The added protection for these trees on Nguyen Van Huyen Street is warranted by the fact that its timber is highly prized and very expensive. 

On Monday, workers sprayed rust-proof paint on the iron fence installed around the roots of the trees, each of which are more than 20 years old. The iron fence is two meters high, 80 cm in diameter and has a support system of several iron pillars planted 70-cm deep into the ground.

Five wide-angle surveillance cameras have also been set up along the row of trees.

Tran Nhat Thanh, a worker in charge of the construction on the Nguyen Van Huyen Street overpass, said the iron frames were built a week ago to "prevent the trees from being sawn and stolen". This was the same design used to protect Dalbergia trees in the city’s Thong Nhat Park, he said.

"Each tree has its own frame to fit its shape and will be maintained until it is firmly rooted. We have sought professional advice, and this is the best solution to ensure the safety of the trees," Thanh said.

Nguyen Minh Khang, 54, who lives near the trees, said, "The frame is very solid, it will be hard for the wind to knock it over, but it is a pity that the trees have lost their natural appearance (when covered by the frame). In the long run, I think the city should move these precious trees to a park and take better care of them," Khang said.

In late April, workers under the management board of the Hanoi traffic construction investment project had pruned and moved the trees three meters closer to the sidewalk to clear space for the construction of the overpass between Hoang Quoc Viet and Nguyen Van Huyen streets.

Scientifically called Dalbergia tonkinensis Prain or hoa sua do in Vietnamese, this tree is a highly prized timber used to make furniture, jewelry and other luxury items. The tree is listed in Group 1A in Vietnam's Red Book and banned from commercial exploitation.

There are more than 700 Dalbergia tonkinensis Prain trees on streets and parks in Hanoi.

This is not the first time iron fences are being erected to protect plants and trees in public places.

Last month, more than 400 Bougainvillea trees on different streets and parks in Nha Trang Town in the central province of Khanh Hoa were also given similar ‘cages’ to keep thieves at bay.

Bougainvillea or hoa giay in Vietnamese (paper flowers) are favored by many for the ease with which it can be grown (with cuttings) and the beauty they add everywhere with a profusion of pink, white, orange and purple blooms.