NEW YORK (TheStreet) --  For investors who want to put the cannabis industry into their portfolios, one investment firm is holding the first ever cannabis investment seminar. Scott Greiper, president of Viridian Capital, which is hosting the seminar Thursday at New York City's Javits Center, said there's tremendous upside in the space, but retail investors still need to be careful.

"Do a little bit more diligence than you might on other sectors," he said. "There are risks that are very specific to this industry." Medical marijuana is legal in just 23 states. Recreational usage is legal in four states, plus Washington, D.C.

For investors who are wary about investing in the cannabis space, there are opportunities in companies that are indirectly related to cannabis.

For example, Greiper recommends the lighting sector as a good way to capitalize on the cannabis business -- companies whose products are used to help the marijuana plant grow.

"A company we like in the sector is called Heliospectra (HLSPY) ," Greiper said, noting that the company is a client of his firm. "They've developed a patented technology that can read how the plant absorbs different spectra of the light so they can actually control the rate of growth."

He also sees opportunities in the biotech arena.

"'In the marijuana space, biotech is very significant," he said, suggesting biotech today is where pharmaceuticals were 30 years ago. "CannapharmaRx (CPMD) , another one of our clients, is a company we think is developing a very interesting positioning in the industry."

Finally, he points to security companies, which even legal cannabis sellers are in need of, "since this is an all cash business." Greiper points to Canna Security America, which he said will be public in the next 30 days. "They have the largest market share of protecting several hundred dispensaries across the United States."

So far, he noted, there's not an exchange-traded fund focused on cannabis. His own Viridian Cannabis Index, which tracks 75 stocks in the space, returned over 38% in 2014 and was up over 23% through the first-quarter of 2015. "We hope to turn that into an ETF at some point," he said. "Ultimately, the liquidity of the underlying stocks in the index have to support a publicly traded exchange, but we think we'll be there by the end of the year."

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.