If your rose bushes are the type that only flower one time each year, they should be trimmed after they are finished blooming. In Arizona, this will usually be in the early summer, before the temperature reaches the extreme highs of July and August. This gives the rose bush plenty of time to grow new branches before the next flowering season. Because of this, you can trim once-flowering roses shorter than other rose bush varieties. Trim the long vertical stems so that they are about one-half to two-thirds of their original size.
Rose bushes that flower more than once during the season should be trimmed before the first blooms have opened up. The general rule is to trim this type of rose bush in the spring, but this can be tricky in Arizona, where the temperatures rise quickly as soon as winter ends. As long as you do not expect a late winter frost, you can begin trimming your rose bushes in February or early March. The bush itself will give you signs that it is ready for trimming. Look for buds that are beginning to swell. Trim out any dead branches that may be in the way of the new growth. New branches should be trimmed to leave only two or three flowers on each branch. This prevents the branches from drooping under the weight of too many flowers. After the season's first flowers have dropped off, go through the rose bush again and pick out any dead blossoms to clear space for the next batch of flowers.
Do not trim your climbing rose bush if it is still young. Climbers need two or three full seasons of undisturbed growth in order to build up strong inner branches and a thick top layer of growth. If your rose bush cannot develop enough foliage on the top, it will not be able to produce many blooms during the flowering season. After the rose bush is fully grown, it can be trimmed so that the side branches are between 4 and 6 inches long. The vertical canes do not need to be trimmed unless you want to keep the height of the rose bush under control.