Marijuana stocks, for the most part, have been outstanding growth vehicles for investors this year. Despite Canada's raging battle with black-market sales and the high level of uncertainty surrounding America's future policy on the drug, investors have been piling into these stocks with abandon in 2019.

So, which pot stocks should investors be keeping tabs on in May? Our Motley Fool contributors think HEXO Corp. (HEXO), Aurora Cannabis (ACB -0.92%), and Canopy Growth Corporation (CGC 3.27%) are three top names in the space worth watching this month. Here's a brief rundown for each stock.

A young cannabis plant in a dark background.

Image Source: Getty Images.

A test case

George Budwell (HEXO Corp.): Canada's HEXO Corp. is the pot stock that has my undivided attention this month. After uplisting to the New York Stock Exchange and subsequently acquiring Newstrike Brands to boost its annual production capacity to a noteworthy 150,000 kilograms per year, HEXO has quietly transformed into one of the hottest pot stocks in the market. The company's shares, in fact, have gained an eye-popping 125% since the start of the year.

Why does HEXO's red-hot momentum matter? Two reasons. First up, stocks in general tend to go into a slumber from May to October. So it'll be interesting to see if HEXO's shares follow this traditional seasonal downturn. If they do, investors can probably expect a slow summer across the entire industry. After all, HEXO still sports the second-most-compelling valuation across the space -- even after its blistering start to the year.

The second reason is that HEXO has a number of major catalysts coming up in the back half of the year. Not only is HEXO's top-line forecast to grow exponentially over the remainder of the year, but the company's joint venture with Molson Coors Brewing -- called Truss -- to develop cannabis-infused beverages should become operational around October of this year. HEXO and Molson plan on being among the first to offer cannabis-infused beverages in Canada once this high-value market segment opens up later this year.

This highly anticipated catalyst should -- in theory, at least -- keep investors engaged during the summer doldrums, perhaps pushing HEXO's shares to new highs. Time will tell.

Too much buzz to ignore

Keith Speights (Aurora Cannabis): I can't think of a marijuana stock to watch in May that's more intriguing than Aurora Cannabis. There's simply too much buzz about this Canadian cannabis producer for investors to ignore.

Analysts are heaping their praises on Aurora. The company recently became one of three winners to receive a cultivation license for medical cannabis in Germany. (To put this accomplishment in perspective, 79 companies were vying for the honor.) Aurora is well on its way to building an annual production capacity of over 625,000 kilograms -- more than any other marijuana producer.

I'm most interested in watching Aurora, though, because it's actively hunting for big partners. So far, Aurora has been left in the dust by several of its peers on this front, including Canopy Growth, Cronos Group, HEXO, and Tilray. However, that could change in the near future.

In March, Aurora announced that billionaire investor Nelson Peltz was teaming up with the company as a strategic advisor. Peltz's No. 1 job is to find partners for the company in industries that could be disrupted by cannabis -- think beverages, cosmetics, pharmaceuticals, and wellness products.

If Peltz can deliver for Aurora, the stock is likely to soar. I'm not sure if we'll see anything happen on this front this month. But I'll definitely be watching Aurora closely.

It's showtime for the largest pot stock in the world

Sean Williams (Canopy Growth): It's May, and there's arguably no event more anticipated than the fourth-quarter and full-year earnings release of the largest marijuana stock in the world by market cap, Canopy Growth.

What makes the company's fiscal fourth-quarter report so interesting is that it'll almost certainly feature the weakest growth in the legal cannabis industry's short history. You see, Canada has been facing a pretty severe marijuana supply shortage, which has resulted in cannabis-store sales declining for two consecutive months. This quarter will feature Canopy's performance during the heart of this sales decline.

One key reason for these supply chain issues has to do with the incredibly long list of cultivation and sales applications that Health Canada is contending with. It often takes months or years for the regulatory agency to review these applications. It's also going to take years for growers to get their production up to full speed. Pot producers were unwilling to spend big bucks on capacity expansion until there was absolute certainty that the Cannabis Act would become law. This didn't leave nearly enough time to get production up to snuff, or to submit cultivation and sales applications.

Back to Canopy, there's a real chance it may deliver a sequential quarterly sales decline. Unlike Aurora Cannabis, Canopy is all in on recreational weed sales, which accounted for more than 79% of total cannabis sales in the fiscal third quarter. Since adult-use sales are being hit the hardest by this shortage, it could be a very challenging quarter for Canopy.

What will really be interesting is how well Wall Street and investors handle a quarter of subpar growth from a company that does have competitive advantages but is also lugging around a hefty premium, considering that it won't be profitable on a recurring basis until 2021 at the earliest. Could we see some of Canopy's premium finally deflate? We'll know very soon.