When you look at a gorgeous Magnolia tree during the growing season, the leaves are supposed to be green not yellow. So a Magnolia tree… yellowing leaves. What gives? I’ve found that much of the time the problem results from alkaline or “sweet” soil.
This is particularly true with the popular evergreen species, Southern Magnolia (Magnolia grandiflora) and its cultivars like ‘St. Mary,’ and ‘Little Gem.’ In San Francisco, Southern Magnolia is the 4th most popular street tree planted. Surrounded by the concrete side walk, (concrete is alkaline and can leach into the soil) and other factors, these street trees can show leaf yellowing.
If you look closely, you might see a textbook problem called iron chlorosis, a symptom of iron deficiency. This is where the leaf is yellow but the veins are green: It looks like a Christmas tree. This same leaf yellowing issue can occur with other species of Magnolia trees. It also happens to many other acid loving plants.
So what to do?
Check Soil pH
1. You may want to check the pH of your soil. Inexpensive soil testing kits are easy to use and kind of fun! You can also send a sample of your soil to a lab for testing.
What is pH: The pH scale, ranging from 0 to 14 is used to indicate acidity and alkalinity. A pH of 7.0 is neutral, values below 7.0 are acid, and those above 7.0 are alkaline. The lower the pH the more acid is the soil. The higher the pH, the more alkaline. Because Magnolia trees do best in soils that are neutral to slightly acidic, if your soil test shows alkaline soil, you will want to lower your soil pH.
How to Lower Soil pH?
Couple of ways. But if you are adjusting mainly the soil pH and making it more neutral to slightly acidic for the Magnolia, you may want to use an amendment that acidifies the soil. You can look for a product at your local garden center, or take a look at the selection of brands at Amazon. I like using Espoma Soil Acidifier. This organic product is safe, using sulfur rather than aluminum sulfate, which can be toxic to plants and soil. The downside though, is that the effects will be slower. I prefer using organic fertilizers and amendments over chemical ones. If you’re not sure what the differences are between organic vs. synthetic fertilizers, you may want to read this post. (Coming Soon!)
Also, you can slowly lower your pH over time by adding good old organic matter (like compost) to your soil. Doing this on an annual basis will also improve your soil structure and increase microbial life: good for the soil food web.
Using both the soil acidifier and adding organic matter to the soil beneath your Magnolia tree annually can be a smart comprehensive approach for long term tree health. You may want to add acid fertilizer to your arsenal. This is great if you have acid loving plants, some of the main ones being: azaleas, camellias, gardenias, hemlock holly, cedar, blueberry, etc..
What can you do to quickly treat iron chlorosis? Pick up chelated iron and spray directly on the leaves. It goes to work fast. 🙂