Ashton Kutcher is putting his green thumb to good use, protecting his friend's home from a devastating wildfire ... by busting out a garden hose.

Video shows Ashton watering plants outside a buddy's estate in the Pacific Palisades in an effort to prevent the property from going up in flames ... in a blaze that's already destroyed tons of homes and businesses.

There are so many out-of-control fires ravaging the area right now, firefighters are stretched thin and folks are taking things into their own hands ... including Ashton.

A house down the street from Ashton's fire-prevention efforts already burned down ... so the stakes are high here.

It may not look like much, but Ashton wielding a garden hose could be a game-changer ... Cal Fire says well-watered plants tend to be less flammable ... and in a blaze like this, anything to curtail the flames helps.

Lots of celebs have already lost their homes in the Palisades ... and Ashton's doing what he can to stop that number from growing.