The Bud Union

Bushes of modern roses are really a grafted combination of two separate rose varieties.  "Rootstock" is the part underground.  It comes from older varieties known not for their blooms, which are usually insignificant, even ugly, but rather for their capacity for massive root development.  To this rootstock a graft is made of the rose variety desired to grow above the ground:  the hybrid.  Where the graft of the two varieties is made, globular bulbous landmark develops that forever bears testament to the marriage.

The bud union is the heart of the rosebush.  It is from there that major new growth will develop when conditions are right.  Also, it is the point from which planting depth is measured.  Assuming you mulch after planting, and I'm going to try very hard to persuade you to , the bud union can be placed above ground, level with the height to which you plan to mulch.  Mulching materials will still cover the bud union sufficiently to protect it from freezing.

If, however, you live in an area where winters are hard and temperatures are extreme (below 20 degrees F.), you can't afford this luxury; for you proper planting usually requires the bud union to be at or below soil surface.