Where the site proposal participants (both the original proposer and quesiton suggesters) on two sites clearly have the same basic audience in mind, I think a merge makes sense, and the tools exist.
But when the groups have a legitimate difference in interests, it needn't turn into a battle of wills. Overmerging of two (or more) unwilling groups just means watching a proposal get re-edited every 5 minutes until the name calling starts in the comments.
It's clear that @Ciaran and his Organic Gardening community are more personally committed to the organic aspect than the gardening one. I'd guess that they'd rather discuss organic laundry than commercial farming.
While @Gnoupi is right that they don't own the proposal, forcing a merge here doesn't make a ton of sense to me, any more than it would make sense to force "Gamers" into a "Leisure Activities" site that includes reading. Maybe it's good for them, but they're not going to stay, because books don't let them get fragged by European adolescents online, and that's what they're interested in talking about.
The organic group is really about organicness (organicity?), not gardening, and it will be big enough to stand on its own, or it won't.