Everywhere one looks in Zeta Cross' yard, it's green, green, green, which is good. But let's get a closer view.

Multiflora rose over there. English ivy down below. Isn't that tree of heaven...and Japanese honeysuckle? She's even got garlic mustard and purple nightshade, porcelain berry and vinca.

Oh, no! Cross has enough runaway stuff in her yard to give an environmental purist like Steve Saffier cardiac arrest. But he's looking healthy, keeping cool, doing his diplomatic best not to embarrass her or make her feel bad.

"These are invasives," he said, describing aggressive plants that are not recommended for home gardens, "but you're not alone. They pop up in most suburban yards."

Saffier is a bird enthusiast who works for Audubon Pennsylvania, part of the National Audubon Society. He's doing an environmental audit for Cross, a real-estate agent who has lived in this 1950s split-level in Erdenheim, PA, for six years.

The audit entails an hourlong walk around her quarter-acre property, during which Saffier looks closely at the landscape, suggesting that Cross gradually take out the problem plants and replace them with native plants. Unlike plants that originated somewhere else, natives long ago adapted to our local soil and weather.

"They thrive in whatever comes naturally," Saffier said.

They're hard-wired to provide food, shelter and nesting space for birds and wildlife. They'll need less water and maintenance, blending seamlessly into the ecological fabric of Cross' yard, neighborhood and region.

Saffier does the audits free of charge as part of the Audubon at Home program. It's a joint effort by Audubon Pennsylvania and Friends of the Wissahickon, a nonprofit group dedicated to preserving the natural beauty and wildness of the Wissahickon Valley and its watershed.

Cross, a Friends of the Wissahickon member, seemed to be a perfect candidate for an environmental audit.

She doesn't use chemicals. She drives a Prius. A certified EcoBroker, she's trained to advise her clients on energy efficiency and other "green" issues that may arise during real estate transactions.

By the time Saffier shows up, in other words, Cross has a pretty good idea of what she wants.

"I'm looking for more plants that offer habitat for birds and wildlife," she said, "and I want a diverse lawn, a healthy lawn."

But like many of us, Cross needs help translating "caring about the environment" into specifics. While she's heard of native plants, she isn't sure which ones will work for her.

"I do know this," she said, pointing to a small stream that runs along her property. "I have two ducks that come every year. I know whatever I do here affects them and whatever's downstream."

Saffier suggested that Cross plant natives like trumpet honeysuckle (Lonicera sempervirens), sweet pepper bush (Clethra alnifolia), arrowwood viburnum (Viburnum dentatum), and red-osier dogwood (Cornus sericea).

She's thrilled.

"I needed a vision for this property, and Steve has given me something. Native plants make sense," Cross said.