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.