Growth stocks make for attractive investments because they can generate significant returns in what can sometimes be a very narrow time frame. And that's what GrowGeneration (NASDAQ:GRWG) and Penn National Gaming (NASDAQ:PENN) have done for their investors. If you had invested $15,000 in either company during last year's market crash, your investment would have grown to more than $200,000.

Have these stocks peaked, or is there still time to invest in them? Below, we'll investigate why they have done so well and ask whether you can still expect to make great returns from them if you buy now.

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1. GrowGeneration

During the early stages of the pandemic in the U.S., on March 18, 2020, GrowGeneration's stock was trading as low as $2.75. If you had invested $15,000 at around that price, that would have been enough to buy roughly 5,400 shares of the company. At a share price of $40, that investment would be worth more than $215,000 today.

Although a hydroponics specialist doesn't seem like it would be a high-growth stock at first glance, investors are excited about its prospects in the cannabis industry. Its products help people grow crops more efficiently, utilizing pipes, pumps, and other components that can also save space -- especially when compared to conventional growing operations. With more states legalizing marijuana for recreational use (Arizona, New York, and New Jersey are some of the biggest ones to do so of late), investors clearly believe that GrowGeneration's products will be in high demand. 

The company is already seeing a surge in sales on its financials. It released its most recent earnings report on May 12, for the period ending March 31. For the first quarter of fiscal 2021, its sales hit a record high of $90 million, up 173% year over year. Acquisitions have played a big role in the company's growth. A year ago, GrowGeneration had just 27 hydroponic and gardening stores. As of its latest earnings release, it had 53 -- nearly double that tally. The business is also growing organically, with same-store sales up 51% year over year.

The cannabis industry is still not legal at the federal level, and the majority of states don't permit it for recreational use, so there is still lots of room for more growth for GrowGeneration. While I wouldn't expect another phenomenal four-digit return from investing in the business today, that doesn't mean you can't still make a great profit from investing in GrowGeneration. Investors love a good growth stock, and that's what you are getting with this unique pot pick. Although the company isn't a licensed producer, it is a fantastic pick-and-shovel investment for people who want to tap into the industry's growth potential.

2. Penn National Gaming

Penn is another business that's made a name for itself in a rapidly expanding industry. The gambling stock fell below $4 during the depths of the coronavirus market crash. At that price, a $15,000 investment would have netted you more than 3,700 shares. With the stock trading at more than $80, those shares would be worth nearly $300,000. That's an even better return than GrowGeneration's, which already looks incredible -- and that's with Penn's stock falling in recent weeks. 

Investors started to get bullish on the stock early last year. In January 2020, the company announced it would be acquiring a 36% stake in Barstool Sports, a digital sports media company, for $163 million. Penn would become its exclusive gaming partner (for 40 years) and would be able to use its brand. With tens of millions of unique visitors to the Barstool site every month, the move allowed Penn to reach more customers and put it in a great position to benefit from the emergence of the sports betting market.

On May 6, Penn released its quarterly results for the first three months of 2021, and sales were $1.3 billion, up 14% year over year. Lockdowns have affected the company's business, but Penn notes that visitations in most of its markets are now back to the levels they were at in 2019. Perhaps more importantly, people are spending more per visit than they were before the pandemic.

Penn is growing at a great pace, and with Barstool to help grow its presence and reach more customers, the sky could be the limit for the company. Sports look to be coming back in full force this year, and with more states looking to legalize sports betting (currently more than two dozen states do), the stock could have another great year in 2021. And that's why despite posting incredible returns over the past 14 months, it may still not be too late to invest in Penn.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.