Horticultural officials in a northeast China city have launched a campaign to save thousands of trees at risk of dying from the effects of a long and severe winter.

About 6,000 trees -- mostly ornamental poplars and willows -- have already died in Harbin, capital of Heilongjiang Province, after a spate of late snowstorms and sub-zero temperatures, said Lu Qingru, deputy director of the Municipal Quarantine Station for Garden Animals and Plants, Monday.

The winter weather was the worst in a decade and the situation for many trees was "very grim," said Lu.

"The last time trees died in such numbers was 1999 when a cold spell hit the city in the spring of that year," said Lu.

The average temperature in Harbin from November last year to February was minus 15.5 degrees Celsius, 3 degrees lower than the same period a year earlier.

In a rare snowstorm in April, snow disrupted roads and air traffic and closed schools.

Lu said the earliest warning of mass tree deaths came in March when the leaves of some pines and spruces in the downtown area turned yellow. In May, some poplars and willows either failed to bud or budded and then wilted.

A field survey found that some poplars that failed to bud had been hit by bacterial parasites that could only attack the bark tissue of weakened or dying trees.

Specially-trained horticulturists had fanned out in the city to spray pesticides and apply fertilizer, said Lu.

The city's horticultual department was also considering introducing other varieties of plants to the city's streets.

Given that introduced plants were more at risk if they failed to acclimatize, more work must be done to raise the survival rate of new landscaping species, said Lu.