In the Garden:
Northern California Coastal & Inland Valleys
January, 2006
Regional Report

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Beautiful hydrangea flowers can be encouraged to bloom in pink or blue, as you prefer.

The Mysterious Hydrangea

I get more questions about hydrangeas than almost any other plant. Ok, the Ficus benjamina is Number 1, but the hydrangea comes in a very close second. I guess the mystery of this plant is why do two hydrangeas, planted in the same general area perform so differently? One will bloom like crazy, while the other will do nothing more than produce an abundance of green leaves.

Temperature
Part of the mystery may be solved by variance in temperature. If a hydrangea freezes, it may never bloom because, in most varieties, the flowers are borne on second-year wood. In a nutshell, the flowers that will bloom this coming summer will grow on wood that grew last year. Keep that in mind when you are pruning. An old Mexican gardener who I worked with in the Napa Park Department told me to remove any wood older than last year's growth to keep hydrangeas blooming. Our plants looked pretty skinny after we were finished with the cut job, but they sure made up for it in the summer when every branch carried a magnificent blossom.

Hydrangea macrophylla, a.k.a. H. hortensia, H. opuloides, and H. otaksa, is our common garden variety of hydrangea. It does well where winters are mild, so the Bay Area is perfect, unless you happen to have a low area in your garden where cold air collects. Not a good place for hydrangeas. They prefer to be protected under a canopy of trees. Some sun is ok, but not where the summers get really hot. Full sun is ok in areas like South San Francisco, or out in the Avenues, where the sun never shines from May through September.

Moisture
Another part of the mystery may be solved by the amount of moisture in the soil. There is a reason that hydrangeas have hydra in their name. They adore and thrive on an abundance of water. Don't even think about planting them if you can't provide ample moisture, or if the soil will dry out between waterings. Incorporate plenty of moisture-retaining amendments into the soil when planting. Peat moss, organic compost, even hydropolymers will help our clay soil hold water during the dry summer months. A thick layer of mulch around the base of the plant is an excellent way to retain moisture in the soil.

All pruning should be done late in the dormant season to protect new growth against damage from frost, but check to make sure which variety you have. Some hydrangea varieties bloom on second-year wood, while others, such as H. paniculata and H. anomala bloom on the current season's growth, so cut harder on these types.

Soil
Pink flowers? Blue flowers? Your color choice is easy to control by adding superphosphate to the soil surrounding the plants if you want pink flowers, or adding aluminum sulfate if you prefer blue. Or, how about creating the dynamic duo by putting some of each type of fertilizer around 1/2 of the base of the plant. Keep in mind that flower color will not be affected unless the fertilizer treatment is made very early in the growing season.

Mystery solved! Keep the temperature even, know what variety you are pruning, and keep the soil moist throughout the growing season to have success with hydrangeas.


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