Day of the agave . . . The 220cm red hedgehog agave towers over Janice Williamson and Bryan Ludemann. PHOTO: RUBY HEYWARD

A Weston couple is stumped by a strange plant that has shot up in their backyard.

Janice Williamson and Bryan Ludemann’s plant had always just existed in a corner of their garden, but recently caught their attention when all of a sudden it grew to 220cm, and was getting taller.

Mrs Williamson had recently moved it into the sun, which had prompted its sudden growth, and she started measuring its height.

Mr Ludemann said it was growing up to 8cm daily, and was not showing signs of slowing down.

“It’s gone from six to seven to eight inches [20.3cm].”

It was so tall he had to use a ladder to measure it.

Mrs Williamson and Mr Ludemann consulted local gardening expert Linda Wilson, who identified it as a red hedgehog agave, or agave stricta f. rubra.

Mrs Wilson said red hedgehog agave budded in either its 10th or 30th year and would die after dropping its flowers. The fallen flowers would put down roots and become new plants.

The mystery was where the plant had come from in the first place.

“It wasn’t born yesterday,” Mr Ludemann said.

With the plant’s appearance leaning more towards fiction than fact, Mr Ludemann joked it was from outer space.

But the running theory was that it was bought by his late wife Joy, who started its surrounding lavish garden.

“She put a hell of a lot into this place,” he said.

As a talented green thumb, the late Mrs Ludemann had filled the yard with everything from fuchsias and dahlias to jasmine, and had probably picked up the spiky plant somewhere along the way.

He remembered dragging the now towering hedgehog-like plant along to the odd flower show for her, but had not paid much attention to it since.

Mrs Williamson’s next big question was what colour the flowers would be.

“When it flowers, it will be spectacular.”