Thursday, June 26, 2008
Lots of factors can cause azaleas to yellow
John Arbogast
Landscape consultant John Arbogast answers your questions every Thursday. Send questions about your lawn, garden, plants, or insects to:
Dear John
5102 Greenfield St. SW
Roanoke, Va. 24018
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Q: Please explain why the azaleas in front of our house have turned yellow, more so on one end of the large mulched planting, and give suggestions regarding what I can do to cure the problem. Our house is situated lower than the street, which slopes downhill making one end of the house more exposed than the other. Our driveway enters from the street at one end of the front yard and the garage is in our basement. There is a decent strip of grass along the street and then there is the sloped bed of azaleas starting near one end of the front of the house and going toward the paved driveway. We used to have Bradford Pear trees in the strip near the street that shaded the azaleas, but winter storms damaged the Bradford Pears requiring their removal. Now, the only shade for the azaleas is from some dogwoods in this landscape bed, which are rather small on the driveway end. I don’t want the yellowing, which is worse near the driveway, to spread up the hill to the rest of the planting. We live in the lower elevations of the eastern U.S. mountains, but it’s hot here now.
A: Several site specific possibilities exist. This does not sound like a disease.
Because of environmental stresses, the yellow-leafed azaleas are probably not taking in enough of the minor nutrient iron. You can temporarily restore a green color by applying a foliar iron fertilizer, such as the one called “Liquid Chelated Iron” to growing azalea leaves during the early morning or late evening hours according to the directions written on the jug. You might find other liquid iron foliar fertilizers available at your favorite garden center. Cooler times of day are required for foliar application. Repeat liquid iron foliar feedings as suggested.
Yellowing of azalea leaves can be caused by incorrect acidity (incorrect pH) in the azalea soil, so take soil tests from the entire azalea landscape bed. Break the area up into two test plots, if azalea yellowing is worse on one part of the landscape bed. Azalea leaves usually indicate pH problems by showing green color along their central vein and yellow in other leaf areas. Soil tests can be taken through your local Extension office or through private labs. Follow directions with the test results for correcting any pH problems identified. Azaleas get the nutrients they need if their soil pH is around 5.1-5.2 on the pH scale of 1 to 14.
Azalea leaf yellowing can indicate too much sun in the location. Correcting this specific problem is likely difficult to do now because it’s too hot to plant more shade trees or expect late growth from your dogwoods. Be creative. Design and build treated wood trellises or arbors, especially over the hottest parts like near the driveway.
The azalea leaf yellowing could be the result of too much heat in your sloped location. Cool the air and provide moisture to the ground by winding soaker hoses that shoot up little sprays of water and hook this up to run early in the morning once per week. Soaker hoses usually do a much better job of cooling the air because of their smaller stream of water size than oscillating sprinklers.
Loss of good green color from azalea leaves can be a result of sucking damage by a little insect pest called the azalea lacebug. Lacebugs appear on the underneath side of azalea leaves and thrive on azaleas growing in a lot of sun. Flip some of the azaleas’ yellowing leaves over and look for evidence of this pest, which might be small black or brown dots of their excrement on the underside of leaves, since the adult azalea lacebugs could have been present since new growth came out in the spring but not too prominent now. Here in Roanoke, there can be a second generation of azalea lacebugs in mid to late summer, so check the underside of leaves again later for their presence.
Q: Is it too late in Roanoke’s growing season to prune a few lower branches off my White Flowering Dogwoods? The limbs are about 1 to 1 ½ inches in diameter. How late in the summer can this pruning be done? I am not concerned about the loss of next spring’s flowers on those branches, but I don’t want to hurt the trees.
A: If you can prune those lower Flowering Dogwood branches off by mid July here in Roanoke, you should be alright. Final pruning cuts should be smooth. Make those cuts back at the “branch collar” where the side limb emerges from the trunk.
