CORVALLIS, Ore. — If your rhododendron or azalea leaves appear yellowish and smaller than normal, they may be growing in soil that’s too alkaline.

Before taking action, test your soil to determine whether acidification is needed.

Rhododendrons, azaleas, blueberries, heathers and hydrangeas require acidic soil to thrive. If the soil pH is too high, these plants are unable to absorb essential nutrients, including nitrogen, sulfur and iron. Poor drainage or overly dry soil can also contribute to nutrient deficiencies, but soil pH is often the underlying issue.

The most effective way to correct iron deficiency in acid-loving plants is to acidify the soil. Cold, wet seasons — such as winter or early spring — are ideal times to begin the process. These conditions give soil amendments time to break down and integrate into the soil.

Before taking action, test your soil to determine whether acidification is needed. You can test it yourself using a home kit or send a sample to a lab. If your soil pH is above 7.5, consider choosing different plants more tolerant of alkaline conditions. Acidifying to the appropriate level — such as pH 5.5 or lower for blueberries — can be difficult in highly alkaline soils. If the soil is below pH 7, achieving the right conditions is more feasible.

To lower pH, apply elemental sulfur or aluminum sulfate:

Start by top-dressing the soil around the plant with ⅓ pound of elemental sulfur or 1 pound of aluminum sulfate.
Water the area well after application.
Repeat monthly applications until you reach a total of 1½ pounds of sulfur or 8 pounds of aluminum sulfate per plant.

Acidifying soil is a slow process, and plant response may not be visible until the following spring or summer. Be cautious not to overacidify, which can lead to further plant stress or nutrient imbalances. Always test the soil pH before continuing treatment.

In soils that are naturally alkaline, ongoing applications may be necessary to maintain appropriate acidity levels for acid-loving plants.

For detailed guidance, including pH targets and application rates, refer to the OSU Extension publication Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden West of the Cascade Mountain Range in Oregon and Washington.

Previously titled Rhododendrons and azaleas need a strong acidic soil