Espoma Organic 6 LB. Soil Acidifier. This natural and safe way to turn hydrangeas blue is water-soluble and slow-release, making it easy to use and effective. It is safe for kids and pets and can be used on all types of plants, including azaleas, barberries, blackberries, bonsai, camellias, cypresses, dogwoods, evergreens, gardenias, hollies, hydrangeas, junipers, potatoes, raspberries, rhododendrons, strawberries, and more.