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A: There are those who suspect Wildflower Center volunteers are the culpable and capable culprits. Yet, others think staff members play some, albeit small, role. You can torture us with your plant questions, but we will never reveal the Green Guru's secret identity.

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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Monday - May 10, 2010

From: Austin, TX
Region: Southwest
Topic: Turf
Title: Did pre-emergent herbicide for lawn kill oak tree in Austin?
Answered by: Barbara Medford

QUESTION:

Your article in today's (May 1st)Austin American-Statesman advised against using herbicides around oaks. Does that include the "pre-emergents" that the lawn care companies use in the Spring? I had a live oak die on me and I suspect it was due to a couple of years of pre-emergent applications.

ANSWER:

A general kills-everything, melts-your-sidewalk herbicide can kill just about anything that can't run away. The pre-emergent that is sprayed on grasses (which are monocots or narrow leaf plants) is intended to kill dicots, which are broadleaf plants. Guess what an oak is? Right, a broadleaf plant. We discourage the use of sprayed-on herbicides because there is always the potential of their moving on to an unintended target. Another downside of the pre-emergent is that many of the "weeds" in your  (probably non-native) lawn grasses are also grasses, often native grasses, but nevertheless unwanted. They will survive and thrive on the pre-emergent spraying, because they are monocots, not the dicots for which the herbicide is intended. Whether this is actually what caused your live oak to die, we can't possibly say, but it sure couldn't have done it any good. And we also discourage letting outside contractors make the decisions about what should be applied in your garden. They may not know what side effects that application might have, they are just doing their job.
 

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