A warm autumn has been blamed for the steep increase in the amount of water hyacinths growing in the Shanghai's rivers, local officials said Wednesday.

The annual floral "outbreak" usually runs from October to December, and local authorities are well versed in tackling it. However, the scale of this year's bloom has been bigger than ever, the officials said.

Since mid-October, about 320,000 tons of hyacinths have been cleared from the city's waterways.

The weight of the haul is already far above the total for the whole of last year.

From the Huangpu River that flows past the Bund to its offshoots in the suburban areas of Jinshan, Songjiang and Qingpu, the plant knows no boundaries.

The leafy greens were spotted as early as mid-July in some river courses, which is nearly a month earlier than usual, the Shanghai Greenery and Public Sanitation Bureau said.

The warm weather throughout the year and especially this fall has promoted the prolific growth, it said.

The work of removing the water hyacinths is likely to continue through January, the bureau said.

In some districts, including Qingpu, Songjiang and Fengxian, the purged plants are being converted into fertilizers.

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