News release: Hydrangeas

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640

Release Date: April 13, 2017
Headline: Hydrangeas

One of the more popular woody ornamental plants is the hydrangea.  These plants have large showy clusters of blossoms, and come in both shrub and vine forms.  They bloom after spring-flowering plants such as azaleas, and thus give color in the landscape well into summer.  Depending on the plant grown, they can range from 4 feet to 50 feet in height (climbing varieties), and can have a spread of 8 to 13 feet.

Common varieties include the big-leafed hydrangea, climbing hydrangea, oak-leaved hydrangea, panicle hydrangea, rough-leaved hydrangea, and smooth hydrangea.  Blooms may extend from summer into autumn, depending on the variety.  Bloom clusters may range from 4 to 10 inches across.  Some can be grown as indoor plants for blooming during the winter.

One interesting trait of a few hydrangea varieties is that the flower color can change, depending on the soil pH.  Plants grown in acid soils have violet or blue flowers, those grown in neutral or alkaline soils have pink flowers.  Sometimes I get questions from someone who planted a pink hydrangea, but through time, it has started making blue flower clusters.  What has happened is that their soil pH has changed.  That means it is time to get a soil test, and find out where the pH is.  All it may need is a little lime to bring back the pink flowers.  However, it’s always best to get that soil test first.  You wouldn’t want to add too much lime.  That could get your soil into worse shape than before.  Not all hydrangea varieties exhibit this color-changing trait.  White hydrangeas will not change color, for example.

Hydrangeas prefer light to medium shade, although some varieties can handle greater amounts of sunlight.  It’s best to plant them in a well-drained soil.  Soil pH can range from 5 (acid) to 8 (somewhat alkaline).  You may want to modify your soil at planting time to encourage the color of flower you desire, assuming you have a variety that responds to pH differences. 

Generally, hydrangeas like water, so keep them well watered during dry spells.  They can easily take 1.5 inches and up to 2 inches per week during the hot summer.  From late August on into the fall, cut back on the watering so that excessive growth is discouraged.  Succulent fall growth is more subject to winter damage.

If you have compost available, this makes an excellent fertilizer, applied in early spring or late fall.  Some people optionally spray each month with fertilizer solutions, but this should be discontinued at least two months before the first frost to discourage succulent growth.

Pruning should not be done until after blooming.  After plants have matured, you should remove the weaker stems, up to approximately 1/4 of the growth.

A number of pests can attack hydrangeas.  These include aphids, mites, nematodes, rose chafers, and scale.  Disease problems include blight, powdery mildew, and rust.  Treatments are available for most of these problems.