English Ivy: Forest Invader
By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
This Naturalist Note is dedicated to Phil Hamilton who passed away on June 12. Phil devoted more than 24,000 volunteer hours helping make Tryon Creek State Natural Area the wonderful place it is today. He played many leadership and resource roles, but is perhaps best known as the ‘King of the Ivy Pullers.’ He not only pulled a lot of ivy himself, but led hundreds of groups of ivy pullers out into the park. Every time I went out with him, he told me something brand new about the forest that I did not know before. He was a great role model, and helped many people get started on the track of volunteering at Tryon Creek. I, and I’m sure many others, will remember this dedicated, knowledgeable and hard-working man forever. Thank you Phil!
If there was a vote on the most despised plant at Tryon Creek State Natural Area (TCSNA), ivy would win hands down. This aggressive, invasive plant outcompetes and displaces many native plants. In an area near the Red Fox Trail where the ivy completely covered the forest floor, I removed and measured the ivy in a three-foot by three-foot plot. In that plot there were 285.8 linear feet of the ivy vine. (Yes, it was really thick, in multiple layers!) If (and thankfully, it doesn’t) this density of ivy covered all of TCSNA there would be enough ivy to wrap around the earth at the equator more than 6 times!
Ivy’s habit of climbing up tree trunks makes it difficult to ignore. Not surprisingly, ivy has many special features that make it so successful.
Ivy: The ingenious climber
Ivy needs sunlight to grow. How does a plant get close to the sun? Most trees, like our Douglas-fir (Pseudotsuga menziesii) develop a thick trunk that lifts their leaves up toward the sun. Building the thick stem takes a lot of resources. A study in western Washington showed that for 47-year-old Douglas-fir, 87% of the above ground biomass was in the trunk of the tree. The major purpose of the tree trunk is to get the needles up into the sunlight where they can photosynthesize. The ivy developed the habit of just climbing the tree trunks that were already there. It saved itself all the energy required to develop a self-supporting stem.
I pulled down an ivy vine that was growing up the side of a tree. The diameter of the ivy’s stem at ground level was 3/4 of an inch. Twenty-one feet up the tree, it was not much smaller, as you can see below:
Since ivy doesn’t need a thick stem to hold itself erect, it uses its energy to grow taller.
In contrast, a western redcedar (Thuja plicata) only 10 feet tall growing along Old Main Trail had a basal diameter of 1.59 inches. The redcedar needs this thick stem to hold itself up, while the ivy doesn’t.
Not that ivy vines don’t grow large, especially when two or more vines merge together.
Ivy only had to develop a method of holding onto the tree. Voilà! The aerial rootlet, which adheres to the tree’s bark:
Using these aerial rootlets, the ivy manages to climb up the trunk of trees into the light without having to expend the energy to develop a big, supportive stem.
Creating a Home for Others
While we rarely envision ivy as a benevolent plant, other organisms may have a different view. As you can see in the photo below, sometimes the mass of ivy stems creeping up a tree is part of a community complex most frequently involving moss or licorice fern (Polypodium glycyrrhiza). While removing ivy from a tree near the Red Fox Trail, I collected the sample (in cross section) shown below. It is a combination of primarily ivy and licorice fern, with a hint of moss.
The mass of roots, stems and miscellaneous dirt measured about 7 cm (~2-1/2 inches) thick.
How much water might this hold? I cut a 2-1/2” by 3-1/4” sample from the tree. I soaked it overnight in water. I weighed the wet sample and then let it air dry completely and weighed it again. I calculated that a square foot of this material would hold slightly more than 2-1/4 quarts of water. This is a mixed blessing. While some of this is a nice reservoir of water for the licorice fern growing in this mass, it is also a significant weight burden for the tree.
To find out how much water might be stored in the mass of ivy roots and licorice fern, I did some calculations. I measured the diameter of the trunk of a large fallen alder tree near the Middle Creek Trail at 10 foot intervals, up to 63 feet above ground, where it started branching out. Based on this data, I calculated the surface area of the tree trunk. If the entire surface of this tree trunk were covered like the sample above, the ivy/moss/licorice fern could potentially contain up to 1,520 lbs. of water. That’s three-quarters of a ton of water. Yikes!
Ivy: It’s Tough
Every species of plant contains nutritious chemicals like sugar, cellulose and dozens of others. This naturally attracts other species that don’t have the ability to capture solar energy to sustain themselves. One of the keys to a plant’s survival is to protect itself from these organisms, which range from molds and insects, all the way to humans. In the picture below, you can see the surviving remnants of leaves on one of TCSNA’s common shrubs.
To find out how effective ivy is in protecting itself, I conducted a survey in the fall of 2016. I examined the leaves of three species of plants, and counted the number of leaves (or leaflets) that were damaged. To minimize the possible effects of humans, I examined sites more than 10 feet from a trail. (Confession: I don’t actually know what caused the damage; it might have been insects, diseases, a hailstorm or whatever.) For each species, I examined leaves in two different places (for example, near Red Fox Trail and near Old Main Trail), to get an “average” value.
The results are presented below:
Number of Total leaves Percent of
Species damaged leaves examined leaves damaged
Red Alder 181 199 90.0%
Oregon grape 375 559 67.1%
Ivy 93 279 33.3%
Ivy has less leaf damage, whatever the cause, than either the red alder or the Oregon grape. Good for the ivy!
Ivy is a Persistent Grower
Every plant has a growing season, and for ivy, it’s long. To determine how long into the fall/winter this plant might grow, I measured the growth of an individual ivy stem along the Red Fox Trail. The data shows that ivy continues growing quite late in the year.
In contrast to the ivy, on September 28, one of the Indian plum (Oemleria cerasiformis) plants I was monitoring in that area was completely bare of leaves, while the other Indian plum in that area had dropped about 98% of its leaves.
Ivy’s Secret Strategy
One of ivy’s secret strategies is that virtually every place along the stem where there is a leaf, there is the potential to grow roots. That is seen in the photo below:
Should the stem of this ivy plant be broken, no sweat! Every part of the stem has its own root system and can stay alive. This is in contrast to most woody plants which only produce roots at a single point in the plant.
English Ivy really isn’t that bad (a tidbit for geeks!)
It turns out that much, if not most, of the ivy that we have at the park really isn’t English ivy (Hedera helix); it’s Irish ivy (Hedera hibernica). Not that the other plants care!
The key reliable morphological feature that discriminates between the two species are the miniature hairs that grow in clusters on the plant. The Irish ivy hairs are in small clusters lying flat on the plant’s surface, while English ivy hairs are in larger clusters and stand erect. The microphotographs below of plants collected at TCSNA shows the difference.
To further complicate things, hybrids of English and Irish ivy have been discovered and…. Okay, I’ll quit now!
The Ivies: Green Success Stories
The ivies in the genus Hedera are very successful plants. They can grow tall without having to use their own stem to support themselves. When hacked into pieces, many of the pieces are able to stay alive and become a whole new plant. They also appear more resistant to disease and predation than many of TCSNA’s other plants. They have a longer growing season than many of our native plants. All of this spells success for the plant, and lots of work for our ivy pullers who are trying to encourage the growth of native plants by reducing the resource competition from the ivy!