The Ivy Complex
A Status Update on San Francisco’s Most Costly Weed
Algerian Ivy, Hedera canariensis
Summary of Contents
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Intro
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Distribution
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Lifecycle of the Plant
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Damage to Built Infrastructure
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Ivy in Trees and Forests
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Ivy in Native Plant Communities
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Techniques
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Goals, Strategies, and the Road Ahead
Intro
The management and control of Algerian ivy is expensive and labor-intensive. Not dealing with this plant, however, has proven to be far more expensive and at its current trajectory will continue to get even more expensive.
While most of what one might find about Algerian Ivy are its attributes as a groundcover, it is the most expensive and costly invasive plant in terms of both dollars and habitat in the San Francisco Bay Area.
According to the California Invasive Plant Council, “Algerian ivy grows vigorously in forests where nothing else seems able to compete and inhibits regeneration of understory plants, including forest wildflowers and new trees and shrubs.”
The effects are wide-ranging, from parks and backyards to built infrastructure to wildland native plant communities.
This article aims to illustrate important facts about the life cycle, distribution, and ultimately the control of this plant.
Distribution
Ivy can be found in every part of San Francisco, including many high-value natural areas:
The highest hilltops of Twin Peaks and Mt. Davidson., willow riparian areas including Lake Merced and Glen Canyon, and wooded parks including Golden Gate Park, McLaren Park, and the Presidio.
The ability for this plant to proliferate beyond the initial invasion is determined by the number of songbirds eating and spreading the fruit.
This seed has evolved to be carried by birds, which are one of the biggest challenges to the containment and management of this plant.
The original introduction of ivy was done intentionally.
Since that time, ivy has spread far and wide beyond areas it was initially intended to grow.
Now here for over 100 years, Algerian Ivy populations have become more robust and widespread than ever.
Its growth has become exponential in recent decades, with old populations spreading at the edges and new populations popping up all across the city.
Lifecycle of the Plant
The lifecycle of ivy begins with a fruit-eating bird.
While the fruit is digested, the seed is passed and ultimately deposited on the forest floor.
A disproportionate number of seedlings are found in areas where other fruiting plants are present.
It is not uncommon to find a diversity of both native and non-native seedlings from fruiting plants sprouting from beneath the same perch.
Over the course of months, seedlings take hold, putting down first a taproot.
They then begin to sprout roots from along the main stems, which eventually become more woody and rooted into the ground overtime.
Two types of ivy can been seen in this photo, with the larger climbing Algerian Ivy above and the smaller-leaved English Ivy acting more as an even groundcover below.
Monocultures like these are found throughout San Francisco’s parks, backyards, and natural areas.
Monocultures of ivy begin along the ground but eventually climb up the trunks and take over the forest canopy as well.
This pictured mass of vegetation is in flower but will now set fruit until the fall and winter.
A very important fact is that ivy primarily only forms fruit once it has climbed a tree and gotten far off the ground. This makes the fruit more accessible to the birds that disperse the seeds from this invasive plant far and wide. However, this fruiting process takes 3-5 years, buying time for better control.
Damage to Built Infrastructure
The invasive roots and woody stems of ivy are destructive and damage built infrastructure. This includes:
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Walls
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Structures
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Underground pipes
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Wires
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Fences
Shown here is a chain link fence that was on its way to being destroyed by a 20+ year old ivy planting effort. Saving this fence took significant work, but not saving it would have cost tens of thousands of dollars.
Ivy encouraged to climb on fences invariably ends up where it is not wanted.
In this case it spread from a fence, where it was planted, onto utility wires. It then eventually climbed up the building, becoming entangled in wires and conduit, and allowing rats shelter and access to the 2nd and 3rd stories of the building.
In this residential scenario, the water meter and sensitive wiring had been completely covered over by ivy and was on its way to being damaged.
In other places it was growing up into the siding of the house.
The new redwood fence was just replaced after being destroyed by ivy.
While this resident has dealt with their ivy, the neighbor has not dealt with theirs, meaning the invasion will continue from their side.
Thankfully this screen grab was not about San Francisco’s Ocean Beach, but this underscores the widespread issue of rats and ivy in urban areas.
Rats and ivy evolved together and ivy is their perfect habitat, providing food, refuge, breeding habitat and climbing vines that serve as highways into trees and uppers stories of buildings.
Ivy in Trees and Forests
Ivy is both ubiquitous and shapeshifting, with many park visitors not even realizing they are looking at it.
Seen here is a tree that was eventually overtaken and killed by ivy. It is now a “mother tree” of sorts.
An individual like this has attained massive size and spread untold numbers of seeds for decades.
Shown here ivy climbs a native toyon tree.
Trees and bushes overtaken by ivy become much less productive and slower growing.
In this case, it means the native tree is producing far less fruit for native birds and wildlife.
Getting the ivy off the base of the tree is the most important, as the rest dries up and eventually falls away.
Ivy removal at this site was done in conjunction with large scale eucalyptus removal.
The phasing of the ivy removal happened before the trees were removed, so as to cover minimal ivy, as it is harder to remove from beneath tree trunks left on site.
We have also followed up here numerous times to continue grubbing out roots, outliers, and maintaining a control line against further spread.
Ivy in Native Plant Communities
Native plant communities can hold their own against ivy when they are intact, robust, and undisturbed.
In disturbed areas however, ivy creeps in from the edges and begins to take over rare, special, and important habitats.
Removing small patches like this early is the most time-effective way to prevent more widespread invasions.
In the advanced stages of a ivy invasion, native plant communities become smothered and are hardly noticeable beneath the blankets of invasive vegetation.
90% of the surface biomass can usually be removed in the first pass.
In following visits, crews work to remove outliers and roots, allowing native plants to recover, become robust, and eventually resist invasion to ivy.
Techniques
Crews working in unison using debris hooks are able to pull and remove large tangles of ivy with far less effort.
Many hands make light work towards an otherwise formidable plant.
In this situation, laborers can use a handsaw or grass knife to cut off masses of ivy runners and vines.
For especially thick and dense ground cover, the reciprocating hedger used in conjunction with potato hooks is an extremely effective removal method.
This monoculture of ivy was effectively removed in a couple of hours.
Fast paced removal is necessary and produces an enormous amount of green waste.
Separating ivy from all other types of green waste is important so as to be able to control growth at the base of the pile.
Whenever possible, larger woodier debris is kept separate from the more herbaceous and viny parts of the plant, which quickly dry up and turn brittle.
Overtime, waste piles are turned over, helping to break down and consolidate green waste while managing resprouts underneath.
As piles dry out, they become lighter and can be moved around easier, merging with other piles or potentially moved offsite with the use of tarps.
Cutting stems at the base can effectively be done with a handsaw, however stems can be so thick that a chainsaw is necessary and always faster.
Perhaps the most bang for the buck task is cutting the stems of climbing vines that are producing fruit and seed up in a tree.
This focused effort does not take much time, but prevents the production and distribution of countless thousands of seeds that would otherwise be spread by birds, rats, and other animals.
Goals, Strategies, and the Road Ahead
One of the biggest challenges in managing ivy is getting everyone onboard for long enough for the effects to start making a long-term difference.
While many large scale ivy efforts have taken place and been effective on a local level over the years, there has yet to be a cohesive citywide strategy and method for controlling this wide-ranging and fast-moving foe.
Following are strategies that will make great strides in the widespread control of this most costly invasive plant:
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Identify and map areas with the highest number of fruiting plants.
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Delegate work forces to sever vines at the base to prevent fruiting and have qualified herbicide applicators on site to treat stumps.
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Create containment lines to manage and limit spread of ground invasions.
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Follow up visits to continue reducing fruit and the spread of seeds
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Allocate funding for continued control.
The effects of a city-wide strike are cumulative and cannot be overstated.
To use the credit card analogy, you do not want to pay down the interest slowly. Rather, the growth needs to be knocked way down systematically all at once.
In the case of ivy, halting its ability to effectively produce the millions of seeds that it does each fall and winter will greatly reduce its spread.
In most cases, it will take years for ivy to begin fruiting en masse again after the largest stalks are severed, buying time to continue to weaken it at the ground level.
In summary, the benefits of a city-wide strike on fruiting ivy in particular include the following:
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Preventing and offsetting damage to built infrastructure and utilities city-wide, both in public, business, and residential settings.
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Reduced habitat, food, and breeding areas for Norway rat (Rattus norvegicus) and Black rat (Rattus rattus).
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Preservation and maintenance of cultural and historic forests, woodlands, urban trees and gardens.
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Reduce the spread of Algerian ivy from San Francisco to other places.
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Preservation and opportunities for expansion of Franciscan native plant communities.
A PSA
from
Habitat Potential
by Consulting Ecologist Josiah Clark & Ray Mullin
