Don't touch this plant! Find out why

2014-08-03T06:00:00Z Don't touch this plant! Find out whySusan Billings Arizona Daily Star Arizona Daily Star

My “sensitive” plant is smart, too.

The mimosa pudica — also known as the sleepy plant or touch-me-not — reacts dramatically when touched or shaken.

When touched lightly, its leaves collapse, two by two, until the whole cluster closes up. The harder you bump it, the faster and more strongly it reacts.

So why does the sensitive plant react at all? 

“When it’s disturbed, it releases chemicals,” says Brad Woltman, a weird-plant specialist at Ecogro in Tucson. Those chemicals cause a drop in water pressure in its cells, he says. The plant then folds up its leaves or even collapses a branch.

It’s probably a defense mechanism against grazing animals, Woltman says. Or perhaps insects?

If you were facing the mouth of a grazing cow, for instance, being able to shrink below the surrounding grasses would be quite useful to keep you from being eaten.

The sensitive plant also reacts to fire — another defense mechanism that likely helped it survive when wildfires swept through its habitat.

At night, this creeping perennial collapses its leaves; morning will find it wide open and stretching its ferny leaves toward a sunny window.

All that’s pretty cool, but what’s even more astonishing is the recent finding that the sensitive plant apparently can learn to tune out touch that is not harmful.

On PRI’s “Science Friday,” food writer Michael Pollan describes the experiment, done by animal ecologist Monica Gagliano of the University of Western Australia. Gagliano set up an experiment to repeatedly drop the plants.

"After five or six drops, the plants would stop responding, as if they'd learned to tune out the stimulus as irrelevant," Pollan told PRI. "This is a very important part of learning — to learn what you can safely ignore in your environment."

When the mimosas were shaken instead, they again reacted — folding up their leaves in a defensive manner. According to Gagliano’s research, the sensitive plants not only learned from experience — they remembered it for weeks afterward.

The mimosa pudica is native to South America and Central America, but is now a pantropical weed. It’s considered an invasive weed in some parts of the world, but won’t survive here in the desert without regular watering, Woltman says. 

Find it and more strange and wonderful plants, plus aquaponic supplies, fish and soil amendments, at Ecogro, 657 W. Saint Mary's Road. Phone: 777-8307; Online:

More food for thought

All of which reminds me of this story, in case you missed it last month:

• Scientists with the University of Missouri have discovered that plants react when they are exposed to the sounds caterpillars make while chomping on other plants’ leaves. The plants produced more chemicals to deter the caterpillars in response, according to researchers Heidi Appel and Rex Cocroft.

And this fascinating read:

• Do plants feel pain? Author Michael Pollan weighs in on the subject of plant “intelligence” in an article in The New Yorker

Go ahead — talk to your plants. You can reach me at

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