A silent army of invaders are floating down the Bronx River and setting up shop in parks citywide. Their ultimate goal of ecological domination would be complete except for a handful of brave souls who donate their weekends to shield the humble woodland violet and demure anemone from certain doom.
Sure, these villains threatening to take over green spaces in the Bronx and beyond are just invasive plant species. How harmful could they be?
Quite harmful, it turns out.
Invasive plant species are any non-native plants that cause significant ecological and economic harm, according to Daniel Atha, a botanist with the New York Botanical Garden. Many of them are carried south on the Bronx River from yards and gardens in Westchester. The invading seeds and small roots find a habitable patch of ground and begin multiplying.
One of the biggest threats to the city’s ecology in recent years is Japanese knotweed, or Fallopia japonica, which can grow up to 13 feet in a season. Its strong root systems can cause quick and expensive damage to sidewalks, plumbing and building foundations, according to the state division of lands and forests. It’s illegal to trade or cultivate Japanese knotweed in the United Kingdom and the sale of property is blocked until the owner proves the lot is free of the plant.
“It’s so aggressive because it doesn’t have any predators here,” Atha said. “It will take over and dominate several types of ecological communities and crowd out native species. So you’ll have a monoculture of one non-native species that the wildlife is not adapted to use.”
Because Japanese knotweed evolved in a slightly warmer climate, it’s active for a longer period than plants that evolved here. It begins taking up resources and drops seeds earlier, often crowding out native plants that then die off.
Before the Central Park Conservancy began regular annual management efforts, Japanese knotweed turned The Ramble’s natural areas into monoculture absent of any native insects.
Animals, insects and bacteria evolved to eat certain kinds of plants. It takes many generations before they see new non-native plants introduced to the ecosystem as food source. If the Japanese knotweed gets its way, it bullies all other plants out of its area. Since all living things like to eat, areas barren of recognizable food tend to be lonely, quiet places.
“We need to prevent it from spreading,” Atha said. “We need to eliminate it, and we need to support the native biodiversity which, in a healthy system, is very diverse. It supports a whole food web up and down, from the very basic microorganisms, all the way up to the top predators.”
Incised fumewort, or Corydalis incisa, is another aggressive invasive species that originated in Asia. It was first found in the Bronx in 2004, Atha said, and its rapid spread in undisturbed wooded areas over the following decade alarmed scientists.
“The plant has an extraordinarily capacity for spreading,” he said. “It can spread vegetatively through the little tubers underground. Animals could disperse those, and it’s also very fertile, so every single flower turns into a fruit.”
Unlike the Japanese knotweed that needs cross-fertilization, it appears incised fumewort can fertilize itself. When ready, seeds burst from the plant with such force, they can travel up to nine feet. The oily seeds are attractive to ants, which carry them to their nests.
After a 2015 study showed the extent of the incised fumewort’s spread from a specific site in Westchester to the Bronx, botanists alerted the Lower Hudson Partnership for Regional Invasive Species Management, or PRISM. Careful management has slowed the southern march of incised fumewort, but other invasive species have taken up residence in Van Cortlandt Park.
“Right now, one of the biggest focuses on invasive plants is on water chestnut, which is a plant that’s purely aquatic,” said John Butler, ecological project manager for the Van Cortlandt Park Alliance. “The issue is that when it comes into a water system, it’s blocking sunlight from reaching down below the water’s surface, and that hurts other aquatic plant life.”
While choking the lake, water chestnut also creates a barrier for the fish to make their way through. Park staff members have seen fish die after being immobilized in the tangle, Butler said.
“But the biggest problem with water chestnut is that it covers a large area and it all dies at once,” Butler said. “As it’s breaking down, aerobic bacteria will propagate and begin to grow within the lake. It begins using up all the dissolved oxygen in the water.”
That can be deadly to plants and animals in the lake, so the group began lending a hand. Several hands, actually.
Over the last three summers, the organization employed a group of 10 or more high school interns who spent six weeks in the lake manually clearing water chestnut plants. Not only does that avoid the massive die off, it also cuts down on the following year’s population.
“So year after year, if you work at it, the idea is that the following year there will be less (water chestnut) because you pull a lot of it out before it creates seed,” Butler said. “And then the same thing the next year. The more you pull, the more you’re depleting that seed source. And over time, there will be less and less in the system.”
Other invasive species, like the Norway maple, is a major problem at Van Cortlandt Park and the area’s smaller parks, said Bob Bender, chair of Community Board 8’s parks and recreation committee.
“As the parks department works on projects in different parts of the park, it tries to combat Norway maples by removing them when possible and replacing them with native species,” Bender said.
Identification and management of invasive species in smaller parks is a parks department goal, he said, but funding is a big hurdle.
“The parks department is certainly aware of the problem, but there is no budget to do extensive removal of invasive plants throughout Van Cortlandt or in other parks because the cost to do so is substantial,” Bender said. “So usually this problem is addressed whenever the parks department does work in a particular area or a particular park.”