It most certainly IS interstate commerce, because you are using home grown to replace what would have otherwise been purchased, thus adversely affecting interstate dealers' sales.
Hey, that argument worked in front of the SCOTUS for home grown wheat used exclusively for personal use, in "violation" of Ag Dept Marketing Orders. Look up Wickard vs. Filburn, 317 U.S. 111 (decided in 1942) and weep.
For an update, look up Blattner.
Of course there's no guarantee that any of this will fly, but it would be good to have the libs fighting commerce clause overreach for once. And getting high is way too near and dear to their hearts for them to just drop the matter - this one is personal.
Re Wickard vs. Filburn........
Different from prop 19 in that the “personal use” amount produced, exceeded the total allowed amount.