It is getting afternoon sun.
Thank you for contacting the Toronto Master Gardeners concerning your hydrangea problem
From your photo it appears as if your hydrangea leaves are turning a purple-brown colour. This could be the indication of either a fungal disease or a nutrient deficiency.
The formation of purple spots on hydrangea leaves as seen in the top leaves of the photograph could be an indicatior of cercospora leaf spot. These small porple spots usually begin near the base of the plant, spreading outward and up the plant to other leaves as the fungal spores are spread by splashing water from either rain or sprinklers. This fungal disease is rarely fatal to the plant, however the spotted leaves may drop prematurely thus weakening the plant. Removing fallen leaves and watering your hydrangea from below can slow the spread of cercospora leaf spot.
On the other hand the purple leaves on your hydrangea could be an indicator of a phosphorus deficiency. If your soil pH is too acidic, that is, a pH below 6.0 this will cause the aluminum in the soil to tie up the phosphorous , making it unavailable to the plant. Alternatively, if your soil pH is alkaline (pH above 7.0) which I believe it is since your hydrangea flowers are pink this could cause the calcium or magnesium in the soil to bind the phosphorus making it unavailable for uptake by the plant.
My first suggestion would be to have your soil tested. We recommend the City of Toronto’s Guide for Soil Testing in Urban Gardens, which outlines a step-by-step process you can follow. Click here to read or print it.
We recently had a similar question on purple hydrangea leaves, our response and how to treat it can be found here.
Good Luck with your hydrangea.
June 30, 2022