How can you turn your hydrangeas from pink to blue? Find out here.

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It is a great time of year for some southern garden hydrangeas. Something about the blue mop head hydrangeas gets the horticulture juices flowing.   

These southern gems are called Hydrangea macrophylla, or commonly known as bigleaf, French or snowball hydrangea. There are more than 500 known cultivars of bigleaf hydrangeas in the nursery trade and are divided into two main groups:  the hortensias with large snowball-like flower clusters, and the lacecaps, which generally have somewhat flat-top flowers, with fertile, non-showy flowers in the center and more showy sterile flowers on the outside.

Depending on the variety, plants sizes vary from 4 to 12 feet. 

My criteria for giving the go ahead to plant these southern beauties are morning sun, afternoon shade and moist, well-drained soil.

It is best to avoid planting them on hot, dry, exposed sites.  

Tips for growing hydrangea

Hydrangea macrophylla is susceptible to freezing temperatures, so finding some protection from the cold is also a consideration. Late spring cold can freeze the leaves and occasionally damage the flower buds. French hydrangea can easily be grown in containers and is an excellent patio plant. Another advantage of growing the plant in a container is that it can be moved indoors on a cold night. 

Bigleaf hydrangea responds to several light applications of fertilizer during the growing season.  A general-purpose fertilizer, such as 8-8-8 or 10-10-10 applied at a rate of 1 pound (2 cups) per 100 square feet in March, May and July is suggested.  It is not necessary to remove the mulch when fertilizing, but do water soon after application to help dissolve the fertilizer and send it into the soil. 

Hydrangea macrophylla is a water-demanding plant best suited for areas where moisture is easily available. Make sure to water it whenever the plant begins to wilt in the absence of rainfall.  It is important to avoid plant stress in the spring when the flowers are forming. Apply 3 to 4 inches of organic mulch, like pine straw, pine bark or fall leaves to the soil surface to conserve moisture and control weeds. 

How to change the color of hydrangea flowers

Did you know that you can actually change the color of the flowers on these hydrangeas from blue to pink?  The science behind the ability to change flower color comes from the plants' ability to absorb aluminum.  When the pH of the soil is lower than 5.5, hydrangeas can better absorb aluminum which results in a blue flower color.  When the pH of the soil gets more alkaline (6.0 and above) hydrangeas can’t absorb the aluminum as well and the color changes to pink. Since most of our soil is acidic, these hydrangeas naturally are blue.   

To gradually change flower color from pink to blue, broadcast one-half cup of wettable sulfur per 10 square feet and water. To make the flowers pink, broadcast a cup of dolomitic lime per 10 square feet and water it into the soil. It may take a year to see a noticeable change in flower color from this treatment. 

Another, quicker way to achieve a change in flower color is through liquid soil drenches. To make the flowers blue, or perhaps bluer during the growing season, dissolve a tablespoon of alum (aluminum sulfate) in a gallon of water and drench the soil around the plant in March, April and May. To make the flowers pink, dissolve a tablespoon of hydrated lime in a gallon of water and drench the soil around the plant in March, April and May. Avoid getting the solution on the leaves because foliar damage may result. 

Pruning on bigleaf hydrangea is best when the flowers begin to fade. Prune out flower heads and leggy branches to encourage fullness. Flower buds will begin forming in late summer for the following season, so avoid pruning after mid-August.  

Sometimes it is necessary to prune plants after a harsh winter to remove damaged foliage. Although this will encourage vegetative growth at the expense of flowers, it is better for the health of the plant and its aesthetic appearance to remove winter-damaged foliage.   

Remember that these shrubs are deciduous, so they will be leafless for a good three or four months during the winter.  

If you can find the right spot, these hydrangeas can be a showstopper in your landscape.