Part 3 – Stop Spraying Thistles!
By: Julianne Mason, Restoration Program Coordinator, Forest Preserve District of Will County
Here are my top reasons for not spraying thistles. I’m specifically talking about Canada thistles (Cirsium arvense), but who knows, maybe this applies to other non-native thistles too.
1. It’s Unnecessary. When native forbs get established, Canada thistles fade away. Read about my experiences at Prairie Bluff below for an example. Note that Canada thistles seem to coexist happily and indefinitely with non-native, cool-season grasses. The fading phenomenon seemingly happens when native forbs get established and displace them.
2. It’s Counterproductive. Spraying thistles causes collateral damage to nearby forbs and can inhibit re-establishment of desirable forbs due to residual herbicide activity in the soil [See Part 2 of post] when herbicides like Transline or Milestone are used in excess. This results in less competition for the thistles and perpetuates their re-establishment.
3. Don’t We All Have Other Invasives to Combat? There are more invasives displacing native species than we collectively have resources to address. We should be focusing our efforts and herbicide on the ones that are displacing natives and reducing biodiversity (e.g., Phragmites, reed canary grass, etc., etc., etc.). Yes, thistles are long ingrained in our culture as noxious weeds because they are problems in ag fields and pastures. But, that doesn’t necessarily mean that they are serious problems in natural areas.
- Let’s Distinguish Between Weeds that are Symptoms of Problems vs. Causing Problems. Thistles are usually symptoms of one of two problems. One is when not enough native forbs are present. This often happens in early restorations, when native communities are first planted in former row crop fields. Or, it can happen when previous herbicide applications cause dead zones or “holes” in the native cover. The second problem when thistles persist is the dominance of non-native, cool-season grasses. Thistles seem to happily coexist indefinitely with non-native, cool-season grasses. In this case, burning, overseeding natives, and addressing the C3 grass problem (we use post-burn application of clethodim herbicide) are the prescription, not spraying thistles. Once the C3 grasses are dealt with and the seeded natives can get established, the thistles fade.
- Combat Thistles with Seed, Not Herbicide. If thistles are symptoms of a lack of native cover, then combat the issue by a dormant seeding of native species, especially native forbs.
Case Study: Prairie Bluff – The Tale of Two Restorations
Here is my perspective-changing experience on thistles from the prairie restorations at Prairie Bluff Preserve in Lockport, Illinois.
Prairie Bluff is a 600-acre former row crop farm. When it was in agricultural leases (prior to 2019), we restricted the use of chemicals that have groundwater advisories, which includes most broadleaf herbicides. As a result, the ag fields became very weedy and Canada thistles were persistent throughout the fields; they were chronically just being suppressed by row cropping. Funding for restoration of the preserve came from two different primary sources. The two different restoration projects, although happening simultaneously, took almost opposite approaches regarding thistle management.
Western Project Approach: Do a good dormant seeding and don’t worry about the thistles.
- Dormant seeded with full prairie and wetland mixes – Feb 2020
- Mowed or spot mowed the weedy parts of the prairie- Summer 2020 & 2021 (I don’t know if this was helpful or necessary, but it made both them and the grant agency feel like they were doing something to help the prairie establish.)
- Prescribed burn – March 2022
Eastern Project Approach: Seed a more limited native matrix and battle the thistles into submission before establishing a more diverse prairie.
- Boom sprayed Milestone herbicide – Fall 2019
- Seeded native grasses, graminoids, and limited forbs – Feb 2020
- Spot sprayed widespread thistles (essentially broadcast sprayed) with Milestone herbicide – Summer 2021
- Prescribed burn – April 2022
- [Summer 2022 – I realized that the western restoration was looking immensely better and decided to change course on the thistle approach in this restoration.]
- Summer 2022 – laid off herbicide and avoided spraying thistles
- Tested herbicide residual in soil – August 2022 [see Part 2 of blog post]
- Overseeded more diverse mixes – February 2023
Western Area Results – Thistles Ignored, Thistles Faded: In the western project area, where the approach was to not herbicide the thistles, there are almost no thistles remaining in the restoration. It is a good-looking early restoration, dominated by Canada wild rye (Elymus canadensis), yellow coneflower (Ratibida pinnata), wild bergamot (Monarda fistulosa),golden Alexanders (Zizia aurea), foxglove beardstongue (Penstemon digitalis), rattlesnake master (Eryngium yuccifolium), field oval sedge (Carex molesta),and many other natives. The diversity of the prairie establishing reflects the original seed mix.
The only Canada thistles remaining in the western project area are present along the trail corridor. There, non-native, cool-season grasses that were planted to be mowed turf along the trail shoulders have expanded out into the prairie. The thistles are persisting in the non-native C3 grass zones, but not anywhere where natives have established.
Eastern Area Results – Thistles Battled with Herbicide, Prairie Restoration Suffers. In the eastern project area, there are still lots of thistles in the restoration. Lots of thistles. The seeded native grasses are becoming established, but besides that, the rest of the vegetation is weeds. Now the restoration has been overseeded with properly diverse mixes and we are avoiding widespread herbicide use to allow the seeding to establish. Hopefully, the recently seeded native forbs will be able to establish well and displace the Canada thistles and other weeds (sow thistles, annual/biennial weeds, etc). I’ll keep you posted on how the restorations are progressing in a few years.
Just for good measure, here’s one more photo comparison of the western project area at Prairie Bluff where the thistles were not treated with herbicide, compared with the eastern project area where the thistles were originally boom sprayed with herbicide. It’s only anecdotal data, but I think it’s time to stop spraying thistles. Restorations are a lot easier and more successful when you view thistles as symptoms of a problem (i.e, lack of native cover) and not the problem themselves.
grassland restoration network 
Excellent advice. The same is true for a host of other invasive species that some managers decide to jump straight to herbicide application rather than thinking about other strategies or dealing with the underlying problem. Sometimes a little patience is all that is needed. If control is needed, there are a lot of mechanical methods that can be used.
I see along the road edge on the southern part of the preserve (Division Street) numerous teasel and poison parsnip (currently blooming). Does the similar herbicide treatment of non-native thistles apply to other aggressive weeds such as teasel and poison parsnip?
Great question! I don’t have any direct evidence that poison parsnip (or other non-natives that are sometimes considered invasives) fade once natives get well established. However, poison parsnip is one that I suspect may be more of a symptom of a problem (usually for us, not enough native forbs and/or too much non-native C3 grass). I have started to mark those areas as “needs overseeding and C3 grass control” instead of scheduling them for broad-leaf herbicide treatments. We have still been herbiciding bolting teasel, but I wonder if the amount of herbicide applied so low to the ground to treat teasel rosettes might not be counterproductive. Always more to test out and learn!
Nonnative C3 grasses are yet another symptom when they occur in remnant prairie or increase in plantings–not enough fire, and it need not be in late spring to control them, just very frequent to volitilize N and keep out the thatch that suppresses many natives. Late spring fire can actually cause other problems by concurrently damaging the native cool season component (which is a major component in remnants especially) and creating a nutrient pulse and open, warm soil surface that promotes new germination and establishment of a bunch of unwanted stuff (sumac–the warmth is a germination cue, Rubus–the nitrate pulse in warm soil is a germination cue, poplars and willows because their seeds need open soil to germinate and are only viable within days of landing). A lot of myths about fire effects (and lack thereof) are based on later spring fire effects (such as those of mostly April burns at Konza, which is like late April to late May in N. Illinois and WI).
This is a great post. I agree with the approach to the specific problem, but even more important is the general point of dealing with underlying problems–often go-to treatments amplify underlying problems even if they address their symptoms in the short-term. It’s not just chemical approaches either.
I really appreciate the posts in this series! Do you have any similar experiences with tall goldenrod? We spend a lot of time trying to diminish this in restorations but I often wonder if, like the thistles, it is a sign of need for more seed and/or soil compaction.
Thanks for the reminder to not just put the blinders on and kill, kill, kill!
Hi Kevin, I have seen Canada goldenrod fade as other native forbs and (in one case, oddly:) Juncus greenii get better established. I haven’t tracked well whether tall goldenrod behaves differently than Canada goldenrod. In general, I have been viewing a lot of tall/Canada goldenrod as a sign that areas may need overseeding.
I know a researcher who has been testing different methods to control tall/Canada goldenrod using base clipping, overseeding, maybe herbicide and/or burning. I’ll reach out and see if she has any results or is ready to write an article for us yet.
Clonal goldenrods also thrive on excessive thatch, because it readily elongates rhizomes and emerges through it. Mid-growing season cutting in conjunction with burning and overseeding (if good stuff is lacking) are the way to go.
Great post Juli!
Bill Kleiman here
Canada thistle: In thirty years I have only sprayed it in CRP brome grass pastures. It does not compete against prairie here.
Weedy goldenrods: We have some prairie planting with some of these established but they don’t take over. They are a common plant of ground that is fallow or seeded too lightly to begin with. When we have those thickets of weedy goldenrods we seed into them, and mow them when goldenrod in flower, if convenient. The turnover from thickets of weedy goldenrod to something better takes a time.
In new plantings we try to keep out for a while the red clover, parsnip, hemlock, all woodies. After the prairie establishes these species usually are kept in check.
Red clover: We say to crew “red clover is an increaser” meaning that it can increase over the years. We tend to say wild parsnip, hemlock, Canada thistle are decreasers in that they will diminish over time on their own or with a bit of a push.
Invasive plants to us in order of priority: Lespedeza cuniata or L daurica, birdsfoot trefoil, reed canary grass, brush, sweet clover, king devil
Fire: Frequent fire good. To me, whether you do fire is more important than when you do fire.
Garlic mustard: Nachusa has very little of it because we burn frequently. We don’t mow, spray or pull mustard.
There are various posts on weeds on the GRN blogsite. There is a search bar there.
Deciding whether to control some weed species can depend on geographic location & site dependent variables. West of Madison, WI, teasel is almost non-existent while wild parsnip is out of control, the exact opposite of the situation in far northeastern Illinois. In the former location, parsnip is seriously invading a high quality mesic prairie remnant that contains no other invasive species. A nearby dry prairie remnant is uninvaded, despite being surrounded by parsnip. Is that due to low soil moisture, a microbiome/native species that repels invasion or parsnip seed pressure not yet reaching an invasion tipping point? The roadsides here are full of it & all the land managers I’ve talked to regard it as a high priority for control.
Blogs like this are helpful for understanding different weed control approaches as long as the authors state the geographic location, community type/condition, etc (as was done with this post) & reminds us what works at one site cannot always be extrapolated to all sites.
What are the general thoughts on mowing/hand pulling Canada thistles to stop the seed from spreading? I’m currently battling a few patches in a nature preserve within areas that have a history of ground disturbance. The area definitely lacks in diversity when compared to other parts of the preserve, but there are some decent species holding on in this area, which makes me worry about too much herbicide use from spot spraying. I’m also considering if this area would benefit from annual fire, as it is adjacent to 2 different burn units and could easily be incorporated into both.
Hi Joni, sometimes we have mowed Canada thistle in prairie restorations. I’m honestly not sure if it was overly helpful: natives in the mowed areas seemed to come in about the same as thistle patches where we didn’t mow. But, it made us feel like we were doing something to help the restoration. In your case, I would be wary of hand pulling since the ground disturbance may also negatively impact the native plants that would otherwise compete with the thistles. But, I think you’re on the right track to focus on management actions that would strengthen the native plant component like more frequent fire, overseeding natives, etc.
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