Investors have jumped onto the marijuana bandwagon in droves. Even the tiniest of cannabis companies are attracting attention, and some of their share-price increases have been nothing short of spectacular. For those who prefer to get diversified exposure to the marijuana sector, the ETFMG Alternative Harvest ETF (NYSEMKT:MJ) has held itself out as a first-moving marijuana-focused exchange-traded fund for U.S. investors. Already, the ETF has attracted more than $700 million in assets, and the recent gains for cannabis stocks make it likely that the fund will bring in even more money, especially given its 50% rise in the past six weeks.

Alternative Harvest holds nearly 40 stocks, including not only marijuana producers but also pharmaceutical and biotech companies looking at the potential uses of cannabis in the medical profession and consumer-oriented companies that have expertise in selling smokeable products. Yet four of those marijuana stocks really stand out for their impressive performance so far in 2018. All four have doubled this year, and they all seem to have potential for further growth in the future.

Indoor growing facility with rows of marijuana plants and light lamps.

Image source: Getty Images.

The leader of the pack

Tilray (NASDAQ:TLRY) is Alternative Harvest's biggest holding. The ETF owns 467,000 shares of Tilray worth $77 million as of Oct. 1, and the position makes up almost 11% of the fund's total assets.

The most impressive thing about Tilray's role in the ETF is that the stock hasn't even been public all year. Tilray had its initial public offering just in July, but already, its share price has soared almost 600% from its first-day IPO price. Investors jumped into Tilray in part because it represented a pure play on marijuana, the first to go public in the U.S. market, and a limited offering of just 9 million shares means that Alternative Harvest currently owns around 5% of the stock's total float. Skeptics believe that Tilray could easily see a huge decline, but for now, traders are happy to use the shares to speculate on the coming legalization of recreational cannabis in Canada later this month.

The fan favorite

Before Tilray came along, Canopy Growth (NYSE:CGC) routinely found itself at the top of Alternative Harvest's holdings list. Even now, the stock is the ETF's fourth-largest position, with a $66 million position representing more than 9% of the fund's assets. The stock has climbed an impressive 110% so far in 2018.

Canopy is best known for the fact that it's attracted the attention of a major consumer products giant. Beverage specialist Constellation Brands (NYSE:STZ) has focused on Canopy with multiple rounds of investment in the cannabis producer, and many speculate that Constellation will eventually buy Canopy outright. That could be great news for the marijuana ETF, although Constellation already holds warrants at below-market prices to take a majority stake in Canopy. Even so, Constellation seems serious about the potential for cannabis to add growth to its lines of popular beer and spirits brands like Corona and Svedka, and with Canopy already showing success with global expansion, the two companies could complement each other quite well.

A new name for a strong stock

Third on the list is HEXO (TSX:HEXO), which recently changed its name from The Hydropothecary Corporation. HEXO makes up about 5% of Alternative Harvest's assets, and the stock has scored gains of 116% so far this year.

HEXO has flown under the radar even among many cannabis investors, but the company has a lot going for it. Like higher-profile players, the Canadian company is set to supply recreational cannabis in the key market of Quebec, with plans to boost annual capacity to more than 100,000 kilograms by the end of 2018. Moreover, a joint venture with Molson Coors to make cannabis-infused beverages could become another growth driver. All told, HEXO is putting itself in position to become a much more important player in the marijuana sector.

A marijuana stock on its way to better things

Finally, TerrAscend (NASDAQOTH:TRSSF) completes this list of marijuana stocks in the Alternative Harvest ETF that have doubled so far this year. TerrAscend makes up less than a 2% stake in the ETF, with about $13 million invested in the company, but its 128% rise so far in 2018 has been a big boon for marijuana investors.

TerrAscend is noteworthy because its most successful move involves a collaboration with another cannabis stock. Late last year, a group of investors led by a private equity company as well as Canopy Growth invested 52.5 million Canadian dollars in a private placement. In return, they got 47.7 million shares of TerrAscend stock, as well as warrants to purchase an additional 47.7 million shares at CA$1.10 at any time through December 2020. As part of the deal, TerrAscend joined the CraftGrow platform, allowing it to sell premium cannabis through Canopy's distribution network. That's worked out well for TerrAscend so far, and investors look forward to how things work out as demand for cannabis ramps up.

What's next for ETFMG Alternative Harvest?

The Alternative Harvest ETF's gains of 24% year to date could have been even greater if the fund had made different decisions. In particular, much of what has held back returns is that the ETF holds stocks that could become involved in marijuana in the future but haven't yet made definitive moves in that direction, especially weak-performing tobacco stocks.

However, the big gains in cannabis producers have now made the ETF much more concentrated on the core marijuana market. If stocks like Canopy, Tilray, HEXO, and TerrAscend keep climbing, the Alternative Harvest ETF is more likely to participate fully in their continued rise.

Dan Caplinger has no position in any of the stocks mentioned. The Motley Fool owns shares of Molson Coors Brewing. The Motley Fool has a disclosure policy.