It starts innocently enough.

A homeowner, taken with the giant hogweed’s striking spray of white blossoms and leaves that can stretch 5 feet across, plants it in the yard.

But this bad seed of the carrot family carries a nasty kick: Its sap can cause burns, blistering — even blindness, leading state environmental authorities to try to eradicate the nonnative plant before it firmly takes root on Long Island.

Summer is the flowering season for giant hogweed — Heracleum mantegazzianum for the science types — so state Department of Environmental Conservation crews have been working to cut the plant back across the state.

“That’s when it makes it most difficult to control, because that’s when you’re at risk of it spreading with seed,” said Polly Weigand, executive director of the Long Island Native Plant Initiative in Hampton Bays.

While giant hogweed is plentiful in western and central New York, it hasn’t gained much of a footing on Long Island, said Naja Kraus, giant hogweed program coordinator and forest health botanist with the DEC. That’s due to efforts by the agency to keep the population under control, she said — largely by digging out the root structures of the plants.

There are 13 known sites on Long Island that have or had the plant, an invasive species that is on the federal noxious weed list and is banned from being propagated in New York.

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In Nassau, the plants were found in East Norwich and Sea Cliff, while in Suffolk, they have been found in Riverhead, East Hampton, Southampton, Brookhaven and Yaphank, Kraus said.

Eight of the sites now have no plants. The DEC was on Long Island several weeks ago to cut down the plants that are still here — all juveniles, she said.

“There were two sites years ago that had 600 and 800 plants. This year they had two and 10, respectively,” Kraus said. “Just us coming and digging it every year, very quickly we can get these sites under control.”

Kraus said most of the instances of giant hogweed on Long Island were from landowners who planted it without knowing of its ill effects.

“We have a couple of sites where people purposely planted it,” she said. “A couple on Long Island, we’re not sure how they got there.”

The plant can spread through its root system or by seeds, which can be dispersed by traveling on streams or when areas containing giant hogweed are mowed.

The sap of the giant hogweed, when it gets on moist skin that is then exposed to the sun, can cause severe burns and blistering. It also can cause temporary or even permanent blindness if the sap gets into the eyes, according to the DEC.

All of the plants on the known Long Island sites are well away from public areas and are not a threat to passers-by, Kraus said.

Health effects aren’t the only problem with giant hogweed, Weigand said.

The plant, which can grow to 14 feet tall or higher, can crowd out other, native plants, she said. Giant hogweed is native to Europe and thought to have been introduced in the United States as an ornamental in the early 1900s.

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“It does outcompete native plants and wildlife,” Weigand said, adding that it’s essential to cut down the plants before they set seed.

Kraus said the DEC relies on public reports of instances of giant hogweed to determine where the plant is. The agency has established an information line and an email address for people to send in photos of possible giant hogweed sightings. If the DEC determines the plant to indeed be giant hogweed, it will send out crews to the area.

“We keep visiting until there are three consecutive years in which there are no plants,” Kraus said. “Then we consider it eradicated.”

But DEC crews also will visit that site once more three years later, she said — just to make sure.

Weigand also urged the public to be on the lookout for the plant on Long Island.

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“There may be more we don’t know about because people aren’t aware of it,” she said. “We are innately drawn to interesting or cool plants because we don’t know what they are. And this is one you don’t want to go check out.”