Some 20,000 female willows and poplars in Beijing will get special injections by May to stop flying catkins.

Catkins, or flower clusters, often wreak havoc on people's allergies and disrupt traffic by reducing visibility on roads.

Horticulturists will burrow up to 2 centimeters into the trunk and inject a liquid growth inhibitor, which will halt reproduction but still encourage vegetative growth of the tree.

"It means that with the inhibitor, female willows and poplars will grow stronger with thicker crowns, but much less or no blossoms," Che Shaochen, a researcher at Beijing Horticulture Research Institute, said.

Since 2001, the city has issued a new regulation that only male willows and poplars can be planted in the city proper.

But statistics with the institute showed that by 2005, there were still nearly 1 million female willows and poplars in the Beijing city proper. Most of them were planted in the 1960s-70s.

Every spring, a large amount of catkins, namely blossoms of female willows and poplars, fly everywhere in the capital city.

Too many catkins can degrade air quality, cause respiratory diseases, and block the heat sinks of automobiles, resulting in their breakdown.

Che said the inhibitor was a "tailored one" which had never been used, but had been successfully trialled last year.

It must be injected once a year during the budding period, which usually lasts from early March to the end of May, he said.

Local horticulturists have experimented with this new method since 2006. They have also tried "transsexual surgeries" on about 100 willow trees by grafting some branches of male willows onto the trunks of female willows.

But grafting was less preferred than inhibitors, Che said, adding the cost was about 10 times higher than inhibitor injections.

In addition, grafting could only be used on willows but not adult poplars because the branches of poplars were positioned much higher, according to Han Yifan, a retired expert at Beijing Forestry Research Institute.