The following information has been gleaned from expert rose growers and Consulting Rosarians who have written on this topic. I have synthesized their experience and put in my own two cents.
What happens if we stop watering our roses during a long dry period? Paul Zimmerman says that roses will drop some leaves, the blooming stops; but they do not die and later after a normal rain, they will recover and be as beautiful as ever. A plant’s defense mechanism during drought is to go almost dormant.
Their other defense mechanism is to push their roots deeper looking for water. These roses may become established and able to fend for themselves. They adapt to changing weather patterns. These roses had been forced to push their roots out of their artificial watering “comfort” zone.
Soil types are different and you should know a lot about your own soil. Sandy soil quickly dries out and will likely not be able to go weeks and weeks without watering. Be sure to use mulch on this soil. Use a moisture meter probe at various points in your rose beds. This will give you an indication of water in the upper most layer of your soil, the shallow root zone. If it is green, then don’t add more water. Observe how your soil type absorbs water and holds this moisture to make it available for plants.
You may be able to stretch out how often you water and here is a simple test you might want to try. Water all the roses and mark the date on your calendar. Then wait and watch. When the roses start to droop note the date, count back the number of days to when you last watered, subtract one day and that is how often you need to water. Repeat this occasionally and you will likely see you will need to water less and less as the roots push deeper into the earth. In other words let the rose tell you when it's thirsty.
When you do water, I recommend doing so by hand with a watering wand. Make shallow catch basins around each plant or rose bush out to the drip line. Fill the basins up with water, then move on to the next. After having done five or six of them, go back and fill the basins again. The second watering helps push the water deeper into the soil where it will last longer for the plant or bush. When watering, apply deeply. Deep watering roses ensures moisture will penetrate down into the root zone where mycorrhizal fungi and the root "hairs" maximize the surface area of the roots and provide the most efficient use of the water. Remember, too, that the root zone extends well down and away from the "drip line" of the rose. Deeper watering is done less often and can be spaced over weeks between applications.
One way to water more efficiently is to use a drip irrigation system that delivers the water directly to the root system without wasteful overspray or runoff. You may recall our speaker Tony Mekisich who gave us a fine course on irrigation and plant needs. You can experiment by watering every two days, but watch your roses carefully. Foliage may droop or even burn as you often see with blooms in hot weather. If they appear to be healthy, try watering every three days, then try four and so on. Let your roses react to the changes and they will guide you in your decision making. If using a drip system, have several emitters around each rose bush. Remember that roses in containers will dry out faster than those in the ground. So keep a close eye on them for water needs.
Keeping the foliage on your rose bushes will help cool the bush during the summer. Foliage is the plant's air conditioning system. Reduce your fertilization schedule on your roses as they enter the summer months. More fertilizer produces more blooms and draws up more water from the soil to create leaves and blooms.
We add well aged compost to the garden each year. Roses love alfalfa which is slow to break down and a mild organic fertilizer. Alfalfa Tea is excellent and can be applied three times a year. After adding compost, give your roses a 3 or 4 inch layer of mulch in early spring and late summer. Don't spread it too thickly or build up around your graft union. We add mulch each year but find that we need to add less as time goes by. The mulch decomposes into humus which is very good for your soil and its biology. As it becomes a part of the soil, we top it up with a new layer. We use fine to medium sized fir bark.
Some roses do well in drought conditions. Rosa ‘Mutabilis’, also known as China rose, has coral-peach buds, opening to yellow-peach in the morning. Some Old Garden Roses and Earth Kind roses are noted for their ability to tolerate dry conditions. Consider adding these to your garden. Well-established old garden roses can be watered once or twice a month, or not at all. If you're watering by hand and in normal conditions, Hybrid Tea and Floribundas like a deep watering once a week. Our good friend Gregg Lowery says that he waters only once a month in the long dry summer. “The trick is to get the water down deeply in the soil where the woody roots have travelled in search of moisture. Frequent light watering results in soils that are dry down deep and moist at the surface. The top 12 inches of soil dries rapidly, stressing roses that are watered this way, and forcing repeated applications of water.” Newly planted roses need more frequent watering but the idea remains the same: encourage the plant’s natural tendency to root deeply. And be sure to use mulch on all your flower beds.
References on Roses in Drought Conditions:
Earth Kind Roses http://aggie-horticulture.tamu.edu/earthkindroses/cultivars/
Paul Zimmerman’s Blog, Fine Gardening Magazine
Pacific Rose Society, Lynn Snetsinger
EBMUD, Plants & Landscapes for Summer Dry Climates of the San Francisco Bay Region
Gregg Lowery, Vintage Gardens Newsletter
Nanette Londeree, Water, Water, Precious Water Marin Rose.org