Herbicide damage is one of the biggest tree killers I run into. Weed-n-feed products are the number one culprits. If you buy a bag of fertilizer from your local do-it-yourself store make sure that the bag does not have weed control mixed in with the fertilizer. Most weed control products will affect boad leaf plants and go unnoticed to grasses. Trees are in the broad leaf plant category.
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The most common chemical used in these weed-n-feed products is atrazine. This chemical shuts down the sugar making process of any susceptible plant that absorbs the product. For a small weed that has a very short life cycle this shut down of photosynthesis results in death within a few weeks. Large, healthy trees can tolerate the chemical. The sugar making process will be shut down temporarily, but once the product is flushed from the system and leached from the soil the trees normal processes will return. This usually takes a couple months from the time of application.
There are a couple of factor of the herbicide application that will affect the severity of damage to the tree. Timing is a big deal. Applying the weed-n-feed while the tree is growing its new leaves early in the spring is the worst time. This will always result in deformed and burned foliage. Ruining a trees new foliage in the beginning of the year leaves it with a crippled canopy for the entire growing season. However, if you apply the weed-n-feed during the summer when there is no growth taking place and the trees photosynthesis is shut down due to heat and lack of water the chemical effects might have very little detriment to the tree.
The next big thing is frequency of application. Any healthy tree will usually tolerate one application of herbicide with little or no visual signs of stress. Two applications in a single season will normally cause a canopy to begin thinning. Treating once a year for three or more consecutive years can kill the tree.
It is possible to control weeds in your lawn without killing your trees. Hand pulling the weeds is the best if you have few weeds. If you have to use chemical then spot treat with Round Up or similar product and do not broadcast weed killers over the entire lawn. Or, hire a pro to do it. These days you can usually hire a professional lawn care company to do your lawn fertilization and weed control for the same price range you would pay to buy the products yourself.