This recent and long dry spell we’ve been in has me spending lots of time attending to the watering needs in my lawn and garden. The most noticeable effects of the lack of rain are with the birch trees in the area, including on my own property. All of my birch trees, which include river birch and white spire, are well established and are all at least 15 years old. But yellowing of leaves and dieback this time of year is something I am well aware can happen if the watering needs of these trees are not met throughout the season. Apparently, due to some situations beyond my control, those watering needs were lacking and several of my birch trees are covered with yellowing leaves that then turn brown and drop to the ground. Luckily, I’m not noticing any dieback on the branches and for at least the last two weeks, I’ve gone full speed ahead on properly watering each and every one.
Yellowing and leaf dieback along with dying branches on birches is commonly caused by insufficient watering but can also be caused by a lack of iron. Iron is a necessary micronutrient that birches need and sometimes, for a variety of reasons, the iron gets bound up in the soil and the tree’s roots are unable to absorb enough. This condition is called Iron Chlorosis. A soil test should be able to tell you if this is the problem you are having.
For the most part, though, the likely problem is drought either from lack of rain, not watering or improper watering. The obvious signs are foliage that wilts and turns brown or may turn a dull gray-green color. Dried brown to gray-green leaves hang on the tree or more commonly, foliage drops prematurely and litters the ground under the tree. Severe drought stress may cause dieback of individual branches starting at branch tips.
To properly water any tree, the water must reach the feeder roots that take in water and nutrients. Those roots are located at the drip line of the tree. The drip line is below the widest part of the tree canopy. That could be several feet from the trunk so watering the truck area isn’t at all efficient. Using a sprinkler that will easily reach the drip line is the best way to water a tree unless you are going to water that area by hand. You’ve watered enough when the soil is moist at least 6-8 inches below the soil surface.
For more information on growing birch trees, visit http://www.na.fs.fed.us/SPFO/pubs/howtos/ht_birch/ht_birch.htm.