Download Emerging Invasive Species in Missouri

Today I’ll be speaking about emerging invasive plant species in Missouri. In this
context, I’m using a fairly broad definition of emerging – encompassing species that
have been dismissed or overlooked for many years and are now increasing at
alarming rates, species known to be invasive in adjacent states that are headed our
way, and even few recent escapees that are only now showing the first signs of their
invasive potential in Missouri or anywhere else in North America.
However, I’m first going to briefly discussing the history of a few of our worst invasive
species, and lessons that we can learn from the past.
This image on this opening slide, taken this past fall at Shaw Nature Reserve,
illustrates this point quite well. In the background is bush honeysuckle, one of the
most widely recognized invasive species in Missouri. In the foreground is burning
bush, another invasive species that is spreading throughout much of the state, yet
remains commonly used in landscaping, and whose invasion is still overlooked by
many.
If we cannot learn from past invasions to prevent the continued proliferation of
invasive species, then there is really little hope for Missouri’s natural areas.
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Speaking of Missouri’s Natural Areas, it’s worth noting that nearly a third of our flora
is non-native, and that ratio is only increasing as new exotic species are,
unfortunately, the primary growth sector of our state flora. However, only a small
percentage of these exotic species might be considered truly invasive, meaning that
they cause economic and environmental damage.
But before I go further with this talk, I need to come clean – I’m not the Santa Claus
of exotic species. I don’t know whose been naughty and whose been nice among all
the little seedlings in Missouri, and who ought to get glyphosate in their stocking. My
work involves rare plant conservation in Missouri and many adjacent states to the
south and east. These travels and my firsthand experience with land management in
East-Central Missouri provide some insights, but I don’t have my finger on the
statewide pulse, so I reached out to the broader field botany community in Missouri,
and it’s these collective insights that informed this discussion.
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So, let’s begin with a little history lesson about Bush Honeysuckle.
It was collected in 1855 during an expedition by the Russian botanist Richard Maack,
whose surname is the basis for the scientific name of the species, Lonicera maackii
A few decades later, it was cultivated at the St. Petersburg Botanical Garden in Russia,
and further shared with botanical gardens and nurseries elsewhere in Europe.
In the final years of the 19th century, it was introduced repeatedly to botanical
gardens in North America, who further disseminated seed to the nursery trade.
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The USDA then further promoted bush honeysuckle, developing so-called “improved”
cultivars, selecting for traits that further contributed to the invasion of the species,
such as increased fruit production.
As early as the mid 1920’s it’s escape from cultivation was documented in Chicago,
but it’s widespread intentional distribution continued for decades afterwards.
In Missouri, it wasn’t documented to escape cultivation until 1983. Only a few
decades later, it is now widespread.
This is a really important point when discussing the threat from emerging invasive
species – bush honeysuckle was already well known to be invasive in other states for
decades before it become a problem in Missouri. Today there are many other invasive
plants that have already become a major problem in other states that are ecologically
and climatically similar to Missouri. The history of bush honeysuckle confirms the
obvious – that it’s simply a matter of when, not if, these regionally invasive plants
also become a major problem for natural areas in Missouri.
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So, I want to stick with the genus Lonicera for a moment, specifically Lonicera
japonica, for our second history lesson.
It was first collected in 1804 for cultivation at Kew Botanical Gardens in London.
Then in 1862, plant breeder George Hall introduced a popular cultivar to the US –
Lonicera japonica var. halliana, or Hall’s Honeysuckle, which was documented to
escape cultivation twenty years late. Virtually all of the invasive honeysuckle in the
eastern US is var. halliana.
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So Japanese honeysuckle is far from emerging – but it is still overlooked by much of
the public and Unlike bush honeysuckle, Japanese honeysuckle is still sold in
nurseries in Missouri.
To add insult to injury, in the past years I’ve encountered several instances in which
efforts to increase awareness of bush honeysuckle have explicitly stated that “that
sweet smelling vine” is not the bad one. Yes, it is. In fact, once could make the
argument that it is worse than bush honeysuckle. Japanese honeysuckle is much
more widely distributed than bush honeysuckle in North America. In the most recent
Non-native invasive plant inventory of the Mark Twain National Forest, only 4 acres of
the inventory were infested with bush honeysuckle, compared to 589 acres of
Japanese, so… yes… that is the enemy.
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Although burning bush is nowhere near as widespread as Japanese honeysuckle, it doesn’t receive
nearly the attention that it ought to. When I discuss this species with people I tend to get two
divergent responses – Sometimes, I get a quizzical look with a response to the tune of “What are you
talking about “emerging” –That’s a well known invasive here.” True, it is already a problem, but many
colleagues have noted a dramatic increase in recent years, and it’s spreading aggressively even in
important conservation opportunity areas like the LaBarque Creek watershed, and in the heart of the
Ozarks.
However, the far more common response, at least in the St. Louis region, is something to the tune of
“Well sure, that’s invasive further north and east, but it’s not a problem here.” I understand that we all
have different personal observations and that this species is nowhere near as abundant as Bush
Honeysuckle in the STL region. I get that. My issue is with this perspective that a species can be a
serious invader in climatically and ecologically similar parts of our region, but that we shouldn’t be
worried about it here until it’s invasion is already widespread. I know we live in the show me state, but
come on, let’s use some common sense.
To make a fire analogy, if the apartments surrounding my apartment were engulfed in flames, and
those flames were beginning to spread into my apartment, I wouldn’t wait until my apartment were
likewise engulfed in flames to determine that the fire posed a threat. That would be absurd, but denial
that the invasion of burning bush and other regional invasive species pose a threat to Missouri is no
less absurd.
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What about sterile cultivars you might ask? The term sterility is often miss-applied to
describe plants that are merely self-incompatible or simply produce less fruit. In the
case of burning bush, studies have examined the invasive potential of different
cultivars with reduced seed production. It’s worth noting that the most popular
cultivar in the trade “compactus” was found to have the highest seed set of all those
assessed. In another study in BioScience, demographic analyses found that even seed
production as low as 12 seeds per plant resulted in population growth and invasion.
The only potentially safe cultivars would have to truly produce zero fruits, and in 2011
a USDA funded study at Uconn did develop triploid burning bush that produced zero
fruit, but they are not yet available in the trade… and even induced triploid sterility is
not always foolproof.
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Another popular ornamental plant that poses a serious threat to Missouri is Japanese
Barberry. Much like bush honeysuckle, Japanese Barberry became a problem in other
states long before it was first detected in MO, but it’s here, it’s spreading, and, like
burning bush, it is also on the increase in the Ozarks.
A common cultivar known as Crimson Pygmy produces less fruit that some other
varieties, and thus is sometimes marketed as “non-invasive.” The same BioScience
paper that assessed Burning Bush cultivars found this claim to be false as well. I might
add that that study was done by some of the most highly respected scientist in the
field of plant conservation and population biology.
So, although Barberry has yet to achieve the invasive abundance it has elsewhere to
the east, keep in mind that lag phase between Bush Honeysuckle's invasion further
east and it’s widespread invasion in Missouri.
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Even with supposedly sterile cultivars that produce zero fruit, we only need to look
once again at the history of past invasions to realize there is still need for caution.
Many so-called sterile cultivars are simply self incompatible, meaning they need to be
pollinated by a genetically different individuals to reproduce.
Such is the case with the many cultivars of Callery pear, which were often marketed
as being sterile. Because all ‘Bradford’ pears are genetically identical clones
originating from a single plant back in 1962, a large planting of Bradford pears cannot
fertilize one another. However, when you plant a bradford near an aristrocrat,
chanticleer, or any of the other numerous cultivars – each can fertilize the other and
produce copious quantities of fruit.
Similarly, damaged or stressed trees can produce sprouts from the genetically distinct
root stock, which can then cross with the cultivar scion of the same tree, so it only
takes one tree to start the process.
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The story of Bradford pear’s invasion is applicable to Miscanthus sinensis. More than
fifty different cultivars of Chinese Silver grass are sold in the U.S. nursery trade. Most
forms set little or no seed due to self-incompatibility, and there is often a
misconception that these cultivars are sterile, but the many different cultivars can
fertilize one another, giving rise a wild type of Miscanthus, which now produces a
significant amount of viable seed that is wind-dispersed.
Although Chinese Silvergrass is better established further to our east, it has been
known from Missouri for sometime and is showing up with increasing regularity.
It’s often found in moist areas like pond edges and streambanks, but it can invade
drier areas also, such as this specimen found at Shaw Nature Reserve growing in the
transition between a glade and adjacent woodland. This is definitely one to watch out
and take action at first sight.
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Another reason to be concerned about Miscanthus sinesis is that many gargantuan
cultivars are being developed for bio-fuels production. If the use of this species for
biofuels expands, this could provide a much, much larger source of invasion than all
the ornamental plantings combined, thus dramatically increasing the invasion
pressure.
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A similar ornamental grass that is an emerging invasive in our region is Ravenna grass,
aka Plume grass (it’s in the genus Saccharum, formerly Erianthus). Unlike Miscanthus,
Plume grass is self-compatible.
I’ll discuss the inadequacy of invasive species distribution maps later, but this species
is more abundant regionally than this maps would suggest.
Plume grass often invades riparian areas and floodplains, but harsh substrates like
glades are also fair game. It establishes with little disturbance, putting intact plant
communities at risk.
Recently it has been observed with greater frequency in east-central Missouri. Given
the massive seed production and wind-blown dispersal, it’s one to be on the watch
for statewide.
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Most plant introductions are intentional, whether they be intended for ornamental
uses, forage, erosion control, or other purposes, but occasionally plants are
introduced accidentally, as is the case with Japanese stiltgrass.
It is believed to have escaped as a result of its use as a packing material for porcelain,
and was first documented in the United States in Tennessee around 1919. It has been
steadily spreading since, and is showing up in Missouri with alarming frequency. With
most invasive species, we are the willing accomplices – we intentionally spread
thousands, or even millions of plants around and provide that critical mass for
invasion. A species like Japanese stiltgrass, which has spread so far without our
intentional assistance, is seriously scary.
Of all the emerging invasive species we face, this might be the one most worth
loosing sleep about. It can thrive under highly variable environmental conditions,
from full sun to deep shade, but it is most common in floodplains and moist
woodlands. It readily establishes and forms dense mats, out competing virtually
everything else in the herbaceous understory.
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Another thing that’s really scary about stiltgrass, is that it responds positively to fire.
A very recent study found that prescribed fire in stiltgrass infested woodlands burned
hotter and longer than in uninfested areas, which led to increased tree seedling
mortality. Furthermore, fire facilitates the growth and spread of stiltgrass, so the only
real options for stiltgrass control in areas with prescribed fire management are to
eradicate stiltgrass before burning or to follow up and spray the fresh new growth
that follows prescribed burns to prevent further spread….. easier said than done.
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I’ve seen what Japanese stilt grass can do in the Southeast, and it’s scary.
My work in the Southeast also brought Nandina domestica to my attention. After
seeing how invasive this species can become in adjacent states, I began paying more
attention to its dramatic rise in landscaping popularity. It was once not thought to be
cold tolerant in our climate, but more cold hardy cultivars have been developed in
recent years and it is now an extremely popular landscaping plant in STL. I was
optimistic that the winter of 2013-14 (among the ten coldest winters on record in
STL) might kill it, but Nandina was unphased. Then, after observing birds feeding
upon the berries until berry-laden bushes were nearly barren….. there is really only
one logical conclusion that can be reached. It will become invasive in Missouri. I’ve
been saying this to anyone who cares to listen for two years now. Then, in
preparation for this talk I learned that Justin Thomas has recently spotted Nandina
escaping in the Springfield area, and the subsequent discussion among members of
the Missouri Native Plant Society revealed that it is known from a number of
locations in the southern tier of Missouri counties. Unfortunately, these are only the
first of many reports to come, so this is an emerging invasive you need to keep an eye
our for. It’s pretty conspicuous.
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Heavenly bamboo has also gotten a lot of attention after several dozen cedar
waxwings were found dead in Georgia a few years back. An autopsy of several of the
birds found their crops were full of Nandina berries, which contain high
concentrations of Cyanogenic glycosides, which release cyanide. The foliage is also
toxic, and considered dangerous for livestock, pets, and children. Michael Dirr’s
renowned “Manual of woody landscape plants” even reports an incident of a puppy
having seizures after consuming foliage. The fatality of cedar waxwings is somewhat
unique and due to their gorging feeding behavior. Most birds simply eat a few berries,
survive, and disperse them, perpetuating its invasion. If that weren't the case we’d
have a lot more dead birds, but not so much of an issue with invasion.
I don’t want to be sensational - there are many plants, including native plants, that
contain toxic compounds that are dangerous if consumed in large enough quantities.
However, the public doesn’t seem all that concerned about (or aware of) Forest
Inventory Analysis data estimating that over 9,000 acres are invaded with Nandina in
Tennessee alone. The reports of dead cedar waxwings are limited but factual, so if
that’s what get the public’s attention, then so be it. Don’t get me wrong, I love cedar
waxwings, and birds in general, but the broader ecological threat to our natural areas
is a primary concern.
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Now let’s talk about a potentially invasive species that is emergent not just in
Missouri, but more broadly in North America. This distribution map of Leather-leaf
virburnum isn't really all that compelling, but published accounts and herbarium
vouchers have now documented the escape of leather leaf viburnum in the past
decade from New York, New Jersey, Kentucky, North Carolina, and Indiana. This past
year I noticed escapees growing along railroad tracks and I-44 near the botanical
garden, flowering and intermingled with Bush Honeysuckle.
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Will this become a truly invasive species, or just a sporadically naturalized species?
Perhaps it will only invade waste places and never pose a threat to natural
communities.
I cannot say for sure, but the science of invasion biology has shown certain life-history
traits to be good predictors of invasion – leather leaf Viburnum has many of those
traits.
The best predictor of a species invasive potential is whether it has become invasive
elsewhere. In Belgium, leather leaf viburnum was first documented to escape
cultivation in 1901, yet in the past two decades this species has begun to increase
dramatically and become truly invasive in regions of Northern Europe.
Such lag-phases of even a century between introduction and invasion are not at all
uncommon for exotic woody species given their long generation times.
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That said, we still don’t know whether leather leaf viburnum will become invasive,
and the same can be said of other recent escapees like Callicarpa dichotoma, purple
beauty berry.
Only a small portion of naturalized exotic species will become invasive. This presents
a conundrum. When detection of invasive potential first begins, eradication is simple
and cost effective. However, by the time a species is a widespread invasive,
eradication is virtually impossible, and only intensive and costly localized
management is possible.
Leatherleaf virburnum is here at the early detection stage, Heavenly bamboo is at the
early detection stage in MO, but much further up on the curve nationally, and bush
honeysuckle, is already at the top of the curve in many areas.
Some countries like Australia use rigorous scientific Weed Risk Assessment to screen
potential introductions, then strictly enforce bans on the importation of plants that
pose a high invasion risk. This program is relatively inexpensive to operate, and one
study found that a similar program in the US would save billions. Unfortunately, our
current societal strategy is to sit on our hands and do nothing, waiting to take action
until it’s really too late.
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An important part of taking action is increasing public awareness, and doing a better
job of documenting the distribution of invasive species.
So the next two slides have a shared theme – Privets are awful, and so is our
documentation of invasive species distributions.
At the Shaw Nature Reserve, border privet is every bit as widespread and aggressively
invasive as bush honeysuckle. We have bush honeysuckle and privet in equal
measures there, and it’s increasing elsewhere. California privet is also present in
Missouri, but let’s look at this map of Border privet for a moment. Is there something
special about the arbitrary geopolitical boundary of Indiana that contains the invasion
– no, of course not, they’ve simply done a better job of documenting invasive species.
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Chinese privet and European privet are also serious invasive shrubs that we need to
vigilant about in Missouri. Once again, looking at the distribution map, does the
Mississippi River somehow prevent European Privet from entering Missouri? No, of
course not. In fact, it’s even been vouchered and included in the revised flora of
Missouri. Steyermark predicted it would escape here, and it has.
So, privets are awful. We all need to be vigilant about anything in the genus
Ligustrum.
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We also need to work at better documenting our knowledge. The Early Detection and
Distribution Maps are the best thing going, but they depend on us to contribute data.
I’m working to help transfer locality data for all US invasive species vouchered in the
massive collections of the MBG herbarium, but we also need to document new
invasions in the field, and making this easier is one objective of the Missouri Invasive
Species Task Force (MIEPSTF). We need user-friendly mobile apps for data upload in
the hands of every Natural History Biologist, Private Lands Conservationist, and Field
botanist in the state. This effort would tremendously help the effort to detect invasive
species early on when we are most capable of containing them.
This also helps convey the message to the general public, which isn’t thinking about
how dynamic invasive species ranges can be. Even a genuinely concerned member of
the general public may look at one of these maps and say, okay, good, that’s not a
problem here, so I’ll plant some more.
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Moving on to discuss several more emerging species in the remaining minutes, Sweet
autumn virginsbower was introduced into the United States as an ornamental vine
and is still in the nursery trade. Based on my personal experience I would have
though that this species has coevolved with chain-link fences in urban areas, but it is
also common on fencerows in rural areas, and is reported to be escaping beyond
fence rows with an alarming rate of increase in the osage plains, so it’s one to keep
an eye on.
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One we should all be very concerned about it Japanese Chaff Flower. It is not known
how Japanese Chaff-flower was first introduced to the United States, but I can’t
imagine that it was for ornamental purposes, ugly little beast that it is. It was first
reported from Kentucky in the early 1980s and is rapidly spreading along the Ohio
River and its tributaries, and now the Mississippi River in SE Missouri.
It is typically found in flood plains and bottomland forests growing in rich, moist soil.
However, only ten years after its initial establishment in Illinois, it is now being
reported from dry-mesic sandstone forests 40 miles north of the Ohio River, so
riparian areas may be merely the conduit for its larger invasion.
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Japanese knot weed is another serious concern, particularly in riparian areas.
Although it is much better established further east, several reports from around the
state noted a considerable increase over the past few years.
It spreads quickly to form absolutely massive, dense thickets and outcompetes native
plant species. Once established, populations are extremely persistent.
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Amur Corktree is another one to be on the watch for. MDC has an information page
warning of it’s potential for invasion statewide, but that it is not yet known from
Missouri.
Indeed, the recently revised flora of Missouri doesn’t include this species as part of
the flora, which is kind of embarrassing since it’s run amuck in several areas at the
Missouri Botanical Garden’s own Shaw Nature Reserve. Yes, it was actually planted
there back in the early arboretum days, but it has since naturalized more widely. This
past summer SNR staff cut down 50 year old trees that had escaped far from the
original arboretum plantings, and seedlings and younger trees have spread further.
Doubtlessly SNR is not the only place in the state in which this species is invasive, but
the experience there proves the invasive threat of this species in Missouri.
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Although Paulownia is pretty well known, I think most in our region still think of it as
being restricted to urban waste places and roadsides, but it has been known to
invade natural communities further east and I received a number of reports of it’s
increasing abundance here in Missouri. Also concerning is that we know from states
to the east that this species rapidly colonizes following fires, or any other sort of
disturbance for that matter.
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If you were to look at this distribution map you’d figure golden rain tree isn’t a
problem here, but in fact it was already documented from several counties
throughout Missouri in the recent revision of the Flora, in which is which the spread
of this species into natural areas was foreshadowed.
More recently, golden rain tree has been reported to be increasing on Xeric sites in
NW Missouri, and may pose a threat to glades.
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Lastly, Mimosa was not known outside of cultivation in Missouri at the time of the
original 1963 Flora of Missouri, but has since spread throughout most of the southern
half of the state.
Although most often associated with disturbed areas, it has a wide range of
environmental tolerance and is increasingly escaping into natural areas.
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In conclusion, it is important that we are vigilant about these emerging invasive
species, since it is at the emergent stage of the invasion process that control efforts
are most effective.
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