Saturday, February 14, 2009

Thistle Logic

--- Thistle seedlings germinating in a group.
A seedhead probably fell here in a previous year. ---

Now is the time to control thistle seedlings. In this posting, I will describe basic strategies to thistle control, especially early season methods, and share some photos of thistle seedlings. Beware: thistles will bore you to tears! It's part of their sneaky plan to take over the world.

---ITALIAN THISTLE seedlings have leaves with narrow greenish-white upward slanting lines and a bright white dot beneath each thorn on the leaf edge. I think of the line and the dot as being the "i" in Italian. They are one of the earliest thistles to germinate in the winter. --->

Thistles can be weeds in yards, gardens, crops and rangelands. In natural areas, non-native invasive thistles spread on their own, take over and reduce native plant diversity. Depending on the type of thistle, time of year and site conditions, effective thistle control can consist of digging, pulling up by hand, mowing, mulching, careful use of herbicides, or more exotic methods like grazing by molasses-bribed cattle or burning. Often, you will switch between methods on the same stand of thistles as the season and your efforts progress.

January through October, I usually spend some of my weekend hours controlling one species of thistle or another in the farmyard, in sensitive areas of the ranch or on other nature preserves. I will share my thistle logic throughout the spring and summer months to help you decide what to do when. Maybe I will even start a thistle cam. Doesn't that sound boring? See, I told you thistles are sneaky.

<--- A brown seedhead from last year easily recognized as MILK THISTLE because of its large size (often greater than 2 inches in diameter) and huge curved spines ---

--- Underneath and nearby are the MILK THISTLE seedlings just starting to get robust leaves with broad 'splashes of milk' running horizontally across the blade. Milk thistles germinate in the winter. --->

Thistle Seedling Identification - With the arrival of winter/spring rains in the Santa Cruz Mountains interspersed with many sunny, warm days, the seeds are germinating and young thistle plants are starting to claim ground. Now is a great time to start popping out young thistles with a hoe, pulaski or whatever is your favorite digging tool. How do you recognize a thistle seedling? Usually, it has leaves with wide flat stems (often whitish) and oval-shaped blades with spiny edges. Use the photos in this blog or links below to help with identification. Look underneath big, brown, spiny thistle plants from last year, and you will start to consistently see the same small green plants which are their newly germinated seedlings.

<--- Cotyledons joined by the first spiny true leaves of a thistle seedling. ---

The first leaves to germinate from the seed are a pair of thick paddles from which the young plant derives its initial nutrition. These first leaves, called "seed leaves" or cotyledons, might be hard to recognize at first. Look for nearby plants which have the same cotyledons but also have their next sets of leaves which should be shaped and colored more like ones you might recognize on the adult plant.

Thistle leaves usually come out of the ground in a neatly organized swirl called a rosette. Much later, a stem will bolt up from the center of the rosette and eventually bear flowers then seeds. Controlling the thistle plant at the rosette stage is a good strategy. It is easier to get enough of the root at this stage to kill the plant and this early action ensures that no seeds will be produced. Some people get good at recognizing thistle seedlings early and pluck them out by hand at the cotyledon stage. Some people eat young thistles.

Control by Digging - Digging or other methods of weed control that turn the soil are sometimes referred to as 'cultivation'. I will be addressing digging methods with hand tools, although in fields or other large tracts of thistle invasion, tractor-pulled cultivation implements could be considered. Start by sharpening the blade of your digging tool. I keep a file near my hoe and pulaski and touch up the edge every day I use the tool. Wear gloves even during the sharpening process. Aim your sharp-bladed digging tool to the side of the plant and strike into the ground beneath it. Your blow or blows into the soil need to sever the root underground several inches below the surface. If you just slice off the leaves or the very top of the root, thistle plants are likely to resprout from the base and you'll just have to rechop out that plant later. Note that purple starthistles in particular need to be cut at least 3 or 4 inches underground or they will slowly resprout.

--- PURPLE STARTHISTLE rosettes have tightly packed, deeply lobed leaves with whitish fuzz in the center. This plant is a biennial with a large root that needs deep grubbing to keep it from resprouting. Purple starthistle plants germinate from early summer through fall. --->

If you dig up a whole thistle seedling and look closely at the shape of the root, you will see a fat part at the top (like a carrot) that transitions into a narrow section or subsections. If you chop the root several inches below where the leaves join the fat root, you sever the active growing buds and won't get resprouting of that plant.

After dislodging a thistle plant, sometimes I reach down with my gloved hands to grab a leaf and flip the plant upside down or into a pile or bucket. This makes it easier to keep track of what plants I have already attacked. If, when I grab the cut plant, it falls apart into many separated leaves, then I know I probably didn't chop deep enough and I aim my tool at the developing hole again. If I grab one leaf and the whole set of leaves comes attached, then I know I probably chopped the root low enough. I shake off most of the dirt clinging to the roots to make sure the plant will dry out and die sooner. If you are chopping out a large rosette, you might discover there are smaller thistle seedlings hiding under the big one's leaves. Get those little guys too.

<--- YELLOW STARTHISTLE leaves change in shape several times as the plant grows. The initial leaves are unlobed, but within a few weeks the leaves are deeply lobed with the terminal lobe in an arrowhead shape. The plant has a distinct flat yellow green color. It germinates in winter but stays small until mid- to late-summer. --->

Persistence - The annual seed germination period of some thistle species does not occur all at the same time and can be spread out over several weeks to months. So don't be surprised if after thoroughly clearing a location of thistle seedlings, more appear a few weeks later. It's not that you need new glasses. Many of those seeds probably germinated after your initial attack because you cleared out a nice open space for them without competition from their older siblings. You will need to pass back through your control area several times during the spring and summer to get the late comers. Don't be discouraged. Many of these late germinaters would have had to wait another year or so before their turn. You are actually stimulating more of the seeds in the ground (referred to as the "seed bank") to germinate in the same year so that the total length of time you have to control thistles at that spot will be less.

<--- BRISTLY OX-TONGUE likes moist soils. It is easily recognized by the white zit-like blisters on the top of the leaves from which the bristles emerge. It germinates mid- to late summer. ---

Several species of thistles can occur in the same general area. You may just finish getting all the rosettes of the early germinating thistles (Italian thistle, milk thistle) when seedlings of the next round show up (yellow starthistle, bristly ox-tongue). Bull thistle is a late summer germinator and bloomer. Purple starthistle keeps germinating for much of the summer, and even seems to increase its germination rate in the fall.

If you start to get overwhelmed, go after the largest plants first to keep them from going to seed, recharge, and then return to the same location in a week or so and go after the largest plants again. Repeat until there are no thistle plants standing. As the season progresses, there are other methods you can use which I will discuss in future blogs.

More Than One Year - How long will it take you to get rid of the thistles? It mostly depends on how many prior years thistle plants have been allowed to mature at that location and drop seed into the soil, and your thoroughness. A seed which was produced one year may germinate the next year, or it may stay dormant in the soil for many years. Some thistle seeds may stay inactive in the soil for as much as 8 years, and still germinate once the right moisture and sun stimulation occurs. A big seed bank takes more years to deplete.

--- BULL THISTLE germinates in late summer and has a soft fuzzy appearance in addition to stiff yellow spines on the leaf edges. --->

Don't be discouraged. Thistle control, indeed weed control or the overpowering of any evil force, takes persistence. You can start humming Dylan's "Where have all the flowers gone?" at this point. When developing your thistle campaign, I encourage you to pick an area that is small enough for you to stay on top of the first year and don't let any thistle plants go to seed in that area ever again. It will be more work than you expected, so start small. Check that area several times during the spring and summer and get any missed plants or late germinaters.

Attack the same area the next year. It actually may seem like more work the second year because all your hard work the previous year is just tricking more of the seeds in the ground to germinate faster. By year 3 or 4, you will probably see a substantial reduction in the number of seedlings and now you are getting the upper hand. As the amount of work declines in your initial attack zone, you can expand your thistle control into surrounding areas, but always try to keep your control area small enough that you have time to prevent any thistle plants from going to seed.

Thistle Logic In Sum - pick a small initial attack area, learn what thistle seedlings look like, use sharp tools, chop out enough of the root in the rosette stage, repeat visits during the growing season, don't let any plants go to seed, attack the same area for several years in a row. You may find that family, neighbors and co-workers ridicule your thistle control aspirations. Just remember these basics to outsmart the thistles and be persistent.

<---A Pop Quiz: Can you tell the difference between the Italian thistle and milk thistle seedlings?

Congratulations on making it all the way through this thistle logic posting. A thistle-filled world is a bore, so do not succumb to the opiate effect of thistle tyrants and save the wildflowers. Here are a few entertaining thistle quotes for your persistence.
Abraham Lincoln: "All my life I have tried to pluck a thistle and plant a flower wherever the flower would grow in thought and mind."
Proverbs: "Who gathers thistles, may expect pricks."
See also:

Thistle Identification & Photos
  • CalPhotos
  • The Nature Conservancy's Global Invasive Species Team
  • Weeds of California and Other Western States, Joseph M. Ditomaso, Evelyn A. Healy, 2007, University of California, Division of Agriculture and Natural Resources. Great photos and descriptions in this new 2 volume set. Expensive but available at some County Agricultural Departments and Weed Management Areas.
  • The Grower's Weed Identification Handbook, Bill Fisher, 1996 with updates, University of California, Division of Agriculture and Natural Resources. Out of print, however, may be available at Univ of California Cooperative Extension Service offices. Includes photos of seedlings, compares to similar plants.
  • A table comparing spiny-leaved thistles from the California Department of Food & Agriculture

Control Methods:
  • Invasive Plants of California's Wildlands, Bossard, C. C., J.M. Randall, and M. C. Hoshovsky. 2000. University of California Press. Berkeley, CA. Complete text available online at the California Invasive Plant Council website.
bristly ox-tongue, Picris echioides
bull thistle, Cirsium vulgare
Italian thistle, Carduus pycnocephalus
milk thistle, Silybum marianum
purple starthistle, Centaurea calcitrapa
yellow starthistle, Centaurea solstitialis


  1. Thank you so much for getting these thistles into one site with all the differences noted, its hard and timely to collect all the info to tell them apart young. Very helpful and quick. God bless you.

  2. Again, seeing this LATE, but THANK YOU SO MUCH! I may need to print these and then range over our property and KILL KILL KILL. This is definitely a service for the planet. =)

  3. What a great primer! Thanks for putting this information out there!

  4. If all thistles are bad guys, I can just take them according to opportunity and inclination. What I would really like to see is a clear way to distinguish the bad guys from ones that we are *not* supposed to take out.

  5. The "lawn" at my new residence is half bristly oxtongue, zits and all. The grass is drought-dead. There is a little pink clover with yellow flowers. Drought favors thistles I guess.

    I will keep digging and cutting; also will plant clover and native wildflowers.

    Those deep roots pull water up from deep down--there's been no water applied for two full summers. So I'm looking for deep rooted natives to crowd out the thistles. Along with a fair amount of sweat.


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