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 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 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.

The second video, shot in early July, shows the doe on the left and two of the new fawns on the right enjoying a leisurely lunch of coppiced knotweed.

The coppiced knotweed in the foreground is short and easy for the fawns to get at. The knotweed in the background has grown to full height since it wasn’t coppiced and is unavailable as deer browse.

In the foreground of the video and 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.

The second video was shot with a motion activated trail-cam. I had set the trail-cam up about a week earlier 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.

In the next video you can see the buck chowing down on the succulent fresh shoots of resprouted knotweed. 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.