Yellowing leaves can signal problem

In the fall, yellow leaves are part of a colorful variety that add beauty to our world. This past week, I saw some hickory trees that were almost golden in color.

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However, yellow leaves are not a welcome sight if they are on our evergreen plants in the landscape. Some of the landscape plants that we love the most can have yellow leaves appear in the cool months. These shrubs are camellias, gardenias and azaleas.

There are several factors that can cause the leaves of these shrubs to turn yellow. The plant involved will determine which of the factors will cause the leaves to turn yellow.

All evergreen plants from time to time will have yellow leaves appear in them, and these leaves will be scattered over the plant. This is due to older leaves getting ready to drop.

Evergreen leaves don't stay on the plant forever. They will stay on the plant one to two years, depending on the shrub. So, if you are seeing older leaves turning yellow and the new leaves staying green, this is normal.

When camellia leaves start turning yellow, it is usually due to an insect problem called Camellia tea scale. The scale insect will form on the back of the leaves and will have a whitish, powdery look. The scales are sucking the sap from the leaves and they will turn yellow or have a mottled look. A horticultural oil spray on the underside of the leaf can control the scale by suffocating it.

Another problem that can cause camellia leaves to turn yellow is a virus.

There are a number of viruses that can cause leaves to have yellow in them. Most of the time, there will be yellow circles on the leaves.

There is no control for viruses. However, the virus usually doesn't harm the plant.

Gardenia is another plant that can have yellow leaves, and the yellow leaves can occur anytime during the year. Most of the time when you see gardenias with yellow leaves, the soil is too wet. Gardenias don't like wet feet, and when the soil gets too wet, like we have now after recent rains, root rot fungi can start attacking the root system.

The last plant that I see a lot of yellow leaves on is azaleas. There are a number of reasons for this, but in most instances it means that the plant is running out of fertilizer, particularly nitrogen. They might have missed their last fertilization in July or early August.

But this might not be the case every time.

I had some azaleas that turned yellow every winter. I fertilized them like our soil sample said, and they still turned yellow. I would fertilize them in late August or early September with the same results.

As I looked at azaleas in the county, I realized that this variety of azalea turned yellow in the winter. When the soils warmed in the spring, they would turn green and flower. So, like gardenias, cool, wet soils can affect azaleas.

This year, you might have azaleas turn yellow that have never turned yellow before. This might be due to the unusual weather patterns we had this past summer. Their fine, fibrous roots like well-drained but moist soils. This year's summer drought, followed by a wet autumn, might have damaged these roots. This is especially a problem in areas not suited for growing azaleas. Azaleas do not like wet or very dry soils, or full-sun locations.

To get the best results from azaleas, plant them in the proper location, which is partial shade or morning sun. They need 1 to 11/2 inches of water per week, and they need a 3-inch mulch layer around them to conserve moisture and protect the root system.

Two other leaf problems with azaleas are iron deficiency and lace bugs.

An iron-deficient azalea will have yellow or white younger leaves. The leaf will be yellow with green veins. This condition usually occurs when soil pH is above 6.0. Iron is tied up and is less available to plants at a higher pH. If iron deficiency is the problem, use a soil-applied iron fertilizer at the labeled rate.

Lace bugs make the leaves look speckled or silvery. The underside of the leaves will be brown speckled. These brown specks are the eggs of the lace bug. They will hatch in late March or early April. There will be two generations in a year. The first is in March or April, and the second is in September.

Camellias, gardenias and azaleas might have yellow leaves now, but they can look great this spring by controlling the insects and following proper maintenance for each type of plant.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. He can be reached at cphillipshort@comcast.net.