So far, 2018 hasn't exactly been a great year for most marijuana stocks. Sure, some marijuana stocks have chalked up huge gains. Most, though, haven't performed very well.

But there are still nearly six more months to go in 2018. I think three marijuana stocks warrant consideration as candidates to buy in the second half of the year: Scotts Miracle-Gro (NYSE:SMG), Canopy Growth Corporation (NYSE:CGC), and Aphria (NASDAQOTH:APHQF)

Marijuana leaf on top of stack of $100 bills

Image source: Getty Images.

1. Scotts Miracle-Gro

Scotts Miracle-Gro's share price fell more than 20% during the first half of 2018. There were two reasons behind this dismal performance. First, California hasn't had its act together in rolling out the legalization of recreational marijuana, seeing high tax rates and slowness for many counties to finalize their regulatory processes. Second, a long winter resulted in the season for U.S. consumers to buy lawn and garden products starting later than it normally does.

But both of these are temporary issues. The California cannabis market should pick up steam. Arcview Research and BDS Analytics project that the state's legal marijuana market should top $7.7 billion in sales in 2022. And the U.S. is definitely in the thick of lawn-and-garden season now.

Scotts is also in a better position to benefit from expansion in the U.S. marijuana market thanks to its acquisition of Sunlight Supply in April. This deal makes Scotts' Hawthorne subsidiary the largest hydroponics supplier in the U.S. With Massachusetts legalizing recreational marijuana and Michigan potentially on the way to doing so, Hawthorne should enjoy solid growth in the future.

2. Canopy Growth Corporation

Canopy Growth Corporation is one of the few marijuana stocks that generated a solid return in the first six months of the year. The Canadian marijuana grower's share price is up 25% year to date.

Can Canopy Growth keep its momentum going? Probably so. The market for adult use of recreational marijuana opens in Canada in October. Canopy already has supply agreements for recreational cannabis with several provinces. The company has also cranked up its capacity to meet what's expected to be a surge in demand.

An even greater opportunity for Canopy lies in global medical marijuana markets. The company reported record sales in Germany in its fiscal fourth-quarter results. Canopy is also expanding into other international markets. And with its big partner Constellation Brands, the company plans to get into the cannabis-infused beverage market. Canopy CEO Bruce Linton recently hinted in a CNBC interview at the potential for launching a zero-calorie beverage infused with cannabis.

3. Aphria

Aphria is one of the Canadian marijuana stocks that has experienced a miserable year so far. Its stock is currently down 39% since the beginning of 2018 -- and that reflects improvement over the last couple of months.

However, most of the same catalysts for Canopy Growth also apply to Aphria. The company should be a big winner from recreational marijuana legalization in Canada. Aphria is on pace to produce 225,000 kilograms of cannabis annually by early 2019. The company also has a solid retail distribution network thanks to its recent deal with Southern Glazer's, the largest wine and spirits distributor in North America.

Like Canopy Growth, Aphria has targeted the global medical marijuana market. If U.S. laws change to prevent interference with states that have legalized marijuana, Aphria should be in great shape to jump into the U.S. market because of its relationship with Liberty Health Sciences. It's even possible that Aphria could join Canopy by moving into the cannabis-infused beverage business: Reports surfaced recently that Molson Coors Brewing is in discussions with Aphria and three other Canadian marijuana growers about a potential deal.

No guarantees

While I think these three marijuana stocks are ones worth investors' consideration, there's no guarantee that any of them will necessarily generate tremendous returns in the remaining months of 2018. Great expectations of growth are already priced into the stock prices for Canopy Growth and Aphria. Challenges in California could continue to plague Scotts Miracle-Gro.

I listed Scotts Miracle-Gro first on the list, though, because I think the company is one of the relatively safest ways to invest in the growth of the cannabis industry. Scotts still makes over 90% of its revenue from its consumer lawn and gardens products, which gives it a stability most marijuana stocks don't have. At the same time, though, Scotts could enjoy solid growth from its status as a go-to supplier for the cannabis industry. 

There's no sure thing with investing. And there's absolutely no sure thing with investing in marijuana stocks. But Scotts Miracle-Gro just might be as good as it gets in the high-growth and high-risk marijuana market.

Keith Speights 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.