General Mills tried to help the bees this spring, and instead it got stung.

Native plant lovers across the country are complaining that the 1.5 billion seeds the company has given away in the last week to encourage people to plant bee-friendly wildflowers are, well, not that bee-friendly.

In fact, native plant experts said that some of the 20 different species in the wildflower mix could be unwelcome weedy interlopers in some parts of the country, and not especially attractive to bees and butterflies in others.

"At worst these things can potentially introduce weedy plants where they might not currently exist," said Eric Mader, a native plant specialist with the Xerces Society for Invertebrate Conservation, which advises General Mills on its pollinator conservation program. "At best … I don't know if there is a best."

The seed supplier, Veseys of York, Prince Edward Island, said the seed mixes do not contain invasives and were selected for their attractiveness to pollinators everywhere.

It's a rare stumble for General Mills, which is widely acclaimed by conservationists as a model for what the corporate world could do to protect insects that play a critical role in the nation's food system. The company has given $4 million to the Xerces Society for pollinator protection, has established thousands of acres of pollinator habitat around its suppliers' growing fields across the country, and is reducing suppliers' reliance on pesticides.

The seed packet giveaway "is an insignificant part of what they are doing," said Mader.

The episode began last week, when General Mills' U.S. Cheerios division joined an ongoing campaign with its Canadian sibling. The Golden Valley-based food company took the "BuzzBee" logo off 10 million cereal boxes and asked its customers to help "bring the bees back" by ordering free packets of wildflower seeds that they could plant for pollinators to feast on.

"The goal was to raise awareness of pollinators," said Mike Siemienas, a spokesman for General Mills.

It was hugely successful. Canadians ordered 134 million seeds. Since March 9, when the program launched in the United States, customers have ordered 1.5 billion seeds — 500 million more than the original goal — and depleted the available supply.

But alarm bells started going off on social media and listservs among native and wild plant advocates across the country. Then the website Lifehacker posted a story citing those concerns, which multiplied across other media sites.

The problem, plant experts said, is that there is no good one-size-fits-all seed mix for all of Canada and the United States, with their vast array of regional ecologies. The packet contains California poppies, for example, which are fine in California but can be aggressive growers in other parts of the country.

Mader said the mix contains both aggressive annuals and slow-growing perennials like New England aster, which will not "play well together." But none of the species is especially harmful anywhere, he said.

"When you mix them together in a packet, you are not going to end up with a cohesive plant community," he said. "Some might thrive, some might fail."

There have been far worse problems with seed mixes recently. Last year, seed mixes sold to Minnesota farmers to benefit pollinators were contaminated with harmful weed seeds that have now been introduced onto hundreds of farms in several Midwestern states. The weeds include Palmer amaranth, one of the most prolific and devastating weeds in the country for corn, soybeans and other row crops. The inadvertent planting on conservation land has greatly accelerated their spread, leading to the discovery of Palmer in Minnesota for the first time.

It's that kind of fear that helped fuel the backlash on the General Mills seed packets. Veseys has been fielding questions from both countries on whether the species were invasive or genetically modified — which they are not, said John Barrett, director of sales, marketing and development for the seed company. The same seed mixes are carried by major retailers throughout the United States, he said.

"It has been field-tested and is known to attract honeybees, bumblebees, and other native bees," he said in an e-mail Tuesday.

Nonetheless, native plant advocates are telling gardeners that it's OK to plant the seeds on private property, but not on public land, where the unruly species could grow without restraint or spread where they are not wanted.

"If you can't use it, don't toss it out on a roadside — put it in the garbage," said Marilyn Jones, co-president of Wild Ones, a native plant advocacy organization in Minnesota.

Josephine Marcotty • 612-673-7394