Answer. Based upon recent news reports, it appears that, subject to certain exceptions, there will be no effort by the federal Department of Justice to seek out and charge violators of the Controlled Substances Act in those states where the medical or recreational use of marijuana is legal.
Thus, it appears that when it comes to enforcement of park rules and regulations, Oregon landlords are on their own; neither the feds, nor the state, will go after persons with lawfully issued medical marijuana cards. Furthermore, if a tenant has a valid card, then arguably he or she has some medical condition that has authorized its issuance. Is the landlord obligated under the Fair Housing laws to make a “reasonable accommodation” for their medical condition, and permit the tenant to continue their use or grow operation? If properly done, the answer is likely “No.” Here’s why:
In January 20, 2011, the U.S. Department of Housing and Urban Development (“HUD”) issued a Memorandum, the subject of which was “Medical Use of Marijuana and Reasonable Accommodation in Federal Public and Assisted Housing.” While the Memo was limited to federal public and assisted housing, it can be regarded as a helpful – though perhaps not a “final” resource – on the issue. It is very complete and helpful for all landlords. It can be found at this link. Here is what the Memo directs:
Public housing agencies “…in states that have enacted laws legalizing the use of medical marijuana must therefore establish a standard and adopt written policy regarding whether or not to allow continued occupancy or assistance for residents who are medical marijuana users. The decision of whether or not to allow continued occupancy or assistance to medical marijuana users is the responsibility of PHAs, not of the Department.”
Thus, HUD appears to be leaving it up to the state public housing authorities to decide whether the refusal to permit on-premises use of medical marijuana constitutes a fair housing violation. Between the lines, it appears that HUD will not directly investigate such claims, leaving it up to public housing agencies on the state level.
While HUD’s pronouncement is directed toward “public housing” is would be hard to believe private housing would be treated any differently. Oregon fair housing law is "substantially equivalent" to federal fair housing law. So, generally speaking, on the issue of medical marijuana, as goes the federal law, so goes state law.
However, in the 2010 case of Emerald Steel Fabricators, Inc. v. Bureau of Labor and Industries, the Oregon Supreme Court held that employers do not have a legal duty to allow employees to use medical marijuana on the job. This case addressed many unanswered questions on the use of medical marijuana in Oregon from an employment perspective. In a subsequent article [found here] by the Fair Housing Council of Oregon it appears that the rationale of the Emerald Steel Fabricators case is helpful for landlords declining to admit new residents with medical marijuana cards – so long as they have an existing policy against the use and cultivation of marijuana in the community.
Thus, it appears that in Oregon, on both the federal and state levels, enforcement agencies are taking a laissez-faire approach to the medical marijuana issue. This means that landlords have it within their control, with little fear of fair housing/reasonable accommodation claims, to enact rules and regulations prohibiting the on-premises medical or recreational use of marijuana.
However, I do not believe the proscription should be retroactive to tenants holding legal medical marijuana cards who have already signed their rental agreements or leases. Like you, I believe that a court would not be favorable to your situation.
It appears that your resident’s medical marijuana card is in order. It must valid and current for Oregon. A California card, for example, would not suffice. [See, State v. Berrenger, 2010].
Conclusion. Yours is a difficult situation. For existing tenants I believe you can legally institute a “no marijuana” policy against recreational and medical use. However, making it retroactive as to persons already holding medical marijuana cards, would be a difficult proposition, since they did not bargain for that when they became residents or when they received their card.
In some instances, and this may not be one, I have seen situations where the resident, under the guise of holding a medical marijuana card, is also selling the drug illegally to others. This situation is most apparent when there are late night visits by unknown persons for short periods of time. If this situation presents itself, and neighbors complain, you may have recourse by issuing a 30-day curable notice of termination for violating ORS 90.740(4)(j) for disturbing the neighbors’ peaceful enjoyment. You do not have to raise the marijuana use, just the noise and disruption. Upon a second similar violation within six months of the date of issuance of the first notice, you can issue a 20-day noncurable notice.
 The exceptions are: The distribution of marijuana to minors; Directing revenue from marijuana sales to gangs and cartels; Diverting marijuana from states where it is legal to other states where there are no laws allowing for marijuana use; Using legal sales as cover for trafficking operations; Using violence and or firearms in marijuana cultivation and distribution; Driving under the influence of marijuana; Growing marijuana on public lands; Possessing marijuana or using on federal property.
Note: This answer is not intended to constitute legal advice. Readers should consult their own legal counsel to determine how to proceed in these cases, as the correct outcome depends upon the specific facts of each situation.
 Note that Oregon has its own set of fair housing laws.