Malta star-thistle Invasion

Malta star-thistle Invasion

Malta star-thistle (Centaurea melitensis L.) was first found in Eddy County around 2003 or so along the truck by pass in Carlsbad.  I carry a hoe in the truck for such occurrence and have rouged out a number of new invasive weed when I see them.  I was too late for this one however; I did rouge out a patch only to find 20 or more patches down the highway.  Since that time this weed has been the target of the Eddy County weed management group who have done their very best to stop this weed.  The fact it has taken 13 years for it to become a major concern is a testament to their work.  But like the Russian thistle (tumble weed) it can now be found in the just about everywhere in the county and is moving from disturbed site such as road sides into fields and landscapes.  It is a winter annual with a spiny yellow flowered head that reaches about 3 feet higher but under good growing condition can reach 4 feet.  The spins are less than  1.5 inches, which distinguishes it from its cousin yellow star-thistle.  It reproduces by seed and can produce 1-60 seeds per flowering head.  The leaves are withered usually by flowering time.  This is a tricky weed though.  It germinates in the fall, like the mustard, as soon as it has two true leaves it bolt and send up one flower that will have 1-5 seed in all less than 3 inches tall.  So it is difficult to mow this flower off and it a guaranteed species survival for another year or more.

There have been six biological control insects released for yellow star thistle.  These insects feed on the seed thus reducing seed production. It is a wait and see if they can also help with Malta, so far as I know we don’t have any in the state yet. 

Chemical control if applied at the right time of year works well.  The systemic herbicides clopyralid or picloram work well when applied between December and April in rangeland or roadside applications. These chemicals will kill trees and other desirable broad leaf plants.  Once the flower is set, chemical application don’t do the job.  In alfalfa fields the use of the mustard herbicides when there are mustard weed present may help.  Clorpyralid and picloram will kill alfalfa and other perennial broad leaf plants like pecan trees, so you cannot use them. 

Sheep and goat like to graze this weed until it gets the spiny flower.  It has no toxic effect but once the spine form they can lodge in the mouth and tong causing problems; however most animals will not try it. Cattle don’t seem to have any desire to feed upon it at any stage.  This weed is almost imposable to control by mechanical methods.  For homeowner in landscape situations all you can do is hoe of cut the tops off catching the seed head and disposing them in a dumpster, but as described earlier there are those survival seeds that are produced without much notice.  Because of flooding in the past there is a lot of seed in the fields and if you do not spry for mustard it will get worse.  The seed will not germinate until late fall.  You can control the mustard and this weed too hopefully with good applications.  

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