Sure, it's the pun of all puns, but the marijuana industry really is growing like a weed. According to cannabis research firm ArcView, the North American legal-cannabis market saw sales increase by 34% in 2016 to $6.9 billion, and they're liable to grow by an average of 26% a year through 2021. That's a nearly $22 billion market up for grabs, and investors certainly want a piece of that pie.

Though Mexico legalized medical cannabis in June, and Canada may be on the verge of green-lighting recreational weed by July 2018, it's still the U.S. market that's viewed as the greatest opportunity for the legal cannabis industry. Since 1996, 29 states have legalized medical cannabis, with an additional eight states having legalized recreational pot since 2012. With recreational weed the greatest source of growth potential for the legal marijuana industry, all eyes are on which states could be next to legalize recreational pot.

As we look toward mid-term elections in 2018, the following five states already have one, or in some cases far more than one, initiative or amendment lined up to legalize marijuana for adult use. The big question is whether or not these pieces of legislation will make it onto the ballot.

Arizona

Of all the states to consider legalizing recreational cannabis in 2018, Arizona is the most logical. Arizona put a recreational-weed measure on the Nov. 2016 ballot, and it wound up failing to pass by a mere 2%. We've seen some of the most progressive states, such as Oregon and California, fail to pass a recreational-pot initiative on their first try, so focused efforts by pro-legalization groups may be able to sway voters to pass such a measure in 2018.

According to Ballotpedia, there are five marijuana-based initiatives in 2018, including one for industrial hemp and the expansion of medical-marijuana access. Perhaps the most intriguing, though, is the Arizona Marijuana Legalization Initiative. This aggressive piece of legislation aims to allow for the possession and use of weed by adults 21 and over, and would allow for the cultivation of up to 48 plants (yes, 48!) with more than a 0.3% THC level. It would also forbid local jurisdictions from passing laws that would keep legally operating weed businesses out. It's unclear if this initiative has a shot at passing, but it's among the most progressive this country has ever seen.

Florida

It took Florida till the second try before it was able to successfully pass a medical-marijuana amendment in Nov. 2016. In Florida, passing marijuana legislation requires changes to the state's constitution. Therefore, a simple majority vote won't work. Instead, a majority vote of 60% is required for amendments to pass. In November 2014, Florida's medical-cannabis amendment fell 2% short of the required vote. However, with pro-legalization groups focusing their efforts on Florida in 2015 and 2016, it passed with ease this go around (71% yes, 29% no).

Now, it looks as if pro-legalization groups are aiming to garner support in Florida for recreational legalization. There are four measures currently be considered for the ballot, although it'll still require a 60% vote to alter the Florida constitution. For instance, the Florida Cannabis Act would legalize possession of up to an ounce of marijuana for adults 21 and up, and also allow residents to grow up to six plants per household. However, only three of those plants could be mature or flowering under this legislation. These plants would also need to be in an enclosed and secure space, so as to keep them away from adolescents.

Michigan

Michigan has long been a state rumored to have a shot at passing a recreational legalization bill. Back in 2016, pro-legalization groups attempted to get a recreational-cannabis bill on the ballot, but fell well short of the required signatures needed to do so. The grassroots effort just wasn't there last year, but with time and money pouring into the state in 2017, there seems to be a reasonable shot at a recreational initiative making the ballot in the upcoming year.

For example, the Michigan Marijuana Legalization Initiative would allow adults ages 21 and up to possess, use, or transport up to 2.5 ounces of marijuana, or 15 grams of marijuana concentrate, and grow up to 12 cannabis plants in their residences for personal use. What might be more exciting is that this initiative would also green-light industrial hemp, which could itself be a multibillion-dollar industry.

If passed, consumers could be facing a 10% excise tax, which comes on top of the state's existing 6% sales tax. Tax revenue collected would primarily head to schools and Michigan's state transportation fund for repair and maintenance of roads and bridges.

Missouri

Missouri might be angling to do something that Ohio attempted in 2015 and failed miserably: legalizing recreational and medical cannabis at the same time. Currently, it's one of 21 states not to have legalized medical pot, and while there are a handful of medical-cannabis initiatives vying for a spot on the ballot in 2018, half pertain to recreational weed or industrial hemp.

The Missouri Marijuana Legalization Initiative aims to remove marijuana from the states' list of controlled substances, allow for its sale to medical and recreational users, and immediately releases prisoners incarcerated for non-violent marijuana-related crimes. It would also forbid the state from using funds to enforce federal marijuana laws. As a reminder, cannabis is a Schedule 1 substabne, and ergo illicit, at the federal level.

I suspect such an aggressive measure that would release non-violent offenders would face a lot of pushback within the state, and I wouldn't necessarily count on seeing a dual medical-recreational bill make it on to the ballot in Missouri.

Nebraska

By a similar token, Nebraska may also be thinking about a dual-legalization effort. Nebraska, along with Missouri, hasn't legalized medical cannabis. In fact, Nebraska and Oklahoma wound up suing Colorado based on allegations that recreational weed in Colorado would be trafficked into their neighboring states. This suit, which the Supreme Court denied in 2016, makes the idea of Nebraska's angling for a dual legalization a complete head-scratcher.

Nonetheless, there's at least a slim chance that the Nebraska Right to Cannabis Initiative makes it onto the ballot in 2018. This measure would create a constitutional right for people aged 21 and up to consume, manufacture, and distribute cannabis for personal or commercial purposes. It'd also allow those under 21 to possess and consume medical cannabis with the written permission of a guardian, and a written recommendation from a licensed physician. Still, I doubt this measure will make it on to the ballot.