What do I put on my hydrangea to make it blue instead of pink?

put some rusty nails in soil around the plant.

Making soil ph more acidic, less alkaline will help.

you need to change the ph of the soil around it

Aluminum sulfate by Lilly Miller works every time!

www.gardenguides.com/68038-make-hydrangeas-blue.html

Yes as Vicki said and if you can;t find that try Miracle Grow Miracid--you need a soil acidifier (not a word but it works) to get blue hydrangeas.

My grandmother also said to put rusty nails around the plant.

Reply a bit late but I've always used coffee grounds to turn blooms blue. Acid soil + blue, sweet soil + pink. ( lime is good for 'sweetening' soil ) I've loved the many shades of lavender blooms I often get as blooms change from pink to blue! Have fun.