Hi Jill,

No, I have never shaped Rosemary into a Christmas tree form. But, I know how I would do it. Actually, it was the two deer who got through my electric fence one night who taught me how to do this type of topiary. They chewed mine down to the stems and I learned that very tight pruning is the answer.
So, here is a suggestion for next year. As you know, Rosemary cuttings will easily root just tucked into the ground during the growing season. I would make a triangular form out of metal window screening. Set it on top of a large clay pot filled with a good draining soil mix. Now, set cuttings of the Creeping Rosemary (trailing form) around the base of the triangle. For a tree one foot tall I would use 20 cuttings. Secure growing stems to screen with a twist of fine brass wire available from hardware stores and keep all tightly pruned as it grows. This should work as I have been creating other forms with this technique.

For a regular Rosemary topiary that you may have, again, try tight pruning through the growing season. The secret is a lot of rooted cuttings. Do not rely on just one plant to do the job for you. If you wish to create a tree trunk then wrap or braid the stems around each other and keep new growth in that area pruned out.

I encourage you to work with the weeping form of Rosemary. It is far more adaptable.