Japanese Stiltgrass – An Introduction To A Horrible Invasive Plant

If you’re reading this, then it is likely that you have just discovered Japanese Stiltgrass. Perhaps you found it through a plant identification app, or discovered an infestation in your yard…nonetheless, you’re probably surprised at how thick the infestation is…and why you never heard of it before. Let’s take a few minutes to understand a bit more about this plant. By the end of this short article you should know

• What is Japanese Stiltgrass
• Where did it come from?
• How to Identify it in all stages of life
• The effect on our North American ecosystems

What is Japanese Stiltgrass?

Japanese Stiltgrass is an annual grass that lives and dies each year. Scientifically known as microstegium vimineum, it grows 18″-36″ tall at maturity in full-sun to part-shade and a wide variety of soil conditions^[1][2][3]. Forming dense canopies of grass that is not favored by North American organisms, it out-competes & smothers native species reducing our bio-diversity.

Many authors have observed that while many invasive species get lots of attention, Japanese Stiltgrass seems to fly under the radar of most people. But, perhaps this makes sense. While a bright flowering Bush Honeysuckle, Callary Pear tree, Dames Rocket, or the ever conspicuous Garlic Mustard are quite noticeable, Japanese Stiltgrass is just, well, a grass. And not a very tall one at that. But studies have observed that Japanese Stiltgrass is much more aggressive in most environments^[10]. Furthermore, the aforementioned species have all been used in residential landscaping at one time or another, which also raises some public awareness.

Related ==> See how to control this invader. Different strategies, methods, alternative plants.

Japanese Stiltgrass wouldn’t appear to be a problem to uninformed observers, as it just looks like lush vegetation. During the growing season it resembles a thick pleasant-looking grass that appears to belong. It takes a plant-curious person to then question why it is so dense, why nothing seems to eat it, and why it is everywhere.

A weak plant, even a toddler can pull Stiltgrass out of the ground with ease. It’s tiny stilt-like roots barely cling to the soil and it makes one wonder how it can be so successful an invader? Because where it gets a foothold, without sufficient competition from natives, it completely takes over forming almost a monoculture. In winter one can spot areas of infestation by the dead draping grass that covers everything!

Quick Reference Table

Scientific Name	Microstegium vimineum
Common Name(s)	Japanese Stiltgrass, Stilt Grass, Japanese Stilt Grass
Native Range, USDA Zone	Japan/Asia, USDA Hardiness zones 4-9
Bloom Time	August
Height	18-36″
Light Requirements	Full sun to medium-shade
Moisture	Moist to slightly dry
Fauna Associations / Larval Hosts	Avoided by mammals, is used by some insects

Origin & Spread

While the official ‘invasion’ point isn’t known, we can point to the first documented specimen. Japanese Stiltgrass made it’s way to North America sometime before 1919, when the first herbarium specimen was collected in Knoxville Tennessee along a creek^[4]. By the 1930’s specimens had been collected through a variety of states including Alabama, North Carolina, Kentucky, Pennsylvania, and Virginia.

It has steadily spread since then, via seed, hitchhiking on the fur of passing deer and mammals. Or getting stuck to hikers clothes and boots, which we unwittingly track to new destinations. And we cannot forget what is possibly the most successful invasive species vector – construction equipment. When new powerline cuts are made, roads are constructed, or logging happens….well, the undercarriages of the equipment doesn’t necessarily get washed. And hence, we have a jump in the spread….

Current North American Range

How well will Japanese Stiltgrass be at spreading further into the Midwest? Well, it will likely have some trouble in the western portion, but I wouldn’t count it out. I see no reason why it couldn’t colonize along a river. Time will tell, but we can hope that it can’t cross the drier western portions.

Identification and lifecycle

The infographic illustrates the key features of the plant. It is an annual that germinates in Spring, grows through summer, then sets seed and dies in late summer/fall. The key factor to identifying this plant during the growing season is to look for the leaf with a vein running lengthwise. Please take note that on Japanese Stiltgrass the vein is offset – aka, it isn’t bisecting the leaf but is offset from the true centerline. Also, look for dense patches of monoculture, or near-monoculture….

Stalk/stem

The overall plant can reach 3′ tall in optimum conditions, but in drier or shadier areas is often 1-2′ tall. The stems (culms) will have branching along the ground, but once rising will be unbranched. The culm/stem is round and light green. One thing to note – if the lower stems (that lay on the ground) come in contact with moist soil, new roots will form and a new culm/stalk will shoot up. This is effectively creating a new plant.

Leaves

When it comes to identifying Japanese Stiltgrass, during the growing seasons there is no easier feature to give it away than the leaves. Leaves are alternate along the stalk, narrowly cuneate or lanceolate with smooth margins. The tip will be pointed and the base tapers almost to a point. The upper surface is medium-green while the underside will be a lighter shade of green.

But the most significant feature is the mid-rib or stripe….it will not bisect the leaf symmetrically but will instead be offset. This is the feature to confirm the ID. This, and probably seeing a lot of plants or at least a cluster.

Flower/seeds

In late August (reference zone 6/7), several racemes of spikelets will develop. They are somewhat short, being 1-3″ long. But when you see these start to form, it is time to get rid of the grass. Cutting the grass to the ground with a mower or trimmer, or pulling it at this time will prevent it from setting seed. This will in-turn help reduce it’s spread.

It’s important to prevent this plant from setting seed, as it can stick to animal fur, clothing, float on water, and can even blow in the wind somewhat. So, take the hour or two to remove the stiltgrass before it does it’s thing and spreads further the next year.

Root

The root system of Japanese Stiltgrass is fibrous. But the roots themselves are very small diameter and shallow rooted. However, they do really look like ‘stilts’ that the rest of the plant rests on. This makes it very easy to pull the plant up in moist soil (and you should do so!).

How Japanese Stiltgrass interacts with our ecosystems

To understand how Japanese Stiltgrass effects ecosystems, we need to consider what our forests look like before an invasion, and how it can effect natural food sources.

Impacts on plant diversity

There are numerous studies that have sampled, measured, and concluded that the presence of Japanese Stiltgrass will reduce plant biodiversity. This has been shown in mixed areas^[5], hardwood forests^[6], and significantly interfere with reforestation^[7]. Disturbance will be particularly bad in areas with new construction or logging.

Anyone who has had the pleasure of hiking remote trails in the Appalachian Mountains has likely noticed how far one can see under a dense stand of hardwoods. What I mean is that the forest floor is often sparsely populated with native Viburnum, Spicebush, and flowering herbs. A primary reason for this is that there are not that many plants that can survive under a dense canopy.

However, Japanese Stiltgrass can thrive under shady conditions, and it’s presence over time will greatly reduce the open areas and plant biodiversity^[8]. Another study found that the increased ground cover allowed rodents to be safer while foraging, and thus would eat more native seeds, which in turn reduces the number of native plants^[9].

This allows it to grow and set seed, slowly or rapidly taking over the entire forest floor. Researchers have documented how the percent coverage of Stiltgrass increased greatly over a four year period, from <5% to over 90% coverage in some cases^[10]. In more open areas and along streambanks where other plants are already densely populated, the Japanese Stiltgrass will be regulated to covering the open, bare, or recently disturbed spots (after construction). And after new construction, Japanese Stiltgrass can invade with a vengeance. Quickly covering an entire area and smothering nearly every other plant out of existence.

Impacts on insect diversity and quantity

The impact Japanese Stiltgrass has on insect communities where it has invaded is more complex. There are some insects that will feed on Japanese Stiltgrass, and as expected, their numbers will increase when it is covering the forest ^[11]. Other researchers have found less diversity of insects ^[12]. And other studies have found increases in diversity, but a lack of evenness or no difference in quantity of insects^[8][13].

For instance, a study from Tennessee forests found increases in the number of grasshoppers, crickets, and leaf hoppers, but a decrease in beetles and ground insects. While a study from Kentucky forests found no increase in the number of insects, but overall increased diversity.

A particularly interesting study in Indiana simulated an invasion over a three year period by planting plots with a native seed mix, but some plots also had Japanese Stiltgrass. There methods were quite extensive in that they repeatedly tilled the land (which was surrounded by hardwood forest) to deplete the seed bank, planted a native seed mix in plots (some with Japanese Stiltgrass), and erected barriers to prevent seed transfer. After three years they surveyed the insect diversity with sweep nets and found the plots with stiltgrass had less diversity than the control plots.

So, needless to say at this point, but direct survey of insect diversity has provided mixed results.

Impacts on mammals

So, from the above two sections we have established that the plant diversity will go down, while the insect diversity may go up or down depending on the particular study. But what about deer, rabbits, and other herbivories?

Well, I can tell you from first hand experience, living with a significant infestation behind my home….in an area that is populated by all Eastern North American mammals except black bear and elk that……nothing eats this grass. There is never any damage to the plant leaves or plants. There are never any bare spots….this grass is untouched. Some of the studies noted the same thing, particularly this one. So, again, no benefit of having this grass around. If anything, the fact that it reduces the native plant diversity means that it will indirectly reduce the amount of food for the deer, rabbits, etc.

Conclusion

We can see from numerous studies, which all shared the same conclusion that the presence of Japanese Stiltgrass will reduce the diversity and quantity of native plants. And when the forest canopy is disturbed, the Japanese Stiltgrass can begin to outcompete the reforestation of woody stems. This is bad for several reasons, as it will impact the amount of available host plants for pollinators and insects. And those insects who lose their host plants will no longer be food for birds, etc. But is it counterbalanced by the increased number of crickets, grasshoppers and leafhoppers? Well, that is a more difficult question to predict, and even more difficult to study. Another second order effect would be the study that found the increased vegetative ground cover led to more rodents, who ate more native seeds, thus also reducing native plant populations. Nature is complex, and has innumerable interactions that we are scarcely aware of.

What are we to make of these studies? Well, we need to start from the position that nature is a complex system, and when you tinker with it by introducing new species, you may be create second and third order effects that are unpredictable. While these effects could be disastrous or amount to nothing, we should consider what little upside we could gain from allowing an invader into our midst. Generally the potential outcomes are asymmetrical, but particularly so in this case as this grass won’t bring any benefit but aesthetics, and possibly feed some insects who weren’t endangered. This is not a stretch of the imagination – it is real.

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