Training Deer to Eat MORE Japanese Knotweed

This post represents a first for PrincetonNatureNotes.org, in that it is written by someone other than me. Mark Nowotarski lives in Stamford, CT, and contacted me more than a year ago after discovering a 2014 post I had written entitled Training Deer to Eat Invasives. Independently, he had begun foraging the Japanese knotweed growing in his backyard (young shoots are edible) and noticed that deer began browsing it as well. Released from any co-evolved limits on its growth, Japanese knotweed has spread across the US and globally, displacing native species and overwhelming any human efforts to counter it. Unlike people, deer are 24/7 land managers. Their appetites decide what can and cannot grow in our yards and woodlands. Training deer to eat a relatively edible species like Japanese knotweed could conceivably, in some situations, relieve browsing pressure on the native plant species deer tend to prefer. As the deer in Mark's backyard continued to consume young sprouts of Japanese knotweed, he sent me photos and text that I incorporated into a post a year ago.

This past growing season, Mark expanded his experiments and observations, exploring how a willingness to browse Japanese knotweed could pass from one generation of deer to another. It's still unclear how much of an impact deer browsing could have on the spread of Japanese knotweed, but it's an interesting inquiry. Along the way, we learn about deer family dynamics, the potential grazability of another uber-invasive, porcelainberry, and even the possibility that deer saliva affects the chemistry of plants. Thanks to Mark for sharing his work with us.

Training Deer to Eat MORE Japanese Knotweed

By Mark Nowotarski

In our post last year, “Training Deer to Eat Invasive Plants – Japanese Knotweed”, I shared observations of white tailed deer (Odocoileus virginianus) browsing a patch of Japanese knotweed (Reynoutria japonica) in my backyard in Stamford CT. The local deer had learned to browse the knotweed continuously from spring to fall resulting in drastically stunted canes. This had allowed numerous native plants to recolonize what had previously been an invasive monoculture. Steve and I speculated that perhaps if we cut the fully mature canes in other nearby patches of knotweed (i.e., coppicing), new shoots would grow and the deer local to those patches might browse the new shoots as well. I’m happy to report that that strategy at least partially worked.

Deer Raising Fawns on Knotweed

Before we talk about manual coppicing, let’s talk about how it’s occurring naturally. This first video, shot in May in my backyard, shows you what deer browsing spring knotweed shoots looks like.

A buck with new antlers approaches several knotweed shoots (Fig. 1). He gives a quick sniff, bites the top portions off, and eats them. The cut stems that are left behind are about 2 feet tall.

This particular buck may well be one of the fawns raised on knotweed in my backyard in the prior summer of 2024. Does and fawns form a family group when the fawns are born in late spring. They normally stay together and browse in the doe’s home range for a full year before dispersing shortly before the doe, if she’s pregnant, gives birth to the next year’s fawns. This is how last year’s fawns learn to eat the new shoots of knotweed that sprout in the early spring. The early spring browsing by last year’s fawns coppices the knotweed so that when it sends out new shoots at a convenient two foot height, the new shoots will be available for the new fawns born in late spring.

This year, our doe was, in fact, pregnant and sometime in June, she gave birth to triplets. Triplets are relatively rare (1 in 10 pregnancies) and indicate that the doe is well fed. By July, the doe and the new set of fawns were out browsing the knotweed previously coppiced by last year’s fawns before they dispersed.

In the foreground of Fig. 2 you can also see several native plants growing up through the coppiced knotweed. On the left is Canadian goldenrod (Solidago canadensis). On the right is Northern lady fern (Athyrium angustum) and sensitive fern (Onoclea sensibilis). The deer would occasionally take a nibble of the natives, but they concentrated primarily on the knotweed. 

This suggests that some of the native species less appealing to deer could begin to coexist with browsed stands of knotweed. I plan on trying this strategy next summer and will keep you posted.

I had set up a motion activated trail-cam to monitor how often, and for how long, the doe and fawns browsed the knotweed over the summer. They returned 4-5 times a week and browsed 5-15 minutes for each visit. This lasted from when I set the camera up in July all the way through late October when the knotweed senesced (i.e., dropped leaves and turned brown). So far this winter, the doe and fawns are still stopping by several times a week. In late October an 8-point buck also stopped by in the middle of the night. After a bit of hide-and-seek with the doe in the knotweed, they mated. We can now hopefully look forward to new fawns this spring.

The cycle of fawns being raised on knotweed and then dispersing in the late spring to new territories may be an important mechanism for the spread of knotweed browsing by deer. The question then becomes, can we spread it even further by coppicing canes ourselves.

Expanding Knotweed Browse with Manual Coppicing

At the same time I was monitoring the deer in my backyard I also set out this past year to explore the knotweed stands in our local parks and land trust sites to see if I could find additional evidence of deer browse. About half of the stands I inspected showed signs of early spring browsing. About half of the spring-browsed sites also showed continued summer browsing. Several of the summer-browsed sites showed a significant expansion of the browsed area versus last year. This was evidenced by areas with tall old canes from last year but only shortened canes this year. The deer hadn’t eaten the knotweed last year and it grew to full height. This year, however, they started in the spring and continued through the summer and fall keeping it short.

To try out the manual coppicing experiment, I selected four sites where there was either no early spring browsing or where there was some spring browsing but no rebrowsing of the new shoots emerging from the coppiced canes. Two of the sites did not show any rebrowsing of the coppiced canes. These sites either had no spring browsing or very scattered spring browsing. The knotweed threw out new shoots which grew to full height despite clear evidence (e.g. footprints) of deer wandering by. Apparently, the deer near those stands had not been sufficiently acclimated to knotweed as a source of food.

In the other two sites, however, the deer did resume browsing the new shoots thrown out by the coppiced canes. These sites had heavy spring browsing and well-worn deer trails right next to the knotweed stands.

The next set of figures shows the history of my coppicing experiment at one of the sites where the deer resumed browsing. 

Fig. 3 is a photo taken in April. It shows what an early spring browsed knotweed shoot looks like. It also shows a new shoot emerging from the cane a few weeks after the initial browse.

Unlike my backyard, however, these new shoots were not rebrowsed. By July they had grown to full height. This, along with the well worn deer trail right next to the knotweed stand, made it an ideal location for the coppicing experiment.

The next photo (Fig. 4) shows a section of the knotweed stand where I cut a 6 foot wide by 12 foot long section of the full grown knotweed canes to about 2 feet off of the ground. 

I cut the canes at a bias so that when I inspected them later on, I could tell whether a cut cane was my doing (angle cut) or a deer browse (horizontal cut). Cuttings were placed where they could not resprout or be washed downstream.

A few weeks later I went back to inspect and, much to my delight, the deer were browsing the new shoots emerging from the manually coppiced canes (Fig. 5). 

Where I had cut the canes, the ends were frayed and the canes turned black down to the next joint. Where the deer browsed the canes, however, the ends appeared to be sealed off and even flared out as if from accumulated water pressure coming up through the knotweed. The canes below the deer browse remained green. It makes me wonder if there is something in the deer saliva that causes a deer browse wound to heal quickly. Perhaps knotweed has evolved this way from browsing by sika deer (Cervus nippon) in their home range in Japan.

Once I confirmed rebrowsing of the new knotweed shoots at this particular site, I set up another trail cam to see what sort of deer were coming by. I half expected another family of a doe teaching fawns to eat knotweed.

 

Imagine my surprise when this magnificent 14-point buck showed up (Fig. 6). It wasn’t a family group. It was a bachelor group with up to four bucks coming through at various times.

This buck and several others came by 4-5 times a week to browse not only the knotweed, but the invasive porcelain berry (Ampelopsis glandulosa) growing over the area in the foreground and hanging off of the uncoppiced knotweed canes in the background. In fact, I suspect that the porcelain berry is the primary reason the deer were there. The knotweed was just an additional food source.

The bucks suddenly disappeared in mid-September after they shed their velvet, no doubt to pursue does during the rutting season. I have not seen them since except for one poor fellow with a missing antler. Given how well-worn the deer trail is, however, I expect to see at least some of them again in the early spring.

Deer in a given area learning to eat Japanese knotweed appears to be a multi-year process. It starts with initial occasional browsing of early spring shoots. The spring shoots are then browsed more intensely in the ensuing years. Eventually the deer start browsing the new shoots growing from the spring browsed canes. If the knotweed is in a doe’s home range, then she starts to raise her young on knotweed and then the process spreads as the fawns raised on knotweed go on to establish their own home ranges.

If you try your own coppicing experiments, please let us know how they work out. I’ve joined a project on iNaturalist called “North American Knotweed Ecology Project” where we can share our observations. iNaturalist is a great tool for finding knotweed stands in your area. If you look carefully in the knotweed photos, particularly those taken in April and May, you can often see a deer-browsed shoot here and there. Sites with browsed shoots would be a likely candidates for coppicing experiments.

In the future I hope to learn more about the general phenomenon of deer adjusting their diets to consume invasive plants; how this affects browsing pressure on native plants; and what impact it might have on the deer themselves. The deer won’t solve all of the problems with invasives, but they may very well be an important part of the solutions.

- Mark Nowotarski

Editor's note: A related initiative is the effort to train cattle to eat invasive species. A woman named Kathy Voth appears to be a leader of this approach. 

Training Deer to Eat Invasive Plants -- Japanese Knotweed

Here's a story and a partially forgotten concept from ten years ago that a commenter on this blog helped me reconnect with. The concept has to do with actively training deer to eat invasive plants. Thanks to Mark Nowotarski of Stamford, CT for reminding me of this concept, and adding his own interesting twist.

Deer manage our landscapes with their appetites. Whether it's your unfenced yard or the local nature preserve, deer largely decide what can grow and what gets eaten down. Introduced plant species can become invasive if they are left uneaten by finicky deer, thus allowing them to proliferate rapidly and overwhelm native flora. Wouldn't it be great if we could train deer to eat invasive species, and thus restore balance to the landscape? 

Ten years ago, I wrote about two ways to potentially train deer to eat invasive species. One is to cut down invasive shrubs along the deer paths and let the stumps resprout, thus presenting the deer with tender new shoots to nibble on. As the deer (hopefully) grow accustomed to the taste and texture of the invasive's new growth, perhaps they would begin eating older foliage as well. 

It's also possible that the deer respond to visual cues. If we repeatedly cut back an invasive shrub, deer may assume other deer have been eating it, and chow down. Deer have reason to revisit a shrub again and again. By eating its foliage, they stimulate the shrub to replace the lost leaves with new ones, much as we do with basil and other vegetables in our gardens. Making their accustomed rounds, deer essentially farm the forest for fresh foliage.

Though we experimented with this at Herrontown Woods mostly with winged euonymus, Mark reports some success with recruiting deer to browse a patch of Japanese knotweed in his backyard:

"I've seen a similar phenomenon where I live in Stamford CT. We have a little bit of woods in the back yard where deer tend to congregate. There is a dense stand of Japanese knotweed down by a small stream. A few years ago, I started foraging the tips of the knotweed in the spring and noticed that the deer continued to browse the knotweed through the summer. Every time the knotweed would send out new shoots, the deer would browse the tips. At first they just browsed where I foraged, but in the past few years now they have expanded the browsed area and are actually beating back the knotweed. If you see any knotweed browsing in your area, I'd love to hear about it."

He sent photos and more text to illustrate:

"About 4 years ago, I noticed that the deer on our property had started browsing the spring shoots of a stand of Japanese knotweed. The knotweed grows down by a swampy stream and has been there for at least 30 years. Each year the deer have browsed the stand more intensely." 

"When the knotweed throws out side shoots after the initial browsing, the deer browse the tips of the side shoots. When the side shoots throw out secondary side shoots, the deer browse the tips of those as well. This continues through the summer."

"The knotweed in the browsed area is kept to about 3 feet tall and is very sparse. Abundant sunlight falls on the forest floor and there has been a substantial increase in the plant biodiversity of the browsed areas." 

"This includes the sprouting of native plants, such as Sassafras albidum (Sassafras) and Impatiens capensis (Jewel weed). I found the I. capensis particularly surprising since this is normally heavily browsed by the deer."

"The deer only browse a portion of the knotweed stand. If a knotweed shoot reaches full size, it’s not browsed. Nonetheless, each year the deer have been browsing a larger and larger area. They originally browsed just an outside edge of the stand where I used to forage knotweed shoots in the spring, but last year they started hollowing out the center of the stand." 

"Based on your experience with winged Euonymus, this leads me to suspect that it might have been my initial foraging that led the deer to continue the browsing. It would be interesting to forage some unbrowsed knotweed in the spring and see if the local deer continue."

Thanks again to Mark Nowotarski for these photos and descriptive text of the interesting dynamic between deer and the patch of invasive Japanese knotweed in his backyard. In our experiment ten years ago with winged euonymus, we found that invasive shrubs ultimately grew back, likely due to our having cut so many that their myriad young shoots overwhelmed the deer's mild interest in their foliage. We also didn't think to try focusing our cutting close to deer paths. 

Anyone managing a sizable nature preserve will soon grow weary of cutting invasive shrubs only to have them grow back. Treating a freshly cut stem with a thin film of systemic herbicide, using a Buckthorn Blaster, is a targeted, minimalist way of actually making progress in a woodland choked with invasive species. 

But especially for the vast majority of woodlands that go unmanaged, the concept of training deer to eat invasive species has appeal. I'm looking forward to harvesting some young shoots of Japanese knotweed this spring, as an experiment. Having trained the deer in his backyard to eat this highly invasive plant, Mark may train me to eat it as well. Testimonials like this one suggest the young shoots are quite tasty. Research the how, what, where and when before giving it a try. Mark recommends sauteing with butter.

Update 1.30.25: Just came across another of my posts from ten years back, entitled Paradox Lost, or, Less Irony in the Woodland Diet, offering a third way to get deer to eat invasive plants. If there are mint-flavored sprays that discourage deer from eating ornamental plantings, maybe there's a flavor of spray that would encourage them to eat invasive plants. 

Who's Been Eating Our Lilies? -- An Update on Deer in Princeton

What's been eating our lilies on busy Harrison Street?! Buds on the lilies lining our front walk had generated considerable anticipation in the household, until they disappeared overnight. We arrived in Princeton a few years after deer began being professionally culled in 2000. Back then, we'd see herds of deer wandering down Murray Ave, or even main roads like Harrison Street.

Though Mayor Marchand took a lot of heat for culling deer, she and township council stuck to their principles, and the result has been dramatically reduced carnage on the roadways, venison for food kitchens, and a rebound of the native flora in local nature preserves. The remaining deer are better fed, finding in the wild a greater abundance of their preferred foods.

Another benefit has been reduced damage to gardens. Our garden had prospered deer-free for years, until one showed up in the backyard over the winter. Evidence suggests there have been visits since then, and now our front yard's centralized setting along Harrison Street is proving no guarantee of safety from their unsolicited landscape services.

All of which prompted an inquiry into how the town's deer culling has been going lately. The temporary lack of an animal control officer and the increasingly quirky climate contributed to a relatively unsuccessful deer cull in the winter of 2016-17. This past winter, however, they were very successful with the culling. According to this update from council member Heather Howard:

"Deer hunt was much more successful this year - in part because of colder weather. This year, 196 deer were harvested, as compared to 63 last year. Also, initial analysis of traffic data shows that deer-related motor vehicle accidents were down. 

Looking ahead to next year's hunt, we are looking for new areas to expand the hunt. There have been an increase in deer complaints in the Riverside neighborhood, which is a hard neighborhood to find suitable areas for hunting, because there are no public properties and private lots tend to be smaller.  

Looking ahead to next year's hunt, we hope to have our annual contract for bow hunting on the Council's agenda by the end of the summer, so they can start with the hunting season in September."

It's important to emphasize that bow hunting is not adequate to control deer numbers. To restore the ecological balance lost when we extirpated the natural predators of deer long ago, it's necessary to have professional hunters who can operate safely in our preserves, to augment what the bow hunters are able to do. One comforting thought is that the deer have lived quality, free-range lives, unlike so many of the animals we eat.

How this relates to the lost lilies in the front yard? Heather's mention of increasing deer complaints in the Riverside neighborhood, along with the surprising appearance of deer in our yard despite deer culling, might suggest that the deer that are eluding the culling are those that spend more time in the densely populated neighborhoods. It's a theory that will be tested by time, and lilies all over town.

A Deer's Landscape Services

Funny. I didn't remember requesting landscape services. Deer hadn't shown up in our backyard since around 2005, when the birth control experiment was still underway and the females were looking elegant and very Princetonian with those identification cards hanging from their ears. Maybe it was taking refuge from hunters, or had had enough of rutting season.

Deer are expert landscapers, trained from birth in the art of pruning. They wouldn't think of driving around town in those giant fossil fuel-consuming trucks that deploy gangs of mowers to quash nature's vertical ambitions.

Deer are too considerate to make a racket with leaf blowers. And instead of releasing a cloud of fumes to sting your nose, they leave behind discrete contributions of fertilizer. They do the landscaping for free, but have a strong independent streak, refusing to adjust their methods to an owner's wishes.

Deer are very sensitive. Even though I stood ten feet back from my bay window, I still had to freeze several times so as not to spook my subject. It's impressive how they are so observant and "in the moment"--a state of mind we too might more easily attain if we were further down the food chain.

When I stepped outside to shoo it away, it didn't flee back over the fence but instead ducked behind some shrubbery in a back corner, waiting for my next move.

Once it realized I was not a threat, but instead just another among Princeton's backyard paparazzi, it regarded me with what appeared to be disdain.

No longer feared, I went back inside without finding out from where it came into the yard.

It was interesting, though, to watch it at work, browsing maybe ten leaves of our oak-leafed hydrangia before moving on to munch some grass. Though I've heard stories of deer wiping out a gardener's favorite plants overnight, my experience is that animals generally lack a thoroughness, whether it be browsers or predators.

My theory with deer is that their alertness to the potential for attack makes a patient devouring of one shrub less adaptive than a few nibbles here and there. And if much of what they eat has some level of toxicity, browsing can help reduce the risk of consuming too much of any particular toxin at any one time. Light browsing also promotes quick recover by the plants, as long as there aren't too many deer visiting the same plant. This suggests that deer could actually be good stewards, if their population was in balance with the landscape.