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Q. Who is Mr. Smarty Plants?

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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Tuesday - May 26, 2015

From: Kerrville, TX
Region: Southwest
Topic: Invasive Plants, Managing Roadsides, Non-Natives, Problem Plants, Herbs/Forbs
Title: How to eliminate roadside thistles
Answered by: Guy Thompson

QUESTION:

When we drive along the highway we see lots of wildflowers and no thistles in the median. How does the Highway Department keep the thistles out? Here in Kerrville, we are overwhelmed by thistles this year. Any suggestions? Thank you.

ANSWER:

TXdot follows certain guidelines in planting and management of roadside wildflowers.  This includes waiting until seeds are set and ripened, mowing no lower than a certain height, and, in some cases, using herbicides.  Details of how areas containing thistles are treated are not available.  Thistles are not among the native species planted by the department.

There are several common thistles in this area, including the very invasive milk thistle and Malta star thistle, and the Cirsium texanum (Texas thistle).  Unfortunately, there is no easy way to eliminate these species.  Organizing a group to pull them up by the roots is effective but laborious.  This is the way it is done at the Wildflower Center.  Slicing off the tap root under the base of the plant with a spade makes this easier. A good pair of leather gloves is required to avoid the prickles.

Malta star thistle, the most invasive of the three mentioned, is an annual, so removing the seed heads before the seed mature is the next best thng to pulling up the entire plant.  But don't act too early in the season because the plant can quickly regrow new flowering stalks.

 

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