Several varieties of Hydrangea macrophylla are color changeable. The flower color is determined in part by the soil’s pH level. A soil sample indicates the nutrients and pH level of the soil, eliminating the guesswork on how much fertilizer or soil additives are needed.

Most major plant nutrients are more accessible at a pH of 6 to 6.5. A pH that is too high or too low can keep plants from absorbing nutrients from the soil.

The nutrients are unavailable — or not absorbable — to the plant because of the soil’s chemistry. This problem can manifest itself in a variety of ways, but in the case of hydrangeas, the bloom color changes.

Color variation in hydrangeas is due to the presence or absence of aluminum compounds in the flowers. If aluminum is present, the color is blue. If it is present in small quantities, the color is variable between pink and blue. If aluminum is absent, the flowers are pink.

Soil pH indirectly changes flower color by affecting the availability of aluminum in the soil. When the soil is acidic (pH 5.5 or lower), aluminum is more available to the roots, resulting in blue flowers.

When the soil is alkaline (pH 7.0 or higher), the availability of aluminum decreases. The flowers in this soil are typically pink.

For pink (rose or red) flowers, broadcast one cup of dolomitic lime per 10 square feet and water it into the soil. However, this method might take a year to see a noticeable change in flower color.

A faster way to achieve a change is through liquid soil drenches. Dissolve 1 tablespoon of hydrated lime in 1 gallon of water. Drench the soil around the plant in March, April and May with the solution. Avoid getting the solution on the leaves, as the solution can damage them.

To gradually change flower color from pink to blue (purple, lilac), broadcast half a cup of wettable sulfur per 10 square feet and water it in. Again, a faster way to make the change is through liquid soil drenches.

Dissolve 1 tablespoon of aluminum sulfate in 1 gallon of water and drench the soil around the plant in March, April and May. Be sure to avoid getting the solution on the leaves.

Hydrangeas respond to several, light applications of fertilizer during the growing season. They are water-demanding plants and are best suited for the moderate water-use landscape zones.

Garden show

A weekend full of food, flowers and fun is scheduled for Baton Rouge on March 9-10 when the LSU AgCenter presents the 11th annual Baton Rouge Spring Garden Show, the seventh annual Louisiana State and Regional Chili Cook-off and the fourth annual Baton Rouge Spring Car Show.

All of the events will be held in the John M. Parker Coliseum on Highland Road on the LSU campus. The garden show will be open from 9 a.m. to 4 p.m. each day, the chili cook-off will be from 10:30 a.m. to 5 p.m. each day, and the car show will be from 9 a.m. to noon Sunday.

Admission is $5 for adults; children under 12 admitted free.

Tomato enemy

Tomato spotted wilt virus (TSWV) is a virus detrimental to tomato plants. Tiny insects called thrips cause it. The thrips that carry the virus and transmit it to the tomatoes cause the plant’s foliage to turn purple. The leaves also have a bronze cast. The fruit has mosaic or mottled light-green configurations on the outer fruit peeling.

If your plants have experienced this problem in the past, there are several varieties of tomatoes available that are resistant to the virus. They include Amelia, Crista, BHN 640, Talladega, Bella Rosa, Quincy and Topgun. Check with your local garden retailer for the availability of these varieties.

Vegetable tags should provide the variety name and preferably what diseases, if any, it has resistance, to help with your purchasing decision.

Fire blight

Fire blight, a common plant disease that is persistent in the Southeast, makes growing pears difficult. Fire blight is caused by a bacterial plant disease that infects trees in early spring, when young tender leaves and flower blossoms begin to emerge. It is most common on pear trees, but can also affect certain types of apple trees and a few other types of plants.

Fire blight produces several different symptoms, depending on what plant parts are attacked and when. The first symptom, called blossom blight, appears shortly after the tree blooms.

In the early stages of infection, blossoms appear water-soaked and grayish-green but quickly turn brown or black. Typically the entire cluster becomes blighted and is killed.

The most obvious symptom of the disease is the shoot blight phase. This first appears one to several weeks after flower petals fall from the tree. The leaves and stems on young, succulent shoot tips turn brown or black and bend over into a characteristic shape similar to the top of a shepherd’s crook or candy cane.

Under favorable conditions, shoot blight infections multiply and continue to expand down the stems, causing the tree to appear scorched by fire. Shoot blight infections can expand beyond the current season’s growth. This causes dark, sunken cankers to form on the older, supporting wood.

If your trees are already blighted, the only option for limiting the spread of the disease is to prune out the affected branches as soon as they appear. Pruning cuts should be made at least 8 to 12 inches below any symptoms of visible infection to ensure complete removal of diseased tissue.

Sterilize pruning-shear blades with alcohol or household bleach between each cut to reduce potential spreading of the disease. Once finished, clean, sharpen and oil pruning tools.

Applications of a copper-containing fungicide/bactericide at or shortly after bud break in early spring will further reduce the number of new fire blight bacteria produced from overwintering cankers. Unfortunately, this will not completely eliminate the problem.

Got a gardening question? Write to Bob Souvestre, horticulturist with the LSU AgCenter, at Burden Center, 4560 Essen Lane, Baton Rouge, LA 70809, or email to, or call Master Gardeners at (225) 763-3990.