
When it comes to planting roses, just because you dig a big hole, doesn't mean you put the rose at the bottom of it.
In mild climate the best height to plant a rose is with the bud union above ground. In cold climates roses are planted with the bud union below ground level so the soil helps protect the union from freezing. But that is not necessary in California. It is better to plant the rose higher so the bud union is exposed to sunlight - some think this encourages basal breaks. A definite advantage to planting with the bud union above ground level is it makes it much easier to determine whether new growth is coming from the bud union (basal canes) or below from rootstalk (suckers.)
When determining proper height to plant, remember due to settling of soil the rose will end up lower. Also after the addition of mulch the rose will be covered up more. With that in mind, I generally try to plant so the bud union is 1 - 2 inches above the soil level. The exact determination is dependent on the length of the shaft or trunk (the area between the roots and the bud union.)
-BRH