Mystery Lawn Weed Identification?

Never let it be said that nature allows us to get bored or complacent when it comes to gardening! In addition to ‘surprises’ like alphabet-busting hurricanes and ravenous rabbits (more about those in an upcoming post), there are also those seemingly innocuous things that she sneaks in on us and dares us to try to figure out. In my case, it’s been a Mystery Weed.

I suspect that the first appearance of the mystery weed was in June 2018. I had recently learned the hard way that I had a Poison Ivy Problem and had called in a specialist to get an estimate for removal. Whilst walking the property with me and advising me that in addition to poison ivy I also had a serious problem with bittersweet, solanum, multiflora rose and wild blackberry, he suddenly stopped, pointed at something in a lawn area, and said “Uh oh, you have [unfamiliar plant name that I cannot recall]. I just went to a seminar last week and learned that it’s been spotted for the first time in [a town about 20 miles west of here]. It’s almost impossible to control once it gets established.”

Having just been verbally hit with a litany of additional and unexpected undesirables in my garden, his comment sounded suspiciously like the prologue to a sales pitch for a “lawn weed control service” to go with the “poison ivy and invasive vine control package” he’d already mentioned. Not having been born yesterday, I refused to take the bait; murmuring a non-committal “Oh, hm, interesting”, I deftly moved on to another subject and put the supposedly-problematic lawn weed out of my mind.

That may have been a big mistake.

Now, I should explain that the non-hardscape, non-ornamentals part of my property isn’t exactly what some would dignify by the title of “lawn.” Yes, it is green (most of the year) and it gets cut once a week during the April-Oct growing season here. And there is a fair amount of some sort of turfgrass, but a uniform monocultural greensward it is not. The grass shares space with a varying amount of violets (which I like), clover (which I don’t mind), dandelions (which I do mind), crabgrass (which I loathe and detest), and a herbarium of other weeds such as hairy bittercress, carpetweed, chickweed, prostrate spurge, ground ivy, bedstraw, henbit and wood sorrel, depending on the season. There are probably a half dozen others I forgot to mention, in addition to the dreaded poison ivy which can pop up anywhere. But the point is that they’re all familiar; or were, until this year which is when I noticed numerous large patches of something unusual which was promptly dubbed the Mystery Weed.

It grows in dense patches of small (about 1/2 to 3/4 inch long) oval leaves of rich medium green, and is noticeably springy when stepped on. This ‘springiness’ was actually what called my attention to it, in fact, because even in my clunky rubber Muck Shoes I could feel the difference as my feet moved from the normal grass/weed surface onto a clump of mystery weed. It felt almost miniature-shrub-like and different from anything else. Over the course of the next few months I began to notice more and more patches of it – some only 6” or so in diameter but many others quite large. Where had it come from, and what was it? Identification, either online or in my copy of Weeds of the Northeast, proved surprisingly difficult. The foliage most resembled Asiatic dayflower (Commelina coelestis) and heal-all (Prunella vulgaris) but I had never seen any flowers and so identification was stymied – so I thought – until spring 2021 when I would make a note to watch for them.

In the meanwhile, I had decided to finally wage all-out war against the crabgrass via application of Tenacity in mid-June. Heal-all was on the list of ‘weeds controlled’ and so I assumed that would take care of the Mystery Weed as well. Although it did turn the tips of the topmost foliage white eventually (Tenacity works by derailing photosynthesis) it did not kill the M.W. outright. Okay, well, I guess it isn’t Prunella

You’re probably curious as to what this stuff looks like, right?

An important caveat: The detail photo was taken in the morning while the dew was still on the leaves and making them look textured, fuzzy, or hairy: They are not. They are smooth.

Subsequent applications of Tenacity did its expected number on the crabgrass but didn’t make a dent in the mystery weed other than a few whitened leaves. One day in late August I was hand-digging some dandelions near a patch of M.W. and thought “let’s see how hard this stuff is to pull up.” I grabbed a handful and gave an experimental tug – and was stunned to see how easily it came out.

In less than five minutes I had plucked out (it really is more like gentle plucking than pulling) a large patch of M.W. with almost no effort at all. I’ve never seen any plant (desirable or weed, either alive or dead) part company with the ground so easily. It only took me about an hour to hand-remove 75% of the stuff from my property, but I still had no idea what it was. Clearly, it kills whatever grass was unlucky enough to be engulfed by it. The new question now was: annual or perennial??

Something Unexpected

The lawn cutting guys did not, for some reason, show up on September 10th as they should have. While stalking around the backyard on the 14th and muttering about unreliability, I suddenly saw a patch of Mystery Weed that I’d missed a couple of weeks ago. It was snuggled up against a slightly raised bed. As Gomer Pyle was fond of saying: “Surprise, surprise!” because this is what it looked like:

My mystery weed is not a weed at all: It’s a Mystery Grass, because it has produced tillers!

This sent me dashing back to the “monocots” section of Weeds of the Northeast but again I could not find a match for the foliage, other than for the Commelina which the tillers proved it was not. Thinking back to 2018, I am positive that the poison ivy control guy (whom I fired after one season when it became clear he was charging $$$ for almost zero effective work) did not describe the 2018 weed patch as crabgrass. But is it?

This is the normal crabgrass that we deal with in our area: Large Crabgrass, Digitaria sanguinalis, freshly dug from my lawn. It spreads out in a flat, starfish shape and is absolutely not in any way “springy” to walk on, as is the Mystery Weed….errr, Mystery Grass. (Tenacity works like gangbusters on common crabgrass, by the way.  If you have crabgrass, Tenacity is what you need but you won’t find it in the big-box stores.)

Healthy clump of Mystery Grass on the left, a small specimen of Large Crabgrass on the right. (It was a challenge to find one, because the Tenacity had already zapped most of them.) Even the green is different.

That leaves the two other species of crabgrass as possibilities: Smooth crabgrass (Digitaria ischaemum) and Southern crabgrass (Digitaria ciliaris; this is not native to the northeastern part of the USA where I live.) But the leaves and habit of those two don’t match the M.G. any better than large crabgrass does.

A third possibility is the Dwarf crabgrass, Digitaria serotina, also called Blanket crabgrass. Normally only found in the Southeast (Florida, Texas, etc.) perhaps climate change has allowed it to move northward. According to Bayer literature, this species differs from other crabgrasses in that

Blanket crabgrass is mat-forming. Its leaves are crowded on stems and the blades are short, about 1 inch long. It is very similar to India crabgrass, but blanket crabgrass has sheaths and blades that are hairy rather than smooth.

That last sentence sent me scurrying for a magnifying glass. No hairs that I can see, so perhaps it’s India crabgrass? (Then again, I can see no hairs on Large crabgrass either) But India crabgrass, like blanket crabgrass, is supposedly a Southern native, so what’s it doing in New York? (other than “well”, obviously!) If mine is either of these, there’s depressing news from the University of Florida Extension Service:

Most [crabgrasses] are summer annuals, but blanket and India crabgrass tend to perennate (live longer than only one growing season) which is why postemergence control is often the only option.

Because these two species are not on the list of three crabgrasses that Tenacity controls, that could explain why it only delivered a ‘transient trauma’ rather than a knockout punch. Hmmmm again.

So, I have a question for all of my American readers: Do you happen to recognize my Mystery Grass, and – for those in the South or Southeast – if you are familiar with either Blanket or India crabgrass, does it feel springy when you walk on it? And does it pull out of the ground with ridiculous ease? (Admittedly I only tried that in late summer and so it may not be so obliging earlier in the year.) The other species of crabgrass have neither of those characteristics. As we all know, the first step to victory is to know your enemy! Many thanks for any identification assistance or experience. 🙂

ETA: A reader comment has reminded me that I forgot to mention what would normally have been my first resource: Taking a sample to my local Cooperative Extension Service for identification. Unfortunately, they have been closed for the past however-many months due to the coronavirus pandemic, so that avenue isn’t available. Bummer!


Thanks to a great “tip” from one of my readers’ comments, I am now almost positive that the mystery invader is Japanese Stiltgrass, Microstegium vimineum. I’d originally dismissed this as a possibility because all the photos via Google showed it as not only a longer leaf but described as having a “silvery midrib” which I did not see on my plants. But after revisiting and researching, my invader ticks almost all of the boxes for stiltgrass. The Illinois Extension Service has an excellent illustrated Field Guide to the Identification of Japanese Stiltgrass PDF which gives these characteristics:

Roots: Thin weak root system; pulls out of the ground easily; rootlets (‘stilt’ roots) descend from nodes along lower section of the stem.

This is their photo illustrating the stilt roots.

A photo of the root system on a sample of mine. Root identification matches on all three counts!

Leaves: 2-4 inches long and around ½ inch wide; both ends pointed; silvery, off-center midrib often present; smooth edges; well-spaced along stem.

Mine match in terms of the pointed ends, smooth edges, and well-spaced. However, I had to hunt for a while before finding just one or two leaves that might be said to have a slightly lighter midrib, and I would not really consider them “off-center” unless it is literally by millimeters sometimes. They are also rarely more than an inch long. However, again: Most photos show mature plants that are growing without interference, whereas my yard gets mowed once a week from May through October – so what I see here may simply be young, rather than mature-length, leaf blades. I do still quibble with the “silvery”midrib although that too may be a feature that develops as the leaf blade gets older. Some sources decribe this as a “noticeable stripe” but not on the ones in my yard!

I also learned that in addition to crabgrass-like tillers (which I only discovered that one time), stiltgrass also has “cleistogamous flowers occurring in short branchlets lower on the stem” – and guess what I found?

This is how stiltgrass seeds itself around even when regularly mowed! The Rutgers (NJ) publication says

Hand hoeing and hand pulling can be an effective means to control stiltgrass as long it is done before it has dropped seed (~ mid August in NJ). Recognize that hoeing or pulling stiltgrass early in the season (before August) disturbs soil which can stimulate germination of new plants from the seed bank. Constant mowing such as in a lawn will cause stiltgrass to grow at a lower height but still set seed.

Another source says: With frequent mowing, [it] adapts and can set seed on plants one to three inches tall. Yep, it sure does!!

In other words, it doesn’t need to make tillers in order to dump hundreds/thousands/gazillion seeds on the soil (the “seed bank”) which remain viable for several years (!) Worse yet, the seeds are easily distributed by contact with shoes, clothing, and pet fur. No wonder it’s such a pest. One thing that surprised me is that no sources mention that mowed stiltgrass in lawn areas feels springy when stepped on, compared to other grasses and weeds. But almost all sources agree on one thing: Pre-emergents do not work on stiltgrass (it purportedly is in the bamboo family…which explains a lot!) although Penn State University says

Pendimethalin (‘Pendulum’), imazapic (‘Panoramic’), and sulfometuron (‘Oust XP’) are effective against stiltgrass. ‘Pendulum’, for example, will prevent stiltgrass establishment when applied two to three weeks prior to germination, but has little effect on plants that are already present. Rainfall or irrigation is needed to dissolve the herbicide.

As far as post-emergent control, the only selective (non-nuclear-option) herbicide that works on stiltgrass is Acclaim Extra (fenoxaprop). Oddly, this is not mentioned in the 2018 Penn State article which cites only nonselectives such as glyphosate which nuke everything.  It also explains why the Tenacity I used this year turned a few leaves white but otherwise did nothing to the stiltgrass. Acclaim Extra costs $90 for a pint (16 oz), which gave me momentary sticker shock until I remembered that I’d paid $65 for the 8-oz bottle of Tenacity!

I probably will not try the pre-emergent next spring, but only because I will have to fill and re-seed the areas left bare by pulling the stiltgrass out. Supposedly, stiltgrass germinates “in late spring, or after disturbance throughout the year.” I will then break out the Acclaim Extra and let ‘em have it, and pull any survivors starting in mid to late August, as per the NY Botanical Garden’s advice:

If you pull in spring or early summer, you will be giving yourself twice the amount of work, because the new plants that germinate will have time to produce another crop of seeds.

Clearly this will be a multi-year control operation (much like crabgrass) but at least now I have a battle plan. (Thanks, bittster! 😊 )

  6 comments for “Mystery Lawn Weed Identification?

  1. October 6, 2020 at 2:07 pm

    Do you have an invasive plant working group (or some such thing)? Or, short of that, how about finding a weed scientist (yes, they exist!)

    I had a similar problem with a “weed” that I went absolutely nuts trying to identify. I finally emailed some photos to one of our weed scientists at our local experiment stations and he came back with–wait for it–it was a tiny shrubby St. John’s wort! So I could have looked until I died at weed resources and never found it!

    Sometimes a weed is actually a wildflower. Who knew?

    As for your grass…I can’t help you. Good luck.


    • October 6, 2020 at 2:27 pm

      I can completely relate to your St. John’s Wort because there were days when I wondered if this was some weird form of mutant Vinca minor! But at least the tillers finally pointed me in the direction of a grass. You could have knocked me over with a feather when I first saw that, though!

  2. October 10, 2020 at 10:56 pm

    Japanese Stilt Grass.
    I noticed it lining long stretches of shaded Pennsylvania woodland roads and then it began moving into the woodlands to blanket the ground. Last year I found a few stray bits along the berm and now it’s thicker and showing up in the garden…
    It’s an annual and super easy to pull, but it’s sooo invasive. My brother has it all over his yard in Stonybrook.

    • October 11, 2020 at 12:17 pm

      That was actually one of the species that I had considered, but because most photos show the stiltgrass blades as longer and narrower, and with a “distinct silvery midrib” I had discounted it. The midrib thing was the main disqualifier in my case but I just did a lot more googling and realized that most photos show examples of larger clumps. So maybe because mine gets mowed every week, the leaves remain juvenile/smaller? I will definitely look for that midrib next year. If it’s indeed stiltgrass, it was just pure dumb luck that I decided to hand-pull it at the ONLY “right time” of year (August/Sept) and not earlier! Today’s googling also explained why Tenacity didn’t kill it: The only selective post-emergent that works on stiltgrass is Acclaim Extra. It’s expensive but I will get some next spring for sure. I was surprised to learn that it’s so widespread, because I’ve been gardening on LI for decades and never run into stiltgrass before. Then again, all my other gardens were on the south shore (near the Bay) so maybe that’s why. Or maybe I was just VERY lucky, lol

      • October 11, 2020 at 8:47 pm

        As it goes to seed it will probably look more typical, like you said it seems to stay bushy if it’s in the lawn and cut every week.
        It only showed up in Stonybrook maybe four years ago? Here it came in last year when they did the work behind our house… I think it will be a forever problem now…

        • October 12, 2020 at 1:58 pm

          Thanks to you, I have now updated my post with a whole new section!It does include a battle plan, so maybe you can take the PA front and I’ll take the LI one. 🙂

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