Woodland Sunflower - Restoration Savior or Threat?

We can loftily say that dense stands of these beauties “are clearly part of a long-term complex process.” But that doesn’t help with the life-or-death decisions we have to make as stewards. 

In restoring savannas and oak woodlands, our goal is recovery for the full diversity of nature. But what is that? Nobody knows, because wooded tallgrass ecosystems degrade rapidly in the absence of fire. They have lost richness and quality for more than a hundred years prior to our recent adventures in biodiversity restoration (See Endnote 1). 

Thus, for savannas and woodlands, we’re starting at a low level of ecosystem health – and a low level of knowing what to aim for. It would be so helpful if we had "healthy reference ecosystems” – if they existed. 

Woodland sunflowers seem to be taking over large parts
 of our savanna and woodland restorations. 
Is this good for biodiversity conservation – or a threat?

At Somme, quality indicators have been rising for decades. Quality graphs seemed to validate our strategies (see Endnote 2), but there were worrisome trends. In the oak woodlands, for fifteen years an increasing infestation by the “malignant” tall goldenrod (Solidago altissima) worried us. We hoped this thug would eventually be out-competed by conservatives. Indeed that seemed to be happening, and the weedy goldenrod is largely gone. But as diverse conservatives increased, so did woodland sunflower, which sometimes seemed even more likely to outcompete the diversity that we hypothesized to represent success. 

Other stewards have noticed the same. In Wisconsin, the late Tom Brock felt the need to control rampaging woodland sunflowers. His excellent website documenting savanna restoration lists woodland sunflower among “Native species that have been invasive”. He experimented with herbiciding it. 

Will woodland sunflowers ultimately "play well with others" in restoration (as, for now, in the foreground, above)? Or will they blot out most other vegetation (as in the background, above, when we looked close)? 

In Vestal, we don’t herbicide, pull, or otherwise control woodland sunflowers. We nervously appreciate them for apparently controlling the tall goldenrod pest and a part of the natural ecosystem. But if, at this stage, they were to malignantly suppress most other vegetation – they’d wipe out decades of work, including the habitats for hundreds of rare animal species. 

Diverse conservatives, laboriously restored. Might woodland sunflower wipe them out? Apparently, no one knows.

Conservative woodland species in the photo above include robin plantain (white with yellow center, right), stargrass (six yellow petals, right), bastard toadflax (five pointy white petals, top), grove sandwort (five rounded white petals, center), shooting star (wide leaves and purplish stems, top left and right), cream vetchling (wide leaflets and tendrils, top half), blue-stemmed goldenrod (pale pointed leaves at top), and the commoner wild geranium (pretty pink petals). 

Even though warned in 2014 by Tom Brock, we’ve continued to mostly ignore the sunflowers. Before this year, I couldn’t even name them. Four similar species can be sorted out by experts, allegedly. But on multiple occasions we’ve asked experts, respected professional botanists. Their answers at first didn’t seem to agree with each other; then after a few more searching questions, we seemed to hear a lot of hedging of bets. 

We’ve tended to refer to the four species of “woodland sunflowers” collectively – without giving them species names. The four, as they appear in books, are: 

Helianthus decapetalus – pale or thin-leaved or forest sunflower 

Helianthus divaricatus – rough or woodland or divaricate sunflower

Helianthus hirsutus – oblong or hispid or stiff-haired sunflower

Helianthus strumosus – pale-leaved or woodland or savanna sunflower

Perhaps one of the reasons that the common names are inconsistent from source to source is that common-name-users give up on these four. Indeed, John and Jane Balaban, the botany-heroes who first mastered the sedges for the North Branch Restoration Project, threw in the towel on this bunch. As John recently wrote, “ I found great difficulty in keying them out … Things just didn't seem to add up.”   

This post does not treat divaricatus, as it's typically grows in sand savannas, and we have no practical experience with it.

Swink and Wilhelm quote the great Henry Alan Gleason who wrote: “H. strumosus apparently passes into H. decapetalus in one direction and H. hirsutus in the other.” Apparently, no clear line divides these “species”. So, do we care? 

Thomas Antonio and Susanne Masi wrestle briefly with this question in The Sunflower Family in the Upper Midwest. They point out that: 

“Members of this confusing group are found in similar habitats and frequently hybridize, making their identification even more difficult … Experienced botanists frequently have great difficulty with this group. Pale-leaved Sunflower is considered the most variable of the perennial Sunflowers, and hybridizes with a large number of species. It has even been termed a “wastebasket” species, because difficult Sunflower specimens are placed under this name when they fit no place else (Heiser et al. 1969).” 

Antonio and Masi also spice up the discussion with this tidbit:

Researchers have found that Helianthus decapetalus is the only Sunflower species in which two different chromosome numbers are present: 34 in general for those growing in the western parts of its range and 17 in the eastern part. An exception to this east-west distribution is found in the Appalachian region where both types of H. decapetalus have been identified (Rogers et al. 1982). Although these plants cannot interbreed, and should therefore be technically considered different species, they are visually or morphologically indistinguishable from one another in the wild (Heiser et al. 1969). For this reason, botanists continue to classify them as the same species. 

Oh God! Oh Darwin! Please help us! In practice, botanical keys define species. On hard-to-identify groups, experts may say: “It depends on your species concept” or “It depends on which key you use.” Every book seems to have a different key – even different editions of the same book: For example, the Helianthus key in Swink and Wilhelm (1979) was changed for Swink and Wilhelm (1994) and again for Wilhelm and Rericha (2017). They keep trying.

But do the differences among these species impact the ecosystem? Do the different "species" behave differently? What is the ecosystem role of the various woodland sunflowers? Motivated by their progressive take-over at Somme Prairie Grove, we realized that for biodiversity conservation, it’s a life-and-death matter whether one species is compatible with (or fosters) sustainable biodiversity in a given situation and the other species is a malignancy that wipes it out. 

As we plan, we check early records for clues. Surprisingly, H.S. Pepoon’s 1927 Flora of the Chicago Region doesn’t quite list woods as habitats for the “woodland sunflowers”:

Helianthus decapetalus – thickets throughout, common

Helianthus divaricatus – common in thickets and brushlands

Helianthus hirsutus – dry soil … rare

Helianthus strumosus – banks of streams and moist borders of swamps, common

These days, as Pepoon’s notes might suggest, after four decades of brush cutting and controlled burns, we see sunflower clumps most impressively on edges. Some clumps are six feet tall, have abundant wide leaves, and little else growing underneath. Others are three feet tall, less dense, often with fewer or narrower leaves, and more diversity in between and under them. Are these different species, or the same species behaving differently? 

Each sunflower clump starts with a single seed, and then spreads densely,
especially if the adjacent vegetation is of low quality. 

Last year, botanist Will Overbeck suggested a way out of our frustrating species confusion. We had been choosing distinctively different plants and trying to key them down, to see which was which. These sunflower plants start with one seed – but then spread mostly by runners. Clumps ten or twenty feet across are often “all the same plant.” The whole clump started from the same seed. Since these species are internally variable, one six-foot tall clump with purple stems and large leaves may be the same species as a nearby clump three-feet tall with green stems and small leaves – just as two people may be different heights and different complexions. 

So, especially if a large proportion of plants are hybrid mongrels, Will suggested we first figure out which clear species were present and then recognize the rest as blends of those. Okay, Will, we decided to buckle down and do that, as best we could. 

We found it helped for us first to put the woodland sunflowers into two groups based on their leaf stems. In two species – strumosus and decapetalus, the leaves gradually get narrower as they approach the stem (technically, a “decurrent” leaf). 

 

To distinguish among the three species above, we first check out the bases of the leaves. The four leaves on the left narrow gradually, just above the stem. The two on the right have "flat bottoms" – without that narrowing. 

Once we’ve determined whether a clump has a blunt leaf base, then we step back and look at a few more characteristics to assess whether that plant is a clear species, or a messy hybrid. 

In the case or our decurrent friends, it’s fairly simple:

The leaves of Helianthus strumosus are more than three times as long as wide. 

If the plant is pure strumosus, it will also have a smooth lower stem and short green bracts beneath the flower.

The leaves of Helianthus decapetalus typically have longer stems, are wider, and often have bigger teeth.
If the plant is pure decapetalus, it will also have long, unruly, narrow-pointed bracts underneath the flower. 

We had a harder time telling the difference between hirsutus and divaricatus. In the end, we seemed to find only one apparently pure divaricatus clump at Somme. Perhaps we have more elsewhere, or in the hybrids. But mostly we have just the other three species. 

The main clue to hirsutus is its coarse hairs on the lower stems (in contrast to the others, which have smooth lower stems). 

Helianthus hirsutus has roughness and coarse hairs everywhere.
Photo by Jan Thomas Johansson.

When you're checking out the leaves, both hirsutus and divaricatus have the blunt (non-decurrent) base.
The two main veins meet the midrib less than 1 mm from the stem.
(Compare with the two decurrent species, above, where those veins end farther up the leaf).

So that's the cast of characters. Now, we can start to answer those nervous earlier questions. Our first Somme Prairie Grove species list did not include any woodland sunflowers. But they were (and are) common in dense patches adjacent to the preserve along Dundee Road, in the occasionally mowed strip just outside the buckthorn death zone. We didn’t plant it. Our strategy in prairie, savanna, and woodland had been to hold back on the most aggressive species until later in the process. Woodland sunflower does not turn up in our initial (1985) Vestal Grove transect sample. It appears in the second Vestal Grove sample (1987) and has been increasing since then. 

Overwhelmingly the most common form in the shady Vestal Grove is that “wastebasket species” – strumosus – with perhaps a little decapetalus (or decapetalistic hybrids) here and there. Very gradually, with us little noticing, like the proverbial frog in the heating kettle, it took over. Now we woke frogs recognize great masses of woodland sunflower, fairly dense in perhaps half of Vestal Grove. Over time we will learn whether these "conservative" and "aggressive" plants are eliminating others - or perhaps contributing to a more stable and conservative diversity.

This year, when we started looking harder outside the grove, hirsutus and hybrids seemed more common, out in that brighter light. Often there is little or nothing under the hirsutus and strumosus-hirsutus hybrids. We have little data on that. It's time to study.

We found little pure decapetalus, which fit with Wilhelm and Rericha’s characterization of this species as “uncommon” (while they rate hirsutus and strumosus as “frequent” and “common”). But we expanded our searches across the street, and there we unexpectedly found decapetalus to be the species springing up most frequently, as part of the recent rapid recovery of Somme Woods East.  

That long-suffering, buckthorn-choked original woodland (in contrast, Vestal Grove and Somme Woods West seem originally to have been savanna rather than woodland) retained some of its spring flora, but little else, we had thought. The recovering summer and fall flora has mostly sprouted from the seed we diligently hunted, gathered, and broadcast. But this year our survey found full-grown (apparently young) little clumps of woodland sunflower, mostly scattered, tens or hundreds of yards apart. These “clumps” were typically just a few stems, unlike the hundreds per clump in and around Vestal Grove. We had not seeded these new clumps. Where did they come from? “Seed bank” seems unlikely for sunflower-type seeds. Might they be the remains of huge old clumps where the roots have survived on the basis of what little light they could gather, with gradually diminishing strength, until we came along? Are they precious summer flora remnants?

Will it help us avoid the dreary tall-goldenrod-stage of early restoration? Or is it a threat that we should control before it builds up too much steam? Is decapetalus more or less thuggish than strumosus or hirsutus? What kind of monitoring should we do, to understand it better? At other sites, woodland sunflower may or may not be a concern (see Endnote 3). 

Gerould Wilhelm has written about the "floristic symphony" of fall color – of asters, goldenrods, and with the woodland sunflowers playing first violin. We’re learning much and will write more on these possibly-dangerous beauties as we experiment, study, and learn. 

Endnotes

Endnote 1

White and bur oak woodlands and savannas developed their biodiversity over the ages with regular burns. Their biota benefits from – and requires it. After the takeover by the Euro-Americans, lightning- and Native American-set fires were suppressed by these farmers, who depended little on the animals and plants of nature. 

Some of the first tallgrass woodlands to resume prescribed burns were at Somme and the Morton Arboretum. Many aspects of these two sites have been studied intensively, although you might doubt it, given that we only now recognized what was happening with woodland sunflower. 

Tallgrass or black-soil savannas are somewhat similar to black oak savannas, which developed on sand deposits. Being less fertile, these areas degraded less rapidly. Also, sand areas – being less valuable “wastelands” – in some cases were regularly burned by kids, hunters, or others, without much repression effort by authorities. So we have some ecologically high-quality ones, for example in scattered areas of Calumet, Illinois Beach, and Pembroke Township. 

Endnote 2

Our basic strategy has been to plant as diverse as possible richness of seeds, especially those of conservative species. The plant community can be expected to go through many evolutions on the way to high-quality sustainability. But if a large component of the species can be restored early on, then they will be able to move and adapt to new niches as they develop. 

Endnote 3

Comments from other sites.

On Aug. 11, John Balaban wrote about his experience at Harms Woods in Glenview, IL:

I found great difficulty in keying them out using Swink & Wilhelm. Things just didn't seem to add up. I recollect that I often ended up at Hel hirsutus even for plants that looked different. I haven't tried keying them in the new flora, but I had it in my mind to give it a try this year. I can't remember any places in Harms where the sunflowers seem aggressive, but I haven't had that question on my mind as I walk around. I will try to look for it next time I'm out. Harms is still full of Solidago altissima (tall goldenrod), so maybe we aren't at the sunflower invasion stage yet.

On August 12, John wrote:

I had a short walk at Harms with Jonathan today. We looked for Helianthus and found a few but not many and did not seem to be aggressive.

On August 12, steward Larry Hodak checked out Linne Woods (in Morton Grove, IL). He wrote: 

This is a relatively high quality woods, and there are three distinct patches of sunflowers. I don't feel they're marching to take over as I don't think they've expanded much over the years, but there is definitely less diversity in at least one of the clones. The best I can key the species out is H. strumosus.  There is also a patch along the railroad that I think is H. divaricatus (not 100% sure on either). I tried both keys but appressed hairs on the internodes stymied me).”

Tom Brock wrote me after seeing woodland sunflower photos in this blog in 2015. He was trying to be helpful, as I still had not focused on this species. He restores savanna at Pleasant Valley Conservancy in Black Earth, Wisconsin.

Cutting did not work for us. If anything, we got better growth. Also, hand pulling was unsuccessful, because it was impossible to get all the underground material.

I spent some time trying to figure out how “our” Helianthus grows. The rhizomes are fairly short. Once a new shoot is formed, the parent rhizome dies, but the new shoot sends out more than one new rhizome. Because of the short rhizomes, the stem density of the patch is very high. There must be almost no light reaching the soil.

Curtis and Cottam did a study in the 1940s or 50s on a perennial sunflower. They had evidence of allelopathy and also autotoxicity. Thus, the center of a clone often dies but the periphery continues to spread. We have confirmed this with marked clones.

Our best herbicide control has been with use of 20% Garlon 4 in bark oil. A single “spritz” at the center of the shoot is enough to kill that stem. (See the attached photo. The red dye shows where the plant was sprayed.) I was successful in eradicating small clones (6-12 ft diameter) using this procedure. Had to return several times after the initial treatment to treat the late-growing stems. Must continue to return the following years to get the few new stems that keep on coming up. 

This procedure is much too time-consuming for larger clones, so we just treat several rows on the circumference to keep the clone from spreading. (Even that is time-consuming.)

In 2009 we did a survey at Pleasant Valley Conservancy in August, during the flowering stage. We found about 75 clones, some small, others quite large. Most of the large clones were confined to mostly open savannas in areas with sandy soil. Unfortunately, since then many of the smaller clones have coalesced into quite large ones.

For more on Tom’s battle with the sunflowers, see http://pvcblog.blogspot.com/2014/06/woodland-sunflower-still-out-of-control.html

http://pvcblog.blogspot.com/2013/06/invasive-sunflowers-helianthus.html

http://pvcblog.blogspot.com/2009/08/woodland-sunflower-invasive-native.html 


Acknowledgements

Thanks to Kathy Garness and Eriko Kojima for proofing, as always.