TUMBLEWEED, one of the enduring symbols of America’s Wild West, is spreading radiation across the country.

 The most contaminated nuclear reservation in the United States has started a huge clean-up after discovering that the roots of the thistle reach deep into underground burial sites for radioactive waste and suck up dangerous pollutants, such as strontium and caesium.

 When the roots of the thistle decay and the spiny, dry skeleton on the surface breaks off and rolls away, it contaminates everything it brushes over, scientists found.

 The cost of cleaning the trails of radiation from the Hanford Nuclear Reservation in Washington State, which made the plutonium for the bomb that was dropped on Nagasaki at the end of the Second World War, could run into millions of dollars, experts say.

 "Our dream is that we have this place tumbleweed-free," Ray Johnson, a biological control manager for radiation protection at Fluor Hanford, the contractor managing the US Department of Energy site, said. A stiff winter wind can push the tumbleweed miles and then "we’ve lost control of our contamination", he said.

 Radiation-control specialists are sweeping the 560-square-mile reservation, using Geiger counters that click when radioactivity is present. When radioactive tumbleweeds are located, they are pitchforked into a lorry. The bushes are compacted and disposed of at an on-site waste dump. The area where they were found is cleaned and covered with 6in of clean soil or gravel.

 The radiation in a large, 3lb contaminated tumbleweed might measure 150 millirads, about one hundredth of the allowable annual dose of radiation per person at Hanford. Uncontaminated tumbleweeds are dumped in an open pit to prevent them from straying into polluted areas.

 Preventative measures to control the growth and spread of the tumbleweeds have been introduced and include backpack, roadside and aerial spraying of herbicide. A bio-barrier, an expensive engineered textile, can also be laid down to block the formation of thistle roots.

 Workers at the site are reluctant to purge the area of tumbleweeds completely. "If we didn’t have them, the West wouldn’t be the West," Mr Johnson said — this despite the fact that tumbleweed is not, in fact, American at all, but was introduced to the country from Russia.

 Hanford was built in 1943 for the secret Manhattan Project. For 40 years it made plutonium for the country’s nuclear arsenal. The last reactor was shut down in 1986.

 Two years ago $2.5 million was spent on a clean-up of contamination spread by fruit flies. The flies had been attracted to a soil fixative containing saccharin being sprayed on a contaminated area. The flies flew to an eating area and spread the radiation to rubbish bins, which later polluted a landfill site.