Gardening columnist Dan Gill answers readers’ questions each week. To submit a question, email Gill at [email protected]
I have a pink hydrangea and would like to know how to make blue flowers bloom. – Andy Gordon
Hydrangeas are exceptional in that the color of their flowers can change depending on the availability of aluminum ions in the soil. Where aluminum ions are freely available for the hydrangea roots to absorb, the flowers tend to be blue. Where aluminum ions are bound and unavailable, flowers tend to be pink.
Aluminum ions are most available in acidic soils and less available in alkaline soils. South of Lake Pontchartrain, soils are alkaline, causing hydrangeas to bloom pink or lavender on the south shore. To get your hydrangeas to bloom blue, apply aluminum sulfate in October and again in March. Follow the package instructions. This makes the soil more acidic and provides aluminum ions. Repeat this treatment every fall and spring until the plants bloom blue.
On the north coast, many soils are acidic and the hydrangeas bloom blue. If you want to turn blue hydrangeas pink, apply lime instead of aluminum sulfate according to the same scheme. This makes the soil more alkaline and binds the aluminum ions. Remember that hydrangeas have already set their flower buds for next summer’s bloom and should not be pruned in the meantime (although any remaining old, unattractive flower heads can be cut off now).
My azaleas are dying. You are about 35 years old. I water them regularly and they get good drainage. Since they died I’ve been feeding them azalea food according to package directions but it hasn’t helped. They started dying about a year ago and up until then have always been well fed, green and very healthy. They get partial sun. The leaves all turn brown, and the entire bush slowly dries up and dies. I’ve tried planting new azaleas to replace the dead ones, but the new ones die off almost immediately. What can I do to make them healthy again? – Donna
Unfortunately, there isn’t much you can do for your azalea planting. Like all living things, plants grow old and die. 35 years is old age for azaleas. Your azaleas lose vigor and weaken due to age, and this makes them vulnerable to attacks from fungal organisms that have never bothered them before. This is not a problem with growing conditions; They have been where they are for decades and happy with the care you offer them. This is not a nutritional problem and fertilizing will not help. This is a problem with the plants themselves.
What is actually likely to cause the azaleas to die off is root rot organisms in the soil that attack and kill the roots. As parts of the root system are killed, the corresponding upper part of the shrub no longer gets the necessary water, withers, turns brown and dies. As root damage progresses, the entire plant dies. These fungal organisms have always been present in the soil. It’s just that now that the azaleas are old, they are more vulnerable to attack.
However, you should be able to grow new azaleas in the area. Once you’ve removed a dead azalea, turn the soil in the area thoroughly and add a 2- to 4-inch layer of compost. The compost encourages the growth of soil microorganisms that can help protect the new plants from root rot.
November would be an ideal time to plant new azaleas. Then planted, the cool weather will make it easy for the newly planted plants. And shrubs have a root growth period in the fall, so the new plants should immediately start sending out new roots into the surrounding soil. Regular rainfall in winter minimizes the maintenance effort.
In the spring, when they begin to bloom and grow, azaleas planted in the fall are already well on their way to establishing themselves. However, the new azaleas will need to be watered more frequently than the old, established plants during dry spells next summer.
Tips for the garden
STOP WEEDS: Apply pre-emergence herbicides to your lawn now to prevent annual weeds such as chickweed, henbit, sticker weed and annual bluegrass from growing on your lawn during the cool season. I am often asked if I should control these weeds in the spring when it is too late to control them most effectively. Look for products like Halts, Dimension, CrabX, Crabgrass Preventer, and other brands and follow label directions.
GARDEN PARTY: The Fall Garden Festival at the New Orleans Botanical Gardens takes place October 8-9. It includes exhibitions and sales of plant and garden products, educational programs, music, crafts, cooking demonstrations and more. The festival is open from 9 a.m. to 4 p.m. on both days. Admission is $12 for adults, $6 for children 5-12 and free for children 4 and under and Friends of City Park members.
AZALEA CARE: Azalea bugs feed on the underside of the leaves, causing small white dots on the top of the leaves and dark brown spots on the back. If you see these symptoms, spray permethrin, bifenthrin, or a light horticultural oil under the leaves two or three times and follow label directions. After treatment, the white spots remain but do not worsen.