Question: Why is it that my pines, azaleas, dogwoods, wisterias and hollies turn yellow? Someone has told me it was a lack of iron.
Answer: If your soil is alkaline (pH of 7.0 or greater), the iron that is in the soil changes form chemically, so that the plants’ roots are unable to absorb it. Obviously, you should always start with plants that are less likely to show iron deficiency if you’re gardening in alkaline soils. Once they’re in the ground and growing, however, you can add supplemental iron in the form of liquid or granular iron products. Keep them off masonry and painted surfaces that could be stained by the iron. It’s also a good idea to use a product containing sulfur, not so much because the plants need the extra sulfur, but more for what the sulfur can do to keep the iron in an “available” form. Sulfur forms an acid when it’s mixed into the soil, lowering the soil pH. Spring and summer will be the best times to apply the iron/sulfur additives, while your plants are growing most actively.