Hydrangeas in NM

The leaves on my hydrangea plants are turning yellow. Do I have a soil problem? Should I worry about this? What is the correction? The plants get plenty of sunlight and plenty of water.

Jim

Answer:

Hydrangea plants are native to a large part of the world. Some are native to very cold climates, and others are native to the tropics. However, those most commonly grown by gardeners are native to Asia.

The common garden hydrangeas are difficult but not impossible to grow in much of New Mexico. One problem is hardiness. Those most commonly sold to gardeners are marginally hardy in central New Mexico. However, in protected areas or southern parts of the state, this problem is reduced. The greater difficulty is the alkalinity of our soils due the high level of calcium in most New Mexico soils. High levels of calcium (and resulting high pH) reduce the availability of some minerals to plants.

The most common garden hydrangea is Hydrangea macrophylla. This hydrangea is interesting in that it changes its flower color as the soil pH changes. In acid soils the flowers (actually the bracts surrounding the flowers) are blue. In alkaline soils the bracts are pink. In acid soils, aluminum is more available to the plants. Aluminum causes the blue color. To have blue flowers in New Mexico is difficult and requires adding aluminum sulfate or sulfur to reduce the soil pH to between 5.0 and 5.5. For gardeners in the eastern U.S., this is feasible. For New Mexico gardeners, it is difficult. It is possible to temporarily raise the pH to that level in pots or heavily treated small garden beds. It is easier to have the pink flowers, which are also very attractive. If the pH of the soil is above 6.0, the flowers will be pink. In New Mexico, most soils have a pH greater than 7.0.

The problem with your hydrangea plants is probably that the soil pH is much higher. At higher pH levels, the plants have difficulty obtaining iron from the soil. This results in yellowing of the leaves in the area between the veins. You will often notice green veins in yellow leaves if the problem is minor. As the availability of iron decreases (pH increases even more), browning and dying of the leaves develops. As the pH gets very high (above 8.0), other minerals may be in excess in the soil (especially sodium). The minerals are called salts, and browning of leaves develops. This is called salt burn.

If your garden is in an area with adequate drainage, application of gypsum and adequate water can leach surplus salts from the soil, reducing the problem of salt burn. Addition of iron sulfate or sulfur and organic matter to the soil can further alleviate the problem of iron deficiency. These treatments may allow you to have healthy hydrangea plants, but their flowers will be pink. Much greater application of acidifiers will be required to have the blue flowers. I hope you like pink hydrangeas.

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Send your gardening questions to:
Yard and Garden, ATTN: Dr. Curtis Smith
NMSU Cooperative Extension Service
9301 Indian School Road, NE, Suite 112
Albuquerque, NM 87112

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.