Q: I need to know why the product label says not to mix horticultural oil and liquid lime-sulfur to spray on plants. I take care of a big rose garden and I know fellow horticulturists who do this all the time.
A: According to Dr. Beverly Sparks, Extension Entomologist, it’s primarily because there is a risk of burning the buds and leaves with the mixture. The horticultural oil smothers dormant insects and the lime-sulfur prevents disease.
It is true that professionals commonly mix the two, and you can too, as long as you do not spray your roses on very humid days. Also, be extremely cautious when handling the liquid lime-sulfur. It is corrosive and can damage your skin and eyes. Wear protective clothing, gloves and goggles while mixing and spraying.
Remember that lime-sulfur is not the same as fungicidal sulfur. Many labels warn against using oils and sulfur together because their chemical reaction does burn leaves.