If your child has been diagnosed with attention deficit hyperactivity disorder (ADHD), you may be wondering how to best help him. ADHD medications are effective for some kids, but not all, experts say, and the meds can have serious side effects.

But there’s one potential prescription that has few if any side effects — more “green time.”

That’s the opinion of some researchers at the University of Illinois at Urbana-Champaign. Since the early 2000s, they have been investigating exposure to nature as a possible therapy for kids with ADHD, a condition often marked by forgetfulness, difficulty paying attention or concentrating, being easily distracted, fidgeting or being in constant motion, trouble finishing things and excessive talking or interrupting.

“I think we're on the track of something really important, something that could affect a lot of lives in a substantial way,” says Frances Kuo, PhD, director of the Landscape and Human Health Laboratory at University of Illinois at Urbana-Champaign. “We're on the trail of a widely available, free potential treatment for a disorder that afflicts one of every 11 children — that is, two or three kids in every classroom."

Related: Parents, Are You Kids Getting Enough Free Time?

Kuo has co-authored several studies on the value of nature in combating ADHD. In research published in the American Journal of Public Health, she and her colleagues reported that spending time outdoors in nature on weekends and after school appeared "widely effective" at reducing symptoms of ADHD in children.

Kuo and colleague Andrea Faber Taylor also found that kids with ADHD can concentrate better after a walk in the park (as opposed to a walk downtown) in a study they published in the Journal of Attention Disorders.

And in 2011, they discovered that kids who have regular "green play" outdoors have milder ADHD symptoms than those who regularly play indoors.

These study results dovetail with those of other researchers, who’ve found a cause-and-effect relationship between exposure to nature and better concentration and impulse control. Kao and others think the findings may offer new possibilities for the millions of U.S. kids who have ADHD.

Related: Can Trees Make You Younger?

Richard Louv, author of "Last Child in the Woods: Saving Our Children from Nature-Deficit Disorder," says immersion in nature benefits all kids, but especially children with ADHD.

“I can’t tell you how many times I’ve heard parents say, ‘Johnnie or Judy are different kids when we get them outside in nature,’ says Louv, “or how many teachers all over the country say that the troublemaker in class becomes the leader when the class is outside in nature. Not just better behaved: the leader.”

Nature Rx

The problem is, getting kids back to nature isn’t always easy. Today, only one in five kids can safely walk to a park or a playground, and access is still more restricted in low-income neighborhoods, according to the Sierra Club.

Here are some suggestions from Louv and the National Audubon Society for filling your kids' "nature Rx."

Revive old-fashioned fun. Get outside with your kids. Catch lightning bugs at dusk; release them at dawn. Collect stones, rocks, fossils or leaves. Build a tree house, hut or fort if you have the space and resources. Walk to a nearby park and play tag or Capture the Flag. Go camping in your own backyard.

Take a hike. Start with easy, short walks and build up. Check out the American Hiking Society or the Sierra Club’s Nearby Nature Initiative.

Walk or bike to school. If you're not able to walk your child to school, other parents may be able to escort him there in a walking school bus.

Create or join a family nature club. Nature Clubs for Families are beginning to catch on across the country. “The idea is that multiple families meet to go for a hike, garden together, or even do stream reclamation,” says Louv, who adds that kids often play more creatively outdoors when there are other kids around.

Plant a garden or join a community garden group. For young kids, pick big seeds for little hands.

Check out the Children & Nature Network, part of an international movement to connect people and communities to the natural world.

Related: 13 Tips for Green Camping

Judith Horstman (judithhorstman.com) is an award-winning journalist specializing in health and science. She has been a Washington correspondent, university professor and Fulbright scholar. She has also written for many publications, including Time Inc.,and is the author of seven books, including four Scientific American books about the brain.