Hydrangeas are an ideal shrub for USDA hardiness zone 7. The hydrangea, with its delicate, lacy flowers, is tough enough to withstand the zone's winter. The average minimum temperature in 7a is 0 to 5 degrees F; in 7b, it is 5 to 10 degrees F. While the temperatures do drop below freezing, it's rare for a bitter, long-term frost to hit the area. According to the Texas Cooperative Extension, only the hydrangeas that naturally have pink flowers will bloom blue, and only if grown in acid soil. Giving your shrub the right growing conditions ensures a healthy, happy blue hydrangea.
Choose an area for your hydrangea that receives morning sun and afternoon shade. Zone 7 is fairly temperate, but its summer sunlight can be quite strong. Do not select a site that gets full afternoon sun or heavy shade (such as under a large tree). An exception to this, according to hydrangeashydrangeas.com, is the paniculata hydrangea, which will grow in full, all-day sun if watered adequately. The paniculatas need at least five hours of sunlight each day for good blooming.
Dig a hole 2 feet wide and 12 inches deep. Because zone 7 tends to have soil that contains more clay, add several inches of compost or humus in the bottom of the hole.
Place your hydrangea in the hole at the same level that it was in its pot. Backfill with soil, tamping down firmly to destroy air bubbles. Zone 7's winters are not severe enough to warrant burying your shrub's crown.
Water thoroughly. Water your shrub at least once weekly during the summer, and more if the plant begins to wilt. Zone 7's annual rainfall can range dramatically from year to year, so ensure that your shrub gets at least 1 inch of water per week in the growing season. You should not need to water during the winter dormant season.
Keep your soil's pH between 5 and 5.5 if you want to keep your hydrangea's flowers blue. Test your soil when you plant, and again in April when you see new growth emerge. According to Clemson Extension, apply aluminum sulfate or sulfur to reduce the pH to this range, if needed.
Apply pine needles as mulch in a 2-foot-wide ring around your shrub's base. The pine needles will not only retain moisture in the soil, but they will increase the acidity as they decompose.
Apply several light applications of 10-10-10 fertilizer during the growing season, Clemson suggests. The ideal schedule would be once each in March, May and July.