I did my homework and I’ve got the hydrangea in the right place but it just will not bloom – on old growth and/or new growth. Will aluminum sulfate coax this plant to bloom?
Thank you for contacting the Toronto master Gardeners with your query on hydrangeas.
Aluminum sulphate is a soil amendment that is used to change the acidity of the soil. Aluminum is the element responsible for blue hues in hydrangea flowers, and when aluminum sulfate is applied around a shrub, it not only lowers the pH of the soil, but also makes aluminum more available to the plant. This will only work on the bigleaf hydrangeas ( Hydrangea macrophylla). At this point I should also mention that changing the acidity of the soil will take more than one season. This link explains the timing and quantity of application of aluminum sulplate.
From you description it appears that you have an Endless Summer hydrangea. Endless summer hydranges are big leaf hydrangeas which bloom on the current year’s and previous year’s growth. There are several factors that could impact blooming, including winter conditions, pruning practices, over-watering and over-fertilizing. It is likely that the combination of weather and pruning practices are the reason why your hydrangea did not bloom this year.
In areas where winter temperatures reach or go below freezing, there is a risk of flower bud loss, since buds form primarily in late summer and early fall of the previous year. For this reason, it is recommended that you do not prune or cut back the hydrangeas after August 1 and, if possible, leave fall blooms on the plants over winter (see more information here). The ice storm that we experienced this April more likely killed off any flowering buds that had formed on your hydrangea.
Non blooming hydrangeas is one of our more frequently asked questions. You can find additional information by typing “hydrangeas” into the seach bar located on the right side of the page to access our archived posts. Also , the following link contains information that has been pulled together from other Toronto Master Gardener ASK posts, which I hope will help: Hydrangeas- Blooming, Growing Pruning