Malta Star Thistle: A Wet Spring’s Unwelcome Bloom

After four years of drought, decent rains brought the flowers back

As I write this report in the middle of May 2015, we’ve had nearly two solid weeks of rain, with good chances for more in the 10-day forecast.  That brings the total for the year in our part of southwest Llano County to 13.5 inches.  After four years of extreme drought, this tropical season is truly a balm for the soul.  The grass seed we planted last winter now has a decent chance of getting established, and I’m hoping that this year we won’t lose any trees because of drought.

Unfortunately, the same conditions that contribute to a spectacular wildflower display (cold, wet winters) also encourage the proliferation of some nasty unwanted weeds such as bastard cabbage (Rapistrum rugosum), and Malta star thistle (Centaurea melitensis).  Both of these weeds are native to the Mediterranean region of Europe and Northern Africa, and probably arrived in North American either in contaminated seed, feed, or even stuck in the fur of animals.  In this report, I’m going to focus on my personal Public Enemy No. I, Malta star thistle.

It’s easy to find many descriptions of this plant on the internet, so let me highlight a just few of its characteristics and then move on to some ideas of how to deal with it.

Why This Plant is So Bad

Malta star thistle is an aggressive annual or biennial with a simple but tough tap root.  It germinates in the fall with the return of rains, and immediately develops a rosette or spreading base before sending up a flowering shoot in late February or March.  Malta star thistle is highly competitive and can quickly out-compete native plants for sunlight, water, and nutrients.  Worse than that, each small plant can produce 1-100 spiny yellow flower heads, and each head can produce over 60 seeds.  In a matter of no time it can form extensive monocultures that essentially smother surrounding vegetation.  The spiny flowers and seeds recruit the unaware rancher to assist in its rapid distribution by sticking to animal fur and tires.  For example, TXDOT has been doing a lot of roadwork along Hwy. 71 near us, and now the expanded right of ways are solid stands of MST that are quickly overwhelming the dwindling stands of wildflowers, and rapidly spreading to the adjacent fields.

How to Get Rid of It

Because Malta star thistle is really good at taking over open, disturbed sites, I used to think that eventually it would be shaded out over time as grasses recovered, but this season has shown me that even as other plants move in, Malta star thistle still has an amazing capacity to proliferate and grow.   Therefore, any treatment to reduce or eliminate it will involve a multi-year effort, using more than one “tool” or activity to knock it back.

Kill, Pull, and Plant

The challenge of getting rid of Malta Star thistle is that the methods that are effective in killing it also kill the surrounding vegetation, which in turn reduces any competition that could help suppress MST (which loves bare ground).  One “tool” is a late winter burn, if the fine fuels on the ground are sufficient. This method could be useful in areas where MST has taken over entire fields, especially if burning was followed by sowing warm season perennial native grasses, or where warm season grasses are already present in decent numbers. Where native grasses are present, a fire could revitalize them while also crowding out MST.  As we have discussed before, sowing may also require several years of over-planting to fill in bare spots, or to add more seed to the seed bank if rainfall was not sufficient in the first year.

The second tool is to mow MST to keep it from producing so many flowers and seed.  However, mowing must be done when the plant is very small and it must be repeated frequently because MST is able to branch a ground level, and a one-time mowing only makes it mad and stimulates it to develop many more branches from the base, each with many more flowers.  Mowing when the plant is tall and in full flower may contribute to spreading it around and certainly doesn’t prevent it from germinating.

A third tool is to spray the plant in January or February when it is in the rosette form with an appropriate post-emergence herbicide that targets broad leaf weeds.  Now this assumes the landowner has learned to recognize what the plant looks like at this stage before it sends up a flowering stalk.  It’s wavy leaves are fairly distinctive, but they also look a lot like gaillardia or Indian blanket, so take some time to learn the difference before you spray, or mark the contaminated area with survey flags so you remember where to look next winter.  Some of the herbicides recommended for thistles that have a limited or short term residual effect in the soil include PasturAll HL or a 2,4,D or 2,4,D and dicambra mix.  It’s very important to read the label carefully on all chemicals before deciding when and how to use them.  Some require restrictions for use around livestock, or a wait period before planting.  These “tools” are the big guns and in my opinion are best reserved for very big infestations.  Also note that once the plant has formed a flowering stalk (which is often when folks first notice it), the chemicals that are used for post emergent treatments of broad leaf plants will not be effective, so using them is a waste of money and worse, eliminates nearby desirable plants.  I spoke with one person who has used glysophate (Roundup) to spot treat small colonies of flowering MST growing in a coastal prairie, with the thought that the surrounding grasses may fill in.  But keep in mind, glysophate will leave a dead zone of grasses and meanwhile, that dead MST stalk will still bear innumerable viable seeds.  In short, early treatment in winter is the best strategy.

Old-time elbow grease is tool No. 4. Pulling up small colonies of plants is feasible for some people.  Personally, I have a zero tolerance for MST where it is growing near places where I have recently had a burn pile and am sowing seed.  I put the plants either on a burn pile or in tall leaf bags for burning later.  Leaving them on the ground just provides seed for next season.

After killing the plant, the next step should be providing more competition to prevent it from completely taking over bare ground.  The challenge there is, that if you disk or till, you may stir up the residual seed of this weed in the soil, resulting in more plants the next year.  Doubling your seed rate or using a no-till seed drill are the only answer to counteracting the proliferation of re-sprouting thistle. 

Finally, one rancher tells me he minimizes the spread of MST by keeping equipment and livestock out of the worse infested areas when the plant is in bloom.  This simple practice may be the first step in what will no doubt be an on-going process of reclaiming our rangeland back from such a nasty invasive thug like Malta start thistle. 

posted on 5/19/2015