"They're opportunistic," says Maria Hernandez, the Conservancy's Director of Horticulture, describing invasive plant species. "Other plants don't have that drive to invade."
"They're there, they'll always be there. Birds bring them in, people bring them in, dogs bring them in,” Hernandez continued. "You take your dog out to Long Island. He comes back with a seed on his fur. He comes into Central Park the next day and drops it. There you go."
Invasives pose a serious threat to the Park's biodiversity and, if left unmanaged, would choke out a wide palette of native species. Managing invasives is hard work, taking up to 75 percent of the Conservancy horticulture team's time in the spring and summer. Many of the Park's most persistent invasives are ephemeral, meaning the window to find and uproot them before they begin producing seeds is only a few weeks long. Once uprooted, Conservancy gardeners must quickly backfill the vacant ground with more benign plants or the invasives will reappear (nine out of 10 times, according to Hernandez). If invasives were pulled to prevent a wider outbreak, rather than cleared for a new landscape project, an annual grass like ryegrass or fescue will be planted until a long-term plan for the landscape is developed.
Though invasives are most easily managed with chemical sprays, the Conservancy prefers manual uprooting except in the most extreme outbreaks. Conservancy volunteers and corporate volunteer groups donate hundreds of hours of work per month to keep the Park's invasive populations under control.
All invasives are not created equal. In general, the Conservancy horticulture team will allow an invasive that's native to New York City to grow in most of the Park as long as its growth doesn't overwhelm the plants around it. Some invasive species, the White Wood aster for example, are pulled in some sections of the Park and cultivated in others. As a woodland plant, the aster fits right into the Park's wilder landscapes, like the Ramble or the North Woods, but would dominate and distract in classically designed landscapes like the Conservatory Garden or Grand Army Plaza.
Other invasive species, like the Japanese knotweed, are non-native and unwelcome throughout the entire Park. The knotweed's dense and quick growth crowds out other plants. Frederick Law Olmsted, Central Park's co-designer, planted knotweed to give the Park a tropical look. Over the last 150 years, the plant's growth has exceeded Olmsted's plan, and Japanese knotweed is now the Park's most persistent and noxious invasive species.
Usually associated with small herbs, invasive species come in many forms, including trees. The Norway Maple, for example, a non-native invasive, has a large canopy that produces leaves before other maples in the spring, and loses its leaves late in the autumn. During that period, nothing will grow beneath it. "That's classic to invasives," says Hernandez. "They will adapt to just about any environment you give them."