They believe Kirklees Council is responsible for the act, which has been described as "incredibly mean spirited". The authority was approached to comment.
Plotholder Debby Fulgoni, who has been at the forefront of the fight to preserve the allotments, said: “It’s devastating.
“It’s the worst thing you can do to an allotment holder: kill off their whole plot so that none of it is salvageable or edible.
“They have broken us as a community.”
She added: “We have pushed as far as we could to save that land. Now we are being punished and made an example of.
“It’s a sledgehammer to crack a nut. They seem to think we’ll go if our stuff is poisoned – that we’ll give up. It’s like a death knell. They are fighting dirty now.”
Allotments at the top of the site at Cemetery Road in Birkby are to be used as playing fields and a car park for the £9.7m Brambles Academy, which is close to being completed.
Plotholders have fought for more than two years to prevent the land being seized. They argue that the car park and playing fields can be sited elsewhere.
However the council has successfully fought the case through the courts. Campaigners now face a £90,000 legal bill.
A metal fence dubbed “Trump’s Wall” was set up in March and in April growers were locked out, unable to access their plots.
They say they the council still hasn’t fully prepared plots for them to “migrate” to or offered replacement plots. Plotholders have been waiting two-and-a-half years for replacement greenhouses and poly tunnels.
At a recent “tense” meeting with the council tenants said they were told: “You have no rights”.
At one time there were around 20 people working allotments at Cemetery Road. That number has now shrunk to a handful.
“There are just some stubborn people left,” said Ms Fulgoni.
“One man is in his 80s. Others are in their 70s. They come here for the community.
“Both myself and my son, who is 10, are devastated. He has grown up with that plot. It took me two hours to calm him down when we found out.
“My Dad was a big supporter of my allotments. When he died suddenly three years ago I took some of his plants and fruit trees from his little garden to my plot to remember him.
“Now they’re locked away – and they’ve probably been killed off by toxic chemicals.”