Roses prone to fungal diseases will be finicky about how and when they get water, but many old roses such as Albas, Gallicas, and Damasks may do quite well enough without any extra water after their first year.
The best method of irrigation is underground or drip irrigation with a soaker hose. These hoses are about 50ft long and are constructed from recycled car tires.
They are full of tiny holes which let water out. They are ideal for watering all kinds of plants, and can be buried just below the surface so as to be virtually invisible.
The rule of thumb is that roses need an inch of water per week. If you live where it rains 1 inch per week every week, then you will not ever have to water your roses.
In places where summers are cool, roses will have somewhat lower water requirements. And in the blazing heat of the Great Plains, two inches per week may be a better number, especially for large plants. In places where roses go dormant during the summer, this also mitigates their water consumtion considerably.
Roses need water. Without it they will die. Budded roses fail frequently to thrive in a new environment because of lack of moisture; their tender feeder roots have been destroyed by the transplanting process. Therefore, careful watering during the first season, especially during the first six to twelve weeks is absolutely crucial to the success of a newly transplanted rose. Own root roses purchased in pots will be just a little more tolerant of moderately dry weather.
Roses, fortunately, are deep-rooted plants. Deep-rooted plants have the capacity to extract water from the sub-soil long after the surface has become dry. Finally, many hybrid teas and teas have relatively few leaves. All of these factors combine to make most rose cultivars fairly tolerant of moderate dry spells once they are firmly established..
This said, a rose will be at its best if it is well watered. It will bloom well and it will be more resistant to disease. Leafy cultivars such as Albas and Rugosas my require more regular watering than more sparsely leafed types.
During the first year after transplant it is imperative that the roses receive an inch of water each week, either from natural or artificial sources. During the first three months water daily. After the first three months, water at least twice a week. Established roses will need watering less frequently, but still an inch of water per week is roughly what a rose will require.