|When do you cut back Roses and what causes the powdery white residue that can form on them?|
|Roses should be pruned in late winter or early spring, just before new growth begins. Cut everything back to abou 18" from ground level, then remove the oldest canes, leaving 3-5 of the healthiest canes. New growth will emerge from these canes and flowering should begin in June in your area. After the flowers are spent you can cut the flower off, along with some of the stem. Cut back to a 5-leaf leaflet and a new flowering stem will emerge.
The white powdery residue is a fungal disease called powdery mildew. This disease is unique among plant diseases in that it doesn't require a wet leaf surface to spread. It can thus thrive during hot, dry weather. The general advice to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces. In the case of powdery mildew, however, you can actually inhibit infection with periodic strong sprays of water (not so strong as to damage the plant.) Here are some general rules for control. Start by making sure that your plants are getting enough direct sunlight. (Eight to ten hours a day is generally the minimum for plants that flower or bear fruit.) You'll also want to make sure that there's enough room between plants for air to circulate freely. Overcrowding not only makes plants more susceptible to diseases, if leaves touch other plants, those diseases can easily be spread. Some people report success with this home-made spray: Mix 1 tablespoon of baking soda and 2 1/2 tablespoons of ultra-fine horticultural oil in a gallon of water. Apply as a spray as soon as the mildew appears and every 10-14 days thereafter. Be sure to coat all surfaces.