Noah Kerner, CEO of Acorns.Adam Jeffery | CNBCSavings and investing app Acorns plans to go public by merging with a blank-check company.The fintech start-up announced a deal Thursday to combine with Pioneer Merger Corp., a publicly traded special purpose acquisition company, or SPAC. The merger values Acorns at roughly $2.2 billion and is expected to close in the back half of this year.When it is finalized, Acorns will trade on the Nasdaq under the symbols OAKS — a nod to the company’s motto and analogy of growing acorns into “mighty oaks.””Now was the time to go public to accelerate our growth, and get the tools of responsible wealth-making in everyone’s hands as fast as possible, when they need it most,” said Acorns CEO Noah Kerner. “We just saw this as an accelerant on that journey.”Institutional investors Wellington Management, Greycroft, TPG’s global impact investing platform, and funds managed by BlackRock also committed to a private placement as part of the announcement. Kerner and Pioneer’s sponsor each plan to contribute 10% of their personal ownership in Acorns as a gift to eligible Acorns customers.The company was last valued at less than $1 billion, and has attracted venture investments from the likes of PayPal Ventures, BlackRock, Ashton Kutcher, Jennifer Lopez, and Dwayne Johnson, according to PitchBook.Irvine, California-based Acorns had been in the process of closing another private funding round, Kerner said, but decided to go the recently popular SPAC route. He pointed to John Christodoro, a PayPal board member and chairman of Pioneer Merger, as the right partner and one reason Acorns bypassed a traditional IPO.”Acorns is not only a category leader but also a category creator. Its value proposition is built around inclusive, long-term financial wellness,” Christodoro said in a statement. “With integrity at its core, the brand has an incredibly loyal following and market leading retention rates.”Acorns’ most popular offerings let customers automatically invest the spare change from debit or credit card purchases into index funds. Since launching in 2014, it has expanded into educational offerings, banking products, a debit card, and an automated retirement account service.Special purpose acquisition companies, known as SPACs, raise money through a shell company to buy an existing company. This has become a popular way for later-stage, venture-backed start-ups to list on public markets quickly this year. New issuances of SPACS dropped off in April though, with just 10 new ones coming to market versus 109 a month earlier, according to SPAC Research.Trading tailwindsThe Acorns listing comes on the heels of record growth for investing apps during the pandemic. Part of that was thanks to a frenzy around GameStop and other “meme stocks.” The trading mania has brought new attention to the markets, and driven millions of first-time investors to platforms such as Schwab, Robinhood and Interactive Brokers.But it’s benefitting passive investment apps, too. Wealthfront and Betterment both notched their best quarters in history to start year. Kerner said the first quarter was also Acorns’ best three months on record with subscribers doubling from the fourth quarter to 4 million. The start-up’s revenue is made up of roughly 80% subscription fees, and 20% transaction fees and brand partnerships.When asked about growing competition, Kerner said “we run our own race.””We’re focused on long term financial wellness and helping customers get and stay committed to their long-term financial best interests,” he said. “Our vision is to build a financial wellness system that enables everyday Americans to save and invest.”Disclosure: Comcast owns CNBC’s parent company, NBCUniversal, and is an investor in Acorns.