Hydrangea Macrophylla, also called Hortensia or Florist Hydrangea, is a variety of hydrangea known for its large leaves and big, globe-shaped blooms. Macrophyllas are not as cold-tolerant as Serratas and are hardy to minus 15 degrees F. at best. Whether grown outdoors or in pots, hydrangea Macrophylla bushes require attention and care.
Position the hydrangea bush in partial shade that receives filtered sunlight. The best spot for the hydrangea is in an area that receives morning sun exposure but is shaded from the afternoon sun.
Plant the Macrophylla bush in soil with a pH range of 5.0 to 7.5. Make sure the soil's pH isn't higher than 7.5 because this will cause yellowing leaves (chlorosis). Amend the soil with lime to make it more alkaline. If the soil is too acidic, adding lime will make the pH more favorable for the hydrangea, allowing the bush to better absorb nutrients from the soil and giving the blooms more vibrant and changing colors.
Amend the soil with an organic fertilizer or compost. Spread a 2-inch layer of organic mulch around the base of the hydrangea bush to preserve moisture.
Water the Macrophylla bush thoroughly each week, keeping the soil moist at all times, but don't make it overly wet. Soggy soil will become compact and lose aeration. In drier times during the summer or during drought, water every day to prevent the soil from drying out.
Adjust the soil's pH and nutrients to change the color of the hydrangea blooms. For example, add aluminum sulfate twice each year to the soil to get blue blooms. Dissolve one tablespoon of aluminum sulfate in one gallon of water and apply it evenly over the roots. Be sure to leave the flower heads until the next spring so that the hydrangea is able to re-use the aluminum.
Feed the hydrangea once each year with a fertilizer containing equal parts of phosphorous and nitrogen, such as a 12-12-4, 10-10-8, or any other with the first two numbers being equal. In early spring, broadcast a complete granular fertilizer using the rate of 2 lbs. per 100 square feet.
Prune the hydrangea during late summer, when the flowers have just faded and shoots begin growing from the lower parts of the crown and stems. Cut back the weaker shoots at the base, but leave several stems of old wood that are still productive.