Canadian Thistle: The Mythical, Cancerous, Post-Apocalyptic, Iceberg-ish, Villain of the Weed World.

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Welcome back to the Wonderful World of Weed Man!  Last week we finished up our ‘The Grass IS Greener Over There!’ series.  I hope you enjoyed it and found it to an informative series.  Although our blog will not always consist of subject series, it will have many.  Before we get into our next series in a few weeks, I wanted to touch on some common concerns our customers are currently seeing right now. This week we will talk about Canadian Thistle!

If you have never had the displeasure of thistle in your yard, you have won the lawn lottery!  I hope you never have to deal with it.  But if you do, or have had in the past, or are currently dealing with it now – there is hope!  Canadian Thistle is one of the most difficult weeds we deal with here in the Treasure Valley.  I hope to open your eyes to answer the question, “Why won’t it just die!”


Canadian Thistle is a perennial weed that will continue to grow and seed and spread for several years.  It does not live and die within a season or single year.  It has a deep and wide root system.  Its creeping root system can extend up to 17 feet horizontally in the soil.  It can plunge 20 feet deep into the soil as well.  The plant itself above surface and grow up to 5 feet tall.  It is most famous for causing pain and suffering to those who walk barefoot unawares in the lawn.


What most people don’t understand with thistle is that many of the surface ‘plants’ are really connected together and are in fact one giant plant.  Let me illustrate with a picture from Purdue University:


From the picture you can see why we would consider this weed much like an iceberg.  The root system is much larger than the surface plant itself.  What you actually see in the lawn is only a small percentage of the plant.

If the size of the plant wasn’t bad enough you also have to contend with its behaviors.  Nancy Muehllehner, from a submission with Columbia University best describes why Canadian Thistle becomes so easily established: “Part of success of the thistle may also be due to its physiological advantage of being able to propagate through two methods, by seed and by cloning itself.  Introduction to new areas occurs mostly by wind-borne seed or sometimes by run-off in ditches. From there it then spreads rapidly by rhizomes (root segments). Lateral roots 3 or more feet deep spread from a fibrous taproot which then gives rise to aerial shoots which are sent up at 2 to 6 inch intervals.  This allows the thistle to dominate an available habitat and utilize all the available resources in a given area.

Not only does thistle spread easily, seeds it produces can lie dormant and still be viable for up to 20 years in ideal circumstances.  The seed is just waiting for those right conditions to germinate!

Thistle also can produce a chemical that will inhibit the growth of other plants nearby.  The more I discuss thistle, I feel like I am speaking about some mythical creature.  It can clone itself!  It hides itself!  Not everything is as it seems with this plant.  It lies in wait! And it has a superpower to stop other plants from growing!  It is villainous!

So what can you do to defeat this lawn villain!  The first thing to remember is do not pull this weed out of the ground.  It is inadvisable with a bare hand as you will get stabbed by its prickly leaves.  The roots are also designed to break and split when pulled out.  When you pull out one weed, two will grow later.  It’s much like a hydra!

Be careful when trying to dig up thistle as well.  If you leave any part behind it can turn into a new plant.  Even root pieces as little as an inch long have capability of staring a new plant.  The more you dig, the more plants you may be creating.  Little zombies coming back from the dead!



The best option when it comes to dealing with thistle is a multifaceted long term approach – it is much like a cancer in your lawn.  You have to wear it down over time with several methods until it is gone.  You want to kill thistle without killing your lawn.  The only way to do that is slowly with varying methods.  Here are some steps you need to take to help eradicate this weed:


  1. Organic options – we mentioned not pulling the weed out, however, it can be cut. Remove the whole top without disturbing the root.  In essence you will be depriving the weed of its source of food.  It cannot make more food without the green leaves.  If you cut the plant before it buds or flowers you will also help in preventing future seeds.  It will also have to expend some of its energy and resources to shoot up new leaves.  Over time it will be a losing battle if you stay determined and vigilant.
  2. Natural Predators – Cattle, sheep, and goats will help with thistle. Though this may not be a good option for your lawn…Some butterflies will feed on thistle as well.  Though I don’t know that would be a very reliable investment.  If you had a large pasture natural predators would be a great method for you.
  3. Make your lawn less inviting to thistle – Thistle likes low fertility soils, open areas, sunlight, and minimal water. Make sure your lawn has a good fertilizer regiment.  Fill in any bare areas with grass seed so the thistle doesn’t setup shop there.  If you keep your grass a little taller 3-4” instead of 2-3” it will shade the soil and low leafed plants a little better.  Don’t go crazy with the watering as you do not want to introduce disease into your lawn, but a little more water will help weaken thistle.  This step will not eradicate, but it will help weaken and even slow the spread.
  4. Weed Control – Thistle is resistant to many types of weed control, including kill all products, like ROUNDUP. Now you wouldn’t spray roundup in your lawn as it would kill the grass around the thistle as well.  I only bring it up to point out how tough it is to even kill with weed control.  It can be done though.  Resistant is not immunity.  One treatment of weed control will not do the trick.  You have to have several.  And depending on how mature the plant or plants in your lawn might be, it may take a season or even two to fully eradicate.
  5. Professional Help -If you have tried these methods it may be time for a professional – This is where many homeowners can get in over their head.  They have tried treating the weed once or even twice with no luck.  This is definitely normal, however, it is advisable to hire a licensed professional when you are dealing with multiple weed control applications on the lawn.  Even spot treating you want to be careful how much you are putting down, and how often.  You do not want to damage a lawn or cause other secondary issues.

If you are running into a thistle problem that you cannot handle, Weed Man would love to help you out.  Give us a call 208-888-9911 or  #wecareforyourlawn







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