The adder's-tongue spearwort is only found in two places in the UK - Badgeworth, near Gloucester, and on Inglestone Common, near Bristol.
At Badgeworth, the tiny nature reserve boasts a small crop of the plant but in South Gloucestershire the buttercup is only spotted once every five years.
Plant expert Richard Lansdown said: "We wouldn't normally reintroduce but Inglestone is down to a single plant."
The species, more commonly known as the Badgeworth buttercup, relies on bare, wet ground that has been churned up by cattle to set seed and thrive.
But with fewer animals grazing the common in recent years, it has become overrun with scrub and the buttercup now "only grows intermittently" at a single pond.
"I sieved out the mud and extracted all the seeds [at Inglestone] and didn't find any from the Badgeworth buttercup - there weren't any left at all," said Mr Lansdown.
"So we've had to use seed originally collected from the site some years ago that was sent to the Millennium Seed Bank."
Using the 270 plus seeds held at Kew, 200 plants were propagated and about 80 planted out at Inglestone with a further planting planned for mid-June
"The two sites have been declining for a very, very long time," said Mr Lansdown.
"But we've planted out 83 plants in different locations at Inglestone and at least half of them are surviving and doing well and have begun fruiting which will add to the seed bank."

Twenty others were also affected after a hot tub on display at JTF warehouse in Stoke-on-Trent exposed people to harmful levels of Legionella bacteria.
Richard Griffin, Harry Cadman, 71, from Stoke-on-Trent and William Hammersley, 79, Chesterton died in 2012.
JTF will be sentenced at Stafford Crown Court on Monday.
It admitted failing to ensure the safety of employees and customers under the Health and Safety at Work Act.
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Not guilty verdicts on two corporate manslaughter charges were accept by the judge at Stafford Crown Court and three other health and safety charges were allowed to lie on file.
A report by the Health Protection Agency (HPA) in 2012 said the hot tub was the probable source of the "significant" outbreak.
In 2015, it emerged that more than £200,000 has been awarded to families after the deaths.
Mr Griffin, 64, contracted the disease while delivering meat to a café in the warehouse in Fenton.
He initially suffered headaches and hallucinations and later lost consciousness at his home in Clayton. He died in hospital from multiple organ failure.
His family said he had never taken a day off sick in 30 years.
A rare buttercup is being reintroduced in South Gloucestershire.
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A firm has admitted a health and safety breach after an outbreak of Legionnaires' disease in a hot tub caused the deaths of three men.