The hide on the estate, which is open 24 hours a day, was attacked by the vandals who smashed two windows and then tried to set it ablaze.
The hide will remain closed for several weeks, just as the summer holiday season begins.
David Thompson, Estate Manager for Dumfries and Galloway at NTS, said: “As dawn and dusk are the best times to enjoy wildlife, our hides on Threave Estate are open 24/7. We want to keep it this way, but regularly we’re finding our hides damaged by attacks like this. Our teams are working hard to repair this vandalism, but precious time and money is being lost in the process.”
Fire retardant paint helped to limit the damage at the hide, where bird watchers gather to spot ospreys, red kites and peregrine falcons.
The attack follows a long list of vandalism at the hide, with bills to fix graffiti, damaged doors and broken windows amounting to around £10,000 over the past few years.
It comes just days after Threave Estate featured in the launch of a new NTS campaign that urges visitors to ‘Love this place, leave no trace’ and aims to educate people on how to minimise their impact on the countryside.
Last month, the Dark Skies Observatory in East Ayrshire was completely burnt to the ground in a suspected arson attack.
Samuel Gallacher, NTS Operations Manager for Glasgow and Ben Lomond, Dumfries and Galloway, said the fire at Threave Estate had come at a “terrible time” as the summer holidays get underway.
Mr Gallacher said: "It’s one of the most accessible hides we have and has wheelchair access.
“It’s a real pity as some of the plans that we had for increased visitors are now ruined. It’s really disheartening, both for the team and everyone who loves and visits the estate. We work so hard to protect these places in the best way possible and to face this situation is a slap in the face for everything that we are trying to achieve.
“The damage will take weeks to repair, everything is taking longer now - due to supply chains and other factors like this. The reinforced glass that we need will take weeks to arrive. On the positive side, we do have fire retardant on the structure and that’s clearly doing its job. It could have been a lot worse.”
Threave Garden is the only Scottish garden dedicated to the training of horticulturists and home of the School of Heritage Gardening since 1960, while the surrounding estate is one of Scotland’s most biodiverse nature reserves and includes the country's only Bat Reserve.
Mr Gallacher added: “Everything our charity does aims to make the experience better for all people. We have recently put in new pathways to make the area more accessible and, as a result, we are busier than we have ever been.
“That’s fantastic for Threave and for Dumfries and Galloway. However, we are seeing an increase in incidents like this, from a tiny minority. Our plea is to please, leave no trace.”