A giant pumpkin will be freed from a Western Australian prison this weekend after five months behind bars.

The enormous 135-kilogram vegetable was planted on Melbourne Cup day at the Bunbury Regional Prison, in the state's south west.

The prison's grounds officer Trevor Bamess says it is the biggest pumpkin ever grown at the prison.

It will be seen for the first time outside prison walls tomorrow when it is taken to Dwellingup Primary School for the annual pumpkin festival.

It will compete against other heavyweights for the title of biggest pumpkin and tough competition is expected.

In 2000, the winning pumpkin weighed in at 220 kilograms.

Bunbury Regional Prison has claimed the Dwellingup title at least twice before.

Mr Bamess says regardless of the outcome, the pumpkin will be appreciated by inmates this winter.

"Pumpkins this big can be a bit stringy to eat but it is perfectly fine for pumpkin soup, so the prisoners can expect a lot of it this April," he said in a statement.

The Bunbury prison supplies vegetables to several other prisons, and Mr Bamess says many inmates have taken a keen interest in tending to the vegetables.

"I had a different prisoner plant each of the four seeds on Melbourne Cup day, which is always the best time to plant them," he said.

"The prisoners didn't seem too interested at first, but as the pumpkins started getting bigger I noticed the men went out of their way to say which one they'd planted."