We are particularly focusing on eradicating fruiting ivy in trees. This is because it’s easy to kill and killing it has a much greater impact than removing non-fruiting ivy, especially ground ivy.

 

Invasive ivy generally doesn’t produce berries unless it’s up somewhere high, and it takes seven to ten years to mature. “Somewhere high” almost always means being in a tree, but sometimes it matures on fences or tree stumps. Examples of mature ivy that aren’t up somewhere high are almost always on deadfalls—trees that had ivy on them that died and fell over. I’ve also seen fruiting ivy on terraced hills one or two times.

 

The two key propositions of the argument are as follows.

 

#1: The damage a fruiting ivy infestation causes per pound of ivy is far greater than other ivy infestations.

 

#2: It takes less labor to kill one pound of tree ivy than one pound of ivy elsewhere. Most fruiting ivy is in trees.

 

#1: Support

 

Every year, a fruiting infestation will grow thousands of berries, most of which are dispersed. Even if a small proportion of seeds from these berries turn into plants, they will cover large areas of ground once they grow. The more dispersal is done by birds versus windfall or rats, the greater the harm.

 

Peer-reviewed research on Hedera helix supports focusing on berry production. Here are a few key findings.

-Genetic studies of large mono-cultures of ground ivy indicate these mono-cultures are made up many different individual plants. This indicates that many different seeds were dispersed there, not that one plant grew to dominate a large area. This is evidence that seed dispersal is important for ivy propagation.

-The germination rate of ivy seeds is very high if the pulp around the seed has been removed. It is unclear exactly what conditions these seeds were exposed to in the lab, and what the germination rate would be in natural conditions.

-One study removed all the ivy from a tree, and weighed it, finding the infestation weighed orver 2,000 pounds. Because the entire ivy plant produces berries when mature, this implies that one tree’s infestation can produce a large number of berries.

 

Although hard data is lacking, anecdotal observations tell us more than nothing. Here are a few.

 

-Recently (May 22, 2024), while walking in the McKay Community Forest in Eureka, I pulled over 20 sprigs of new ivy on the side of the path in a ten minute period, without seriously looking for them.

-I often see many new sprigs of ivy all over the middle of the forest in the McKay Community Forest when patrolling for invasive plants. Five to ten sprigs are often clustered near a tree, presumably because a bird was sitting above dispersing seeds.

-When I come across an ivy patch I didn’t know about immediately after berry season, all or nearly all of the berries are gone, presumably eaten by birds.

-I personally have never seen an ivy berry patch swarmed by birds, but I often hear from people who have such patches close to their yards that they see the birds swarm them.

-Residents frequently tell me they see birds sitting on tree branches in their yards pooping, and that many ivy sprigs come up under where the birds sit.

 

The rate at which fruiting ivy will multiply in a given time period is far greater than the rate ground ivy will multiply. We of course don’t have hard data on many of the variables we’d need to know about to compute these growth rates, but the following constitutes a rough estimation.

 

For the sake of argument, say that the largest fruiting ivy tree infestation at the site I cleared on May 17, 2024 had ivy berries starting at 20 feet and going up to 70 feet, and I estimated it to weight 2,000 pounds. Say also the tree itself was three feet in diameter while the ivy was bushing out another 3.5 feet on each side (i.e., defining a 10 foot in diameter cone). That makes a total volume of of 3,575.25 cubic feet (3,928.5 – 353.25 for the tree, 3,928.5 = 50 foot tall column * 78.57 area circle, etc.). We estimated 6,593.4 berries in a 5 x 5 x 10 volume (250 cubic feet), most of which was air around the berries. That comes to 94,292.21 estimated berries in the tree. Let’s assume just one seed in each berry (they can have up to five). If 1% are eaten and dispersed by birds and grow to one pound plants in two years, that would constitute a 21.3% annual growth rate. This doesn’t take into account the fact that the ivy in the tree will also continue to grow. I don’t know the rate at which a mono-culture of ground ivy grows, but it’s far lower than that.

 

If the growth rate of fruiting ivy is so high, why aren’t we inundated with ivy now? A plausible explanation is because the vast majority of invasive ivy isn’t fruiting. Again, we don’t have hard data on this, but my extensive observations lead me to believe it is true.

 

#2: Support

 

The time it takes to kill one pound of fruiting ivy in a tree is much lower than the time it takes to kill one pound of ground ivy.

-Example: About 20 people show up to Sequoia Park Ivy League events. Let’s assume on average they work two hours each of the four hour event. Let’s be generous and say that they pull 2,000 pounds of ivy each time. (The ivy at events is deposited in a dumpster, which is weighed, and the weight is sometimes reported via email. It is often a weight far less than 2,000 pounds.) This implies that 50 pounds of ground ivy is being produced for every hour of labor. Note that this is ground ivy removed from mono-cultures of ivy, where it is much faster to remove than sporadic ivy that is spread out across the forest and among native plants.

-On May 17, 2024, I went to a private residence alone and worked for 3.5 hours, girdling the ivy on 17 trees. I estimated the weight of ivy in each tree, which totaled 9,650 pounds. This estimate is obviously going to contain a great deal of error. That comes to 2,757 pounds of ivy killed per hour, which is 55 times more ivy per hour than the estimated amount of ground ivy removed at Sequoia Park. Perhaps I work faster than the average volunteer, and perhaps I overestimated the weight on the trees. Whatever the case, there is still a huge difference in efficacy.

 

WHY HASN’T FRUITING IVY BEEN FOCUSED ON IN THE PAST?

 

If there is so little fruiting ivy, and it does so much damage, and it’s so easy to kill, why hasn’t it been specifically targeted in the past? From what little I’ve observed, people who work professionally to get rid of invasive plants are responsible for small plots of land, such as a park or nature preserve. They are quit sensibly responding to the incentives their job demands by focusing on taking care of that area, not trying to confront widespread problems in the region.

 

Another reason this strategy hasn’t been pursued may be because of the complications involved in getting permission to kill fruiting ivy from private property owners.

 

CHALLENGES

 

There are challenges with the strategy of going after fruiting ivy. First off, some unknown fraction of fruiting ivy is on stumps, fences and deadfalls. The efficiency of killing ivy is far lower in these situations compared to when it’s in trees. Second, a lot of fruiting ivy is on private property, which means that permission has to be obtained from each property owner.

 

NON-FRUITING IN TREES?

 

The above discussion brings up another question. If we should focus on fruiting ivy in trees, should we then ignore non-fruiting ivy in trees?

 

There are many considerations when answering this question. The obvious reason to kill this ivy before it fruits is because it is more likely to someday fruit than the average patch of ground ivy. If one is at a site where there is fruiting ivy in trees, the transportation cost and “permissions cost” of getting there has already been paid. There is also a chance the ivy will kill the tree before it fruits.

 

There is also the “political” consideration of being able to say that you’ve killed all the ivy on the trees at that site. It’s still far easier to kill ivy in trees than on the ground and the tree ivy is still taking light and water from other plants. To generate good will, doing a little more sometimes goes a long way.

 

The obvious argument against killing tree-ivy that isn’t fruiting is because of the opportunity cost. If one has many sites with fruiting ivy lined up, spending time on non-fruiting tree ivy will result in less fruiting ivy killed. One can gauge how thorough one should be in terms of how backed up the list of targets is. As more people get involved, optimal strategy changes to being more thorough.

 

For now, I girdle the ivy on all the trees at a site if I’m there anyway, although I recognize that this might not be the most effective use of time.