Cameron Diaz & Nicole Richie: Sisters/Wives Plant Family Roots

Cameron Diaz and Nicole Richie are defying that old adage about in-laws -- these sisters by marriage not only get along ... they're taking on household chores together.

Joel and Benji Madden's typically glam wives dressed down and busted out their green thumbs and went shopping for plants together at a Culver City, CA nursery this week. We're told they're both looking to overhaul their respective backyards, and had consulting sessions with landscapers. It's Hollywood, after all.

It's all sunshine and rainbows ... until they both wanna host Thanksgiving.