Think Out Loud

Invasive English holly not so jolly for Pacific Northwest forests

By Gemma DiCarlo (OPB)
Dec. 11, 2023 2 p.m.

Broadcast: Monday, Dec. 11

A cluster of bright red berries sprouts from a branch with spiky, dark green leaves.

English holly, photographed near the border of Saint Edward State Park in Kenmore, Wash. While certainly festive, the invasive plant poses a significant threat to Pacific Northwest forests.

David Stokes


English holly certainly looks festive this time of year, with its dark, spiky leaves and bright red berries. But as recently reported in High Country News, the invasive plant poses a significant threat to Pacific Northwest forests. Much like English ivy, English holly is shade-tolerant, meaning it can thrive in the dense forests of western Washington and Oregon. It forms a dense undergrowth that can overtake native species, significantly changing forest composition and reducing biological diversity.

David Stokes is a professor emeritus of ecology and conservation biology at the University of Washington, Bothell. He joins us with more details about the spread of English holly in the Northwest and what’s being done to combat it.

Note: The following transcript was created by a computer and edited by a volunteer.

Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. English holly certainly looks festive this time of year with its dark spiky leaves and bright red berries. But as recently reported in High Country News, the invasive plant poses a significant threat to Pacific Northwest forests. Much like English ivy, English holly is shade tolerant, meaning it can thrive in the dense forests of Western Washington and Oregon, outcompeting and overtaking native species. David Stokes is a professor emeritus of ecology and conservation biology at the University of Washington Bothell.

He has studied the spread of holly and he joins us now. Welcome to the show.

David Stokes: Glad to be here.

Miller:  How did English holly, which is native to Europe and North Africa, get here?

Stokes: Well, as is the case with a lot of non-native species, especially attractive ornamental plants, it was brought intentionally in the mid-1800′s for horticultural reasons. It was, as you said, a very attractive plant, probably reminded people of home back in Europe. And so it was brought out in the mid-1800′s purposely.

Miller: And was the idea in general that, let’s just plant this to have it around for our own, as a hedge on our property, or let’s grow it commercially?

Stokes: I can’t speak to whoever brought it out for the very first time, but relatively quickly, an industry developed around the plant. And by the mid-1900′s there was a multimillion-dollar business centered on holly, and I think in Washington and Oregon there were about 50 commercial growers by then.

Miller: When did people first start to notice that it was spreading in an unhealthy way?

Stokes: That’s an interesting question. Land managers, people who are managing parks and forest lands in general, had been noticing this for several decades, but there was no record of it in the scientific literature. Nobody had really examined it. So there was a lot of anecdotal information that holly was invasive, but no real scientific proof…

Miller: When did you decide to study it?

Stokes: I decided to study it shortly after 2006, when I came back to the Northwest from and I had a position in California, but I came back to the Northwest in 2006, and as I was walking around in the woods, I started noticing holly in places where you wouldn’t expect to see it. And it really started as kind of a question, like what’s going on here? Is this thing spreading or what? And at what rate? And that’s really the genesis of my interest. I also was looking for a project that I can involve students in, in field research. And this is sort of a perfect project for that.

Miller: Where did you decide to focus for that research?

Stokes: Well, since the overall interest was, what’s the state of the invasion in Washington? We were looking for a fairly typical location, and the real question was, is it invading forests? So we wanted a site that was typical forest, where we knew that holly did exist. And the place that we settled on was Saint Edward State Park, which is a really beautiful, largely intact forest land in the greater Seattle area, surrounded by suburban development.

So it’s a typical forest, typical of all of Western Oregon and Washington, but in a sort of urban or suburban context. And it’s got a distribution of species that are pretty typical. If you looked at the forest, you’d think, well, that just looks like a really healthy forest. You wouldn’t have even really noticed the holly until you started looking at it. But that’s what we did. We started looking closely and there was quite a bit of holly there.

Miller: You say, “looking closely.” Can you describe what you and your students actually did?

Stokes: Yeah, we took a really straightforward approach. We basically identified an area of about 20-acres in size, which is pretty large for a field ecology survey of this kind. And we walked every square meter of that 20-acres. It took two years to do this. Our field season was only in the winter because that’s when holly is most visible. And we basically look for every individual that we could find, whether it was one-inch tall or 50 feet tall.

And for each individual, we pulled the plants that we could pull, we cut down and herbicided the stumps of the plants that were too big to pull, and we took a sample of each one at ground level, so we could count the rings, the annual rings, that the tree put on and determine when it established. And we also mapped all of those sites with GPS. So at the end, we were able to assemble a database of all of the holly that existed on that 20-acre plot and when the various individuals became established. And the end goal being to be able to characterize the history of the invasion.


Miller: And what did you find when you graphed it, with the oldest one that you could find, and then the increase in population over time? What did it look like?

Stokes: This is what was really interesting. Like I said, if you just looked at the forest, if you just went out in the forest, you would hardly notice holly. It’s just not that conspicuous when it’s at that density. But what we found was that, first of all, the oldest tree we found in our study area was established in 1966. And then there was a period of very slow establishment for the next 15 years or so. And then the rate of establishment took off in an exponential curve, and a really rapid increase in the numbers of trees, in the size of those trees and the amount of canopy there was.

And so we got some very alarming projections of what that density would look like, even in as little as 10 years. We found that the population was doubling approximately every six years. So it was fairly alarming, actually, the rate of increase that we discovered.

Miller: How do you explain that? What made holly after, I guess that establishment time, when that first tree was getting going, how do you explain the really significant spread after that?

Stokes: Well, first of all this pattern of a lag time is very typical of a lot of invasive species. They don’t seem to take off right away. They sit there for a while just hanging on. That could be because of the insufficient density of individuals, could be pollen limitation. Holly is a dioecious tree, meaning it has separate sexes. So you need male and female trees to grow the population. So that could be a number of reasons why it wouldn’t take off right away. But once it reaches that critical mass, it was apparently really well- adapted to these conditions.

Certainly didn’t have any competition from other holly because there was no holly there to begin with. It appears to do really well in our climate, which is similar to where it exists in Europe and North Africa. And basically, it just didn’t have any limitations - no natural predators, no competition from other similar plants. And so it was really in an unrestrained condition, and may still be, we don’t know how dense it can get, ultimately.

Miller: You noted just now that you’re not sure how dense it can be, but you are part of a team that visited a forest around Lake Youngs, a reservoir that provides drinking water to the Seattle area. Can you describe the density of holly there?

Stokes: That’s maybe a little window into what the future could be for other locations where holly has invaded. It’s a place in the reservoir system for the city of Seattle, and it’s been off limits to people, but somehow holly got established there and it’s been growing along, undisturbed, for at least several decades. And what you see when you go out there is a Douglas fir forest overstory, but underneath that, there’s a 15-to-30 foot high growth of holly, really, call it a thicket, with very little of anything else growing underneath that. So the whole native understory in some areas of Lake Youngs has been replaced by this tangle of holly, sort of jungle of prickly leaves and dense criss-cross branches.

Miller: You noted that there are still Doug fir above that - older, larger trees. But do you imagine that Doug fir saplings could take hold and find enough water or sunlight to actually become new trees given the understory of holly?

Stokes: That’s one of the really concerning things about this situation. It’s not simply an aesthetic question or a conservation question, but it’s also an economic issue. We did not see any young Douglas firs coming up underneath this holly. It’s unlikely that Douglas fir would establish underneath a canopy. Douglas fir doesn’t really like to establish under any kind of canopy, but it would almost certainly not come up under holly.

And so there is a potential for large economic damage from this invasion, when you consider that this is not just happening at these little parks where I and other researchers have happened to see it. It’s happening all across the Northwest and there’s a forest service researcher, Andrew Gray, who’s done surveys over large areas of federal land in Oregon and Washington, and found very similar rates of increase in those areas as well, similar to what we found. So the potential ultimately could be a significantly reduced timber harvest.  And this is a long, long way out, but it is a potential end point for this sort of phenomenon.

Miller: What are the challenges to removing holly?

Stokes: Well, it is a tough customer, I’ll tell you that. As I said, you can normally pull up small ones, and probably many of your listeners have had to do that in their gardens because it tends to spread everywhere. So, if the plants are small, you can pull them up. But once they get to be  two, three, four-inch diameter at the base, then they’re almost impossible to pull up, at least with manual means.

And at that point, you have to switch to herbicide – Imazapyr is the favored  herbicides for killing the tree - and the most efficient way to do that is to use an injector, where you inject little capsules of the Imazapyr into the base of the trunk, but you put several shells into each one. It’s a very labor-intensive process, and that’s another problem with this invasion, it’s not an easy thing to stop. It’s widespread, and each tree requires individual treatment. So it’s a challenge.

Miller: Compared to something like Scotch broom or English Ivy, which I mentioned at the beginning, holly grows relatively slowly. Does that make it harder to get the public to pay attention to it?

Stokes: That’s a really good point. I think that it’s easy to see invasions of things that happen very rapidly. But for a lot of forest invaders, and English holly is a great example, the invasion is happening too slowly for humans to really easily, immediately understand. But when you figure that a holly forest isn’t gonna really appear for several decades, but several decades is really rapidly changing when you consider the time scale of the forest – that is, an old-growth forest takes several hundred years to develop. So something changing the forest over a matter of a few decades is really rapid in ecological terms. But as you say, people can’t see that happening. That makes it hard to get attention.

And that was one of the reasons why we, in the paper we produced, showed the data in the way that you could see the time scale of the invasion. And you can see the exponential shape of the curve, hopefully getting the attention of at least the readers of that paper that this is changing really fast and exponential growth is, by its nature, takes people by surprise. So, that’s a challenge. I will say the managers of Saint Edward Park, at least, were very receptive, and when they saw our results, they actually applied for and got support from the agency to begin holly control efforts. And they’ve done quite a bit. They haven’t been able to get rid of the plant, but they’ve done quite a bit of control over the past few years.

Miller: Efforts to get holly added to the list of noxious weeds in Washington State have been blocked by a member of the Washington State Noxious Weed Control Board, who is himself a holly farmer – an obvious vested interest there. What would it have meant to have it added?

Stokes: Well, I think, first of all, it wouldn’t solve the problem by itself. That’s just words on a page, when something is listed. However, it does have a practical effect, several practical effects. One is, as you alluded to earlier, it’s higher in the public mind, that this is a plant maybe to avoid. If you’re thinking about putting a plant in your yard, you’re less likely to choose something that’s listed as a noxious weed. It also offers more avenues of control by the state who could make recommendations or guidelines about what nurseries could sell to customers. And third, it makes it a lot easier to get funding to control something if it’s listed as a noxious weed than if it is not. So it’s not a panacea but it would have a positive effect.

Miller: David Stokes, thanks very much.

Stokes: Thank you.

Miller: David Stokes is professor emeritus of ecology and conservation biology at University of Washington Bothell. He joined us to talk about invasive English holly.

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