Animal lovers are being warned of a heavy crop of deadly ragwort growing next to the road between Colchester Zoo and Heckfordbridge.
The plant, which has yellow daisy-like flowers and ragged leaves, can also be found in smaller clumps along the verge almost to the roundabout on Maldon Road.
It is growing in a field by the turning to Stanway and a field on the outskirts of Tiptree.
Kay and Keith Amos of Hardy's Green, who own two horses and who noticed the plant, said even the smallest amount could be dangerous to all animals.
Mrs Amos said: "It is now dying off - which is when it is at its most dangerous. It can make horses go blind, cause liver complaints and although the death process is not long it is unpleasant. We are concerned for other horse owners in the area."
Colchester Zoo said it was unaware of the ragwort growing outside its premises. "None of our animals has access to it," a spokesman said.
Barry Layzell, of Colchester Council, thank the Amoses for bringing it to his attention. He said: "Officers will be arranging for contractors to deal with it."
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