Start with a hydrangea that was blue when you bought it. Some kinds never turn blue; white varieties always stay white. Some pinks turn purple instead of blue.

Plant and grow blue hydrangeas in acid soil mix. When planting in containers use a commercial soil mix designed for camellias and azaleas. When planting in the ground, amend the native soil in the planting hole with the same acid planting mix, according to package directions, or use two-thirds premoistened peat moss. Mulch with leaf or redwood.

If planting in the ground, check the root run of established plants to make sure the soil is not too alkaline. Use a soil test kit to test the pH of the soil. A pH of 4.5 to 5.0 yields blue flowers, a pH of 5.5 to 6.5 yields mauve, and a pH of 7.0 to 7.5 yields pink.

Maintain the desired pH with aluminum sulfate. Use 1 tablespoon aluminum sulfate per foot of plant height, or 1/4 teaspoon per potted plant. Mix this in water and apply it as a drench several times in spring and fall, beginning in September.

Never use fertilizer that contains phosphorus. Phosphorus is alkaline, so the use of it will raise the pH of the soil and turn blue hydrangeas pink.