Why grafted passionfruit vines can leave you pulling out weeds years later

When Hobart gardener Sally Catherall bought passionfruit vines for her garden, she didn't think she'd be pulling out roots over a decade later. 

Her vine used blue passionflower (Passiflora caerulea) as rootstock. It's a popular choice because of its resistance to disease and ability to thrive in poor conditions. 

But that also makes it hard to control — and the flower is considered a weed in natural areas.

Sally keeps a keen eye out for new shoots; when she spots them, she says they don't have a "snowflake's hope in Hades" of survival. But it's an ongoing battle.

It's a story gardening expert Jennifer Stackhouse has heard many times. Here is how you can identify and manage the wayward plant.

A surprise guest

Sally describes her battle with a mixture of admiration and exasperation. She estimates she's pulled 240 blue passionflower shoots from her garden in the last decade.

As a lover of passionfruit, Sally was filled with joy when two black passionfruit vines grew magnificently and produced fruit prolifically. So, she bought another.

But this time, the vine looked a little different. Intermingled in the usual three fingered leaves of passionfruit (Passiflora edulis) were darker, five-fingered leaves and "prettier" flowers.

The rootstock had grown from the base alongside her passionfruit vine. 

When the passionfruit vine eventually died, Sally was left with a tasteless parting gift.

Blue passionflower produces a round orange fruit. At best it is tasteless; at worst, it may contain traces of the toxin cyanide.

What to know about growing passionfruit

When purchasing and growing passionfruit, Ms Stackhouse's advice is to be vigilant and check plant labels.

The gardening expert is a regular guest on ABC Radio Hobart's gardening talkback segment and says passionfruit problems are "up there" with citrus problems. 

Traditionally, grafted plants grew better but Ms Stackhouse has noticed less of a difference in growth between the two options in recent years.

• The label on a passionfruit plant will indicate if it is grafted and should include what rootstock has been used. 
• Non-grafted plants can achieve a similar life span to grafted plants if grown in a warm, north-facing and sheltered position with good soil that is well drained.

If purchasing a grafted plant, monitor growth to ensure rootstock is not growing in place of the grafted plant or from the base alongside a healthy grafted vine. Remove growth as it appears.

How to spot passionflower

Blue passionflower leaves are five-fingered, dark green and leathery in appearance, in comparison to the softer, light green passionfruit leaves.

Once a grafted passionfruit plant dies, take action swiftly.

Remove roots from below the soil, instead of cutting top growth, to stop the plant from shooting.

If using herbicide, a wetting agent is needed for proper absorption.

A spokesperson for the Department of Natural Resources and Environment in Tasmania recommends spraying the entire plant or to cut blue passionflower at the stem and paint the exposed section with a concentrated herbicide mix within 10–15 seconds of cutting.

All removed sections of the plant should be bagged and disposed of appropriately to prevent spread into other parts of the garden or areas of natural vegetation.

In natural areas, blue passionflower can smother trees and shrubs.

Another plant that can cause problems is the banana passionfruit (Passiflora cinnabarina/Passiflora tarminiana), a common environmental weed that was originally recommended as a good passionfruit for cooler climates.

It is recommended the plant and fruit is removed to avoid further spread.

'It would always be a problem for gardeners'

With persistence and a sharp eye, "Eventually, I guess, you'd be able to get rid of it," Ms Stackhouse says.

Of her own experience with blue passionflower, Ms Stackhouse says she had it at one stage but moved house.

"It wasn't because of the passionfruit," she says.