We planted a river birch tree in our backyard three years ago and I noticed that the leaves have a strange looking yellow cast to them. When we looked more closely, there were some leaves turning brown too. What can we do to help our tree?
— Burt Davison, North Chicago
From your description, it’s likely your tree is suffering from chlorosis, which is a yellowing of green leaves due to a lack of chlorophyll. Symptoms of chlorosis could appear in a few branches or the entire plant. After three years of growth, the tree’s roots are likely established in your garden’s native soil, which may lack the needed nutrients for the tree. The initial sign of chlorosis can be a subtle paling of the green color of the foliage, which may go unnoticed at first. This off-green color can progress to a general yellowing over the course of a growing season or over several years.
There may be a combination of factors causing your tree to decline. In the Chicago region, some of the most common causes of chlorosis among trees and shrubs include nutrient deficiencies that are related to soil alkalinity (high pH), drought, over watering, poor drainage and compacted soil. Chlorosis will be more pronounced on birches when they are planted in heavy clay, compacted soil and high pH soils, which impede the tree from absorbing iron and manganese. The affected leaves can be stunted and have browning along the edges. Soils that are compacted and poorly drained, roots that have been damaged by construction and drought can affect root growth and contribute to a chlorosis problem. Hot and dry weather is hard on trees and can bring on increased chlorosis symptoms later in the season.
River birches generally develop chlorosis in alkaline soils with a pH higher than 6.5. You can have a soil test done to determine the pH of your soil, but it is most likely higher than 7. The tree’s roots cannot absorb iron because of the high pH of the soil. Other nutrient imbalances in the soil can cause chlorosis symptoms too. Birch trees also perform poorly in compacted soil because they have shallow root systems, and compacted and heavy soils impede the tree’s roots from absorbing nutrients. Heavy watering of your lawn coupled with a heavy, poorly drained clay soil can bring on chlorosis too. It’s been a rainy spring so far.
Weather and your cultural practices can affect the development of chlorosis. Birch trees prefer moist soil that is well drained — soils that hold water and stay wet for extended periods of time can cause chlorosis. River birches tend to develop yellow leaves in the summer during hot and dry periods without supplemental watering. These trees will also shed dead leaves in response to these hot and dry periods. To help green up the foliage in the short term, you could have the tree’s foliage sprayed with chelated iron, but this is not a long-term solution. You could also have the soil around the tree treated annually with elemental sulfur to help lower the pH of the soil at a rate of 1.5 pounds of sulfur per 100 square feet of bed in spring and fall. It may take a couple of years to see any results. There are also other commercial products that arborist firms can use to treat the soil or inject the trees with to green up chlorotic plants. It is difficult to lower the pH of the soil enough to dramatically improve the performance of a birch tree. Results depend on the type of soil you have and its pH level. You will get better results when you start a program of applying sulfur and other soil amendments as soon as you see chlorosis in the leaves.
Since your tree is young and already having issues, you may want to replace it with a different tree that will be more adapted to your site’s growing conditions. Selecting the right plant for a garden’s growing conditions and design objectives is an important first step for success.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
