HUNTSVILLE, Alabama -- We have lived in this house for 24 years and have enjoyed the beautiful azaleas that were here. A couple of the plants have turned yellow. What causes this, and is there anything that I can do? Thanks for your help. -- Nick L.
I think we can tackle the problem of azaleas turning yellow, but I am going to give you several suggestions and courses of action to take to make sure we cover all the causes for the symptoms you are seeing.
I imagine that there are several things in play, beginning with their age, the crowded soil conditions, the pH of the soil, possible nutritional deficiencies and, lastly, a pesky little bug that just loves to infest our beautiful azaleas. I think addressing all of these possible causes will ensure that we get your azaleas back in tip-top shape.
Since these azaleas have been in the ground for over 20 years, I suspect that the roots have done a pretty good job of filling in the planting area that they are occupying. This looks like a bed between the house and a sidewalk, and I imagine that it may be - for lack of a better word - tired.
This fall I would suggest raking out all of the old mulch and fallen leaves from underneath the bed and adding a fresh layer of organic matter to the top of the soil. If you make your own compost, this is the perfect place to use that compost - 1 to 2 inches across the top of the soil. If not compost, then any other organic matter will do just fine.
At this time you should also take a soil sample and, at a minimum, have the pH tested in this bed. Azaleas like to grow in an acid soil with a pH range of 4.5 to 6.0; without doing a test, you haven't a clue as to what your pH actually is.
Make sure to report that azaleas are growing in this bed so that the proper recommendations can be made to bring your pH into the optimal range. My guess is that it is above 6.0 and that you will need to add sulfur to bring the pH down to the appropriate range. Bringing the pH into proper range will also ensure that all necessary nutrients will be available for the plant to take up for use.
The primary symptom your plants are expressing is what is termed chlorosis. This term is derived from the word chlorophyll, the green pigment found in leaves; therefore, the lack of this pigment results in a yellowing of the leaves and a chlorotic effect.
In azaleas we often see two types of chlorosis - one caused by the lack of nitrogen (the nutrient needed in the greatest amount) and one caused by the lack of iron (nutrient needed in small quantities). Nitrogen chlorosis is manifested in a yellowing of the entire leaf, while iron chlorosis shows up as a yellowing of the leaf blade, but the veins within that blade are still green; this is referred to as interveinal chlorosis.
The easy answer is to add more fertilizer, but with it getting late in the growing season, be hesitant to apply a lot of fertilizer to these plants. You can use a liquid fertilizer and apply it directly to the foliage so that it can be absorbed and help correct the problem at the source.
There are several liquid iron products available where you mix one tablespoon per gallon of water and spray on the foliage. Other liquid fertilizers like Miracle Gro or fish emulsion can be used, but I would recommend using it at half-strength; we want to try to correct a nutritional deficiency without forcing an abundance of growth.
Next spring, just after flowering, use a good slow-release fertilizer, preferably an organic one that will provide the necessary nutrients for season-long growth. I really like Holly-tone from Epsoma, which is excellent for acid-loving plants like azaleas.
Lastly, I am afraid that your azaleas have been attacked by a pesky little insect called the azalea lace bug. These tiny insects are found on the underside of the leaf and are very small. They have a needle-like mouth part that pierces the leaf and literally sucks out the chlorophyll, thus yellowing the foliage.
Often we think this is a nutritional problem when actually the insect is causing the damage. If this is the problem, apply a sys