You can watch the ads repeatedly and stack the duration (up to 12h, apparently). 8 ads a day isn't a great burden, so keeping it at full production continuously for free does seem viable.
Which makes the greenhouse the equivalent of about 2.7 farms running Producer (about 20% more food production), or 0.45 of a farm running Super Producer. That's definitely something, especially for those of us without the gold to run Super Producer nonstop. The less gold production you have, the better it gets - if all you can do is Harvester, it's the equivalent of almost 4 farms (more really, since a 3-hour production time you can't really keep going all night).
So it's something that helps newer players most, reasonably developed players appreciably, and long-term vets not much. That seems pretty good to me actually.
But yeah, going for full production off of anything other than ads is only for the innumerate.