose the excess powder. Create a planting hole in the medium that is deep enough to hold the leafless part of the Norfolk pine stem.
4 Slip the cutting into the prepared planting hole, taking care not to scrape off the rooting hormone. Hold the cutting upright and push the medium snugly against the stem until it can stand up on its own without leaning. Mist the cutting liberally with water.
5 Cover the pot with a large plastic freezer bag to hold moisture around the Norfolk pine cutting. Secure the bag around the pot with a rubber band. Make sure the bag doesn't rest against or touch the cutting because it may damage the foliage or cause it to rot.
6 Set the pot in a bright location indoors near an east- or north-facing window. Shield the cutting from direct sun exposure. Lift the plastic bag daily and mist the cutting. Moisten the medium if it feels nearly dry 1 inch below the surface. Always let the medium dry out slightly on the surface.
7 Tug on the base of the cutting in eight-to-10 weeks to feel if it has rooted. Remove the plastic bag and transplant it into a larger pot filled with a peat-based potting mix once it roots. Continue to mist the Norfolk pine cutting regularly to keep the foliage healthy.