Allbirds Inc. has already beaten the odds in the startup world by raising a quarter of a billion dollars and turning earth-friendly wool sneakers into a legit product category. But the company behind Silicon Valley’s favorite footwear now faces another daunting challenge: impressing investors ahead of a potential initial public offering.

That’s coming after its unicorn valuation took a hit last year. In September, Allbirds raised $100 million in a Series E round from large investment firms like Franklin Templeton and T. Rowe Price. According to researcher Pitchbook, the company’s valuation nudged down to an estimated $1.7 billion from $1.73 billion in January 2020. CB Insights, another data provider, shows the value plummeted to $1.14 billion, according to state filings it sourced. The company declined to provide any details on a possible IPO, valuation or performance.

Allbirds is part of the boom in direct-to-consumer brands that act as maker and retailer, avoiding the low margins of selling wholesale. Investors fell in love with the business model and threipw cash at the sector, ushering in a flood of companies selling everything from underwear to toothbrushes.

But now, more than half a decade after that initial enthusiasm, there are questions about how big these brands can get. Growth is often driven by massive spending on marketing that isn’t sustainable. And after success, copycats quickly emerge. In one high-profile example that’s raised doubts, Casper Sleep Inc., a fast-growing mattress brand, went public a year ago and has largely struggled with its stock still below the IPO price.

To prove that it can get bigger and broader, Allbirds—a brand built through its own e-commerce—will boost the physical stores it operates by 50% this year to three dozen, including its first location in Minneapolis, co-founder Joey Zwillinger said in a recent interview. The company is also coming out with items at lower prices and trying to expand its green bonafides by partnering with footwear giant Adidas AG on a sneaker that promises to have almost no carbon footprint.

“That’s the goal here—not to be a niche player selling premium products to rich people who care about the environment,” Zwillinger said. “We have sustainability for the masses.”

Zwillinger and co-founder Tim Brown, who share the chief executive officer role, had grand plans for Allbirds early on. In 2016, the year the first shoe debuted and gained a following among tech workers in the company’s hometown of San Francisco, they unveiled internally what the brand would look like a decade later. “It was very, very ambitious,” said board member Dan Levitan, co-founder of Maveron, an early investor. “I was like, ‘wow, if they accomplish half of this, they would have been very successful.’”

Allbirds is following the playbook of other DTC brands that have broadened their audience by moving from digital marketing and e-commerce into brick-and-mortar and traditional advertising. Just last month, Allbirds started spending on television ads for the first time with a focus on reruns of sitcoms with wide appeal such as Friends and Two and a Half Men, according to iSpot.tv, a TV measurement and analytics firm. The company has so far this year spent the most on marketing in the U.S. across digital, TV and print among the 175 DTC apparel and footwear brands tracked by MediaRadar, an advertising intelligence platform.

“People don’t even know about us yet,” Zwillinger said. “We have to continue to methodically build our brand, brick by brick.”

Therein lies the promise of Allbirds for Levitan, who started Maveron in 1998 with Starbucks founder Howard Schultz. The firm has backed several winners in the consumer space, including EBay and Pinkberry, and he sees Allbirds on the cusp of breaking out beyond its core audience. In May, the company pushed into running, one of the largest categories of the athletic shoe market. The Tree Dasher, which retails for $125 and has a mesh upper made from eucalyptus wood pulp and a sole crafted with sugarcane, was its biggest product launch to date in terms of sales. Its apparel line now spans no-show socks to puffer jackets, displaying its ambitions to become a full-body brand. And it already has expanded outside the U.S., selling in more than 30 countries and opening stores in large markets like China and Japan.

“Do I think this company has the fundamentals to be a super large, enduring consumer business?” said Levitan, whose firm first invested in Allbirds in September 2016, just six months after its first shoe hit the market. “Yes, absolutely. Is it going to be hard? Yes.”

The company’s first breakthrough was making footwear deemed easy on the feet. Having a good publicist helped, too. When its Wool Runner launched the brand almost five years ago, Time magazine wrote a glowing piece titled “The World’s Most Comfortable Shoes Are Made of Super-Soft Wool.” Later, the New York Times mused about the footwear becoming a mainstay in Silicon Valley’s minimalist uniform. In 2018, the New Yorker dedicated two stories to the company, including a profile in the magazine that focused on its green ambitions.

It’s that last storyline that will decide whether Allbirds becomes anything close to Gen Z’s version of Nike, which is approaching $40 billion in annual sales after more than four decades as a public compnany. Allbirds is a certified B Corp., meaning its board is legally bound to balance profit and purpose and publicly share an impact report on how it’s improving society or the environment. If the company has an IPO, that designation will be central to its pitch, Zwillinger says. That could make it an attractive stock for money managers on the hunt for companies in the growing sustainable investing sector.

For consumers, the company’s advertising is blaring the message that it’s all about fighting climate change. One ad’s kicker is simply “reduce your carbon footprint.” Another spot proclaims “when nature wins, we all win.” That’s a far cry from the “Just Do It” tagline made famous by Nike.

Weaving shoes and apparel so directly to saving the earth is a bold move. History shows that the overwhelming majority of consumers aren’t swayed by environmental concerns when it comes to fashion, according to Simeon Siegel, a retail analyst at BMO Capital Markets.

“At the end of the day, people buy the footwear that makes them comfortable and makes them feel special,” Siegel said. People like to believe they are “consumer activists” on issues like climate change, but they often aren’t, he said.

However, marketing can do wonders. And surveys of 20- and 30-somethings show that some want to buy products that are made sustainably, according to Matt Powell, an analyst for market researcher NPD Group. But there’s a catch.

“It’s a smaller audience.”