Until daytime temperatures cool down in a month or so, we need to
keep watering our roses frequently and deeply to ensure their survival
through the rest of the summer. About now you may notice a new problem affecting your bushes -- the appearance of crispy brown leaves.
Even though we occasionally apply extra water to wash accumulated salts well below the roots, dissolved salts still remain in our city water and enter the roots, then travel through the plant to the leaves. As water evaporates from leaf surfaces, salts are left behind, causing leaf edges to become brittle, then often
whole leaves turn brown.
There is little we can do to prevent this process, short of cutting back on water -- which is not an option if we want our roses to survive.
About all we can do to alleviate the problem is to rinse off the roses frequently with a strong spray of water to help wash away salt deposits on the leaves. This not only gives your roses a cooling bath but also adds humidity and is the best nonchemical control for pests such as spider mites that hide beneath lower leaves. Early morning rather than evening is recommended for spraying, because it's a little cooler for the person doing the spraying and the leaves will be dry overnight, thus discouraging any powdery mildew from returning when
nighttime temperatures finally begin to decrease.
If your roses haven't been fed lately, chances are any residual fertilizer from the last application has long since been washed away by frequent watering. In addition to a balanced rose food such as Magnum-Grow or Miracle-Gro for Roses, for a timely boost in vigor, toss under each bush a cup of Epsom salts (magnesium sulfate) to reduce soil acidity. If leaves are chlorotic (yellowish), 3 tablespoons of iron chelate (such as Organo's KeRex) per plant scratched into the soil should help restore a greener color. Be patient awaiting results, because chelated iron usually takes about three weeks before deeper green foliage is noticeable.
It's important to read and follow all product directions and remember to always water well before applying any fertilizer to avoid burning roots. Rose foods containing insecticides are to be avoided, because they sterilize the soil for many years, killing beneficial microbes and earthworms (and eventually perhaps even the rose itself). In addition, the presence of insecticides within leaves can cause pets that chew them to become seriously ill. A far better practice is to apply specific pesticides directly to problem areas as needed, rather than to introduce poisons into the soil.
When the weather cools in September, a light fall pruning may be done.