I have two climbing hydrangeas on the north wall of my Toronto house, one on either side of a window. The dry soil conditions adjacent to the house apply to both. The one on the east, planted probably 30 years ago is vigorous. The one on the west, planted about 20 years ago is not so vigorous. In the last 5 years or so by mid summer many of its leaves begin to go yellow near the veins and steadily the yellow spreads. For the last few years I have fed it iron chelate, 7%, once in the spring and once early summer and have mulched it with pine needles. It hasn’t improved. Is there anything else I could do that would help it?
Thank you for contacting the Toronto Master Gardeners.
From your description I am assuming that you are referring to interveinal chlorosis. Interveinal chlorosis refers to the yellowing of the leaf but not the veins. You are right in thinking that an iron deficiency can be the cause of interveinal chlorosis, however it is not the only cause: magnesium deficiency, alkaline soil ( high pH), compacted soil, wet soil, high salts, phosphorus can all produce the same symptoms. The application of iron sulfate or iron chelate may result in a temporary improvement, but for long term results, you should test your soil’s alkalinity to see if that needs improving, in which case aluminum sulfate may be the answer. See Purdue University’s discussion of the issue here.
Simple soil testing kits to test the soil for pH, nitrogen, phosphorus and potassium can be purchased at any garden centre.
The Ontario Ministry of Agriculture and Rural Affairs (OMAFRA) has a list of accredited soil testing sources www.omafra.gov.on.ca/english/crops/resource/soillabs.htm
In addition the City of Toronto Public Health Department has published an excellent Guide for Soil Testing in Urban Gardens. https://www.torontomastergardeners.ca/wp-content/uploads/2012/02/Guide-for-Soil-Testing-in-Urban-Gardens.pdf
There are many places that offer soil testing. A reliable facility for testing is the University of Guelph. The University of Guelph’s Agriculture and Food Laboratory information can be found here.
Soil is a complex medium. Improving your soil will take time and may require a variety of approaches to get the results you like. The soil testing should help you on that path.