The Société Jersiaise is reminding people not to leave anything - including offerings or debris of spiritual practices - at historic sites.
It follows damage to La Geonnaise passage chamber near Greve De Lecq at the weekend.
An area of the central grass has been scorched and a circle around the edge cast with flour, which volunteers have not been able to remove.
Debris included decomposing fruit and flowers, which the Société's Botany Section says pose a potential risk to native plant species.
The prehistoric site is monitored as part of the National Plant Monitoring Scheme, to gather important data about the site of natural habitats.
There are concerns pesticides and preservatives on the fruit and flowers could damage the fauna, insects and worms.
In a social media post, the Société says:
"This is no way to treat an ancient monument.
"Please do not leave these sort of offering at these precious ancient sites. The result is the absolute opposite of what I'm sure you wanted to honor.
"Leave nothing - take nothing."
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