Staff and visitors at a tourist site said they have been terrorised by drug addicts and anti-social behaviour.
Staff said there had been a catalogue of incidents at the Sir Joseph Banks Conservatory at Lincoln's Lawn complex.
A pregnant worker was threatened with a knife and a visitor with her grandchildren said she saw two addicts injecting themselves.
Lincoln City Council said it monitored anti-social behaviour complaints and had security measures at the Lawns.
'Disturbing and frightening'
Rachael Everett, who works at a nearby shop said: "We had two teenagers who were drunk swimming in the pond and it's not very deep, it's not very easy to swim in.
"We had a streaker in here as well... which was, well, it's amusing to look back on but at the time it was disturbing and frightening.
"The drugs - it's not a huge problem but it's getting bigger. I know the public toilet across the road is where they tend to go and to think that happens in the conservatory is frightening really, especially around children."
Workers also said a donations barrel in entrance had been stolen.
The conservatory was set up as a memorial to explorer and botanist Sir Joseph Banks.
Lincoln City Council said it hoped to organise a meeting with the operator of the conservatory to discuss the problems.