We're already getting some questions like this that have heavy cross over - what should the policy be? We should probably decide sooner rather than later in order to have a clear aim going forward and avoid confusing users.
The gardening SE attracts a lot of conventional answers that decidedly are not sustainable. I think if a question is about permaculture, forest gardening or sustainable (as in chemical free, polyculture) gardening then it should live here. The asker is far more likely to get the answer they are looking for here. Furthermore, a large proportion of experts in sustainability are experts in Permaculture. Growing food sustainably in a distributed way in urban and suburban environments is a massive part of what they do. If we exclude the topic of growing food, we'll be excluding them and we'll lose a huge portion of our potential experts.
 I'm going to expand this to further explain some of the reasoning and point to a number of questions that have been closed as off-topic, but that I think should we should carefully reconsider whether or not they should be on-topic.
Currently there are many questions that are being closed as off-topic gardening questions because they don't explicitly mention sustainability. But of course people aren't going to mention sustainability explicitly, they don't have to! It's assumed. They came here to ask the question instead of Gardening because they want answers from people with Sustainability on the brain. They don't want to have to wade through a whole bunch of conventional or unsustainable answers. And they don't want to be the ones to have to judge whether an answer is unsustainable. That's really key. They want answers they know will be the most sustainable ones, or at least will take sustainability very much into consideration. And by asking here, that's what they expect to get. At Gardening, they will have to be the ones to judge whether each answer is sustainable and they are probably asking the question in the first place because they are not qualified to be that judge.
Many of the questions that have been closed are implicitly about sustainbly growing things. Sustainable agriculture deals with forest gardening, permaculture and polycultures. So questions about encouraging connections between species (https://sustainability.stackexchange.com/questions/234/what-to-plant-with-asparagus), (What species of mycorrhizal fungus do well with apple trees?), (How do I innoculate a 3 year old tree with a mycorrhizal fungus?) are about using interconnections between species to increase the robustness of an ecological system and thus reduce the inputs required to sustain the system. These are about forest gardening and polycultural gardening. These should be decidedly on topic
Beyond that questions about actively growing food in suburban environments (https://sustainability.stackexchange.com/questions/274/replacing-a-tree), or even in urban ones (How do i start with vertical gardening?) are also integral to sustainability and implicitly about growing things for the purpose of being sustainable. When people come here to ask these questions they are looking for answers about how to do it in a sustainable way. The vertical gardening asker doesn't want to be told to buy a plastic window tower produced in China using petroleum products by forced labor. The tree replacer wants to be told how to reuse the material from or move the existing tree. If we move these questions over to Gardening, they will not get the answers they are looking for and they will have to be the judge of sustainability.
As someone who is actively experimenting with fully sustainable urban lifestyles and who runs in a large community of people doing the same using various styles and methods, growing food is at the very core of what we do. A massive amount of our time is spent figuring out how to grow our own food, in every nook and cranny, in a way that requires minimal inputs (labor, fertilizer, chemicals or pesticides). We usually land on high labor inputs, and then hope to minimize them over time. But this is easily more than half of what our time is spent on. It is absolutely inseparable from a sustainable lifestyle. If we exclude these questions, we will be potentially alienating and excluding a huge portion of the community of experts on this topic. So for this SE to succeed, we would do well to include these questions and answers.
 Here's another great example question: What acid soil friendly nitrogen fixers won't be affected by pine allelopathy in zones 6-7?
This is a question about permaculture, that is to say, sustainable agriculture. Guilding, planting things together that support themselves through interconnection, is a permaculture concept. And yes it doesn't explicitly mention "I'm doing this to be sustainable" or "to grow sustainable blueberries". But it shouldn't have to, because it's about sustainable agriculture. Look at the answer Chris gave. It opens with a discussion about whether or not it really is sustainable to change your pH when attempting to create micro-climates. That's exactly the sort of stuff people are looking for when posting here. And something they won't get over in Gardening. Because people in gardening aren't thinking about Sustainability.
With any of these obliquely-related subjects, it is really easy to draw this line, but it is sometimes difficult to explain to someone after the fact why their question isn't about the subject of sustainability. So please handle with care and always provide thorough guidance when closing these questions.
The Acid Test
Ask yourself —
"Is this question primarily related to the subject of 'Sustainable Living', or is the subject only coincidentally of interest to the audience who happens to frequent here?"
It's easy to forget that this isn't a general support group. This site should focus on building a canon of knowledge about the SUBJECT of sustainability. It cannot reasonably become about any subject that just happens fall with the interests of the folks who frequent here.
Consider a question like this:
How to get rid of greenfly in roses and grapes without toxic chemicals?
(About Sustainability? — YES)
A forum of gardening experts will know how to get rid greenfly — they may even know how to do it without chemicals — but it is difficult to vet if the authors (and voters) are completely versed in the deeper issues of sustainability that drive this site. You're the experts here. It's easy to imagine the "best" answers being wrong, if not asked in the context of this site.
But consider this question:
How can I save seeds from a tomato for replanting?
(About Sustainability? — NO)
I don't see anything that puts this in the purview of a sustainability expert. I could just as easily ask why my tomatoes are dropping blossoms or how to haul 500 lbs of compost, but that doesn't make it suitable for this forum.
I don't think there will be a single line: there will be a crossover area, within which there are questions that would sit equally well on either site. But that just leads to the next question - where is that crossover area?
I'd have thought that if a question's motivation is primarily about the sustainability of the gardening, then it can live here, as well as there.
Gardening.SE is full of gardeners; if you have a gardening question, and specify that you're looking for organic / sustainable / [synthetic] "chemical free" solutions, you will get those kinds of answers. (Some wiseguy may chime in with "just use napalm", but they'll end up downvoted to the bottom of the page.)
Compost probably fits equally well in either community; the end goal of the asker is probably how to decide where to ask.
An example of a question that would be more appropriate on gardening.SE:
at its essence, this is not a question about "sustainable living", it's a question about growing stuff.
I am going to defend a very lenient rule regarding permaculture on this site and also explain what I see as a bit of a problem with the way stack exchange is set up that makes these things more of an issue than they need to be.
In Defence of Cross-Posting and No Single Best Site to Ask a Question
The simple fact is that if you ask a question in different sites, you get different answers. Very often these are different aspects of an answer, and where you ask the question determines which aspects of the question are emphasized. If I ask about non-1NF designs on DBA I will get a very different answer than if I ask on programming. The latter will be far more theoretical and the former far more real-world (I have done this) and thus the issues discussed will be different issues. Often times it's good to be able to see similar questions being asked on different forums to get a sense of the issues at stake.
As the real example, I asked questions of non-1NF db design for modelling accounting constraints that don't fit well in relational models in both forums, and the answers I got back were very different. On DBA, it was mostly "well, this might work but it adds the following complexity that you might not want" and a few alternatives all of which were at least as complex. On Programming I got back answers entirely relating to the role of normalization theory in database design and how non-1NF designs are always bad. The point is that the two different communities took the question from very different angles and the same will happen if you ask a question here vs in gardening.
I think the stack-exchange phobia of cross-posting is misguided. For cases on the penumbra of several different sites, the sorts of answers given are qualitatively different and often a union of the answers is more helpful than selecting only one.
Permaculture == Sustainability
I personally think all permaculture questions should be welcome here because permaculture is one road (possibly the best road) to sustainability. Narrowly defined, permaculture is simply building sustainable gardens (even those who practice larger scale permaculture are more likely to call their projects gardens than agriculture), where the output of one system is the input of another. However, this is not actually how permaculture is actually practised. One thing several permaculturalists have told me is that "all of life's problems can be solved in a garden" and this provides what I think is the more or less total convergence of permaculture and sustainability. Because of this permaculture necessarily leads to sustainable living and, if we are serious about sustainability, sustainability necessarily leads to permaculture too.
If you think about it, virtually all of the sustainability questions boil down to basic permaculture questions, just applied in a different sphere, namely "how do we find something useful to do with X" or "how do we take this output and turn it into an input somewhere else?" or "what inputs do I need to do X?"
I am a huge fan of permaculture training for the simple reason that it provides basic training in sustainability for families and communities and how to think about sustainability of a depth that currently can't be found anywhere else. All permaculture questions IMO should be welcome on this site.
Edit: More evidence
As additional evidence I went and checked https://gardening.stackexchange.com/questions/tagged/permaculture and compared to https://sustainability.stackexchange.com/questions/tagged/permaculture
We already have twice as many permaculture questions as Gardening does and they don't have a great track record regarding multiple answers in this area. I would suggest this is further evidence that we need to see permaculture as perpetually on-topic here.
Edit: Even more evidence
I asked the asparagus question on gardening.se and the only answer I got was from a regular of this forum. The simple fact is that we are outperforming Gardening.se on quality and quantity of answers, and permaculture generally is integral to sustainable living (just as sustainability is integral to permaculture). I think this should be seen as pretty close to proof positive that all permaculture questions should be seen as on-topic here.
The line is not easy to draw, because there are many areas when various proposals and SEs overlaps. Gardening is not the only SE that overlaps with that SE. Survival-related questions could as good be asked on The Great Outdoors or Seasoned Advice.
One of my questions about cooking wild snails have been migrated from Outdoors to Seasoned Advice. Running questions could be as good asked on Outdoors as on Physical Fitness.
So how to decide where to ask some question? Assuming it is on topic on both (or more) proposal, first you have to analyse, when it is more on-topic and when less? Would this question be considered core or margin?
But you ask questions to get answers. So the second criteria: On which site I'm most likely to get the answer I'm waiting for?. The answers from gardening would be more biased towards the art of gardening, on seasoned advice more biased towards consumption issues, and here? Probably more biased towards the independence from food suppliers. You want to have food from beautiful garden, you want to have tasty food from the garden, or you want to have food mostly (or only) from your garden?
I'm going to be a contrarian say that 95% of gardening questions wouldn't belong on a site title "Sustainable Living." Why? 1% of the population can be very ecologically friendly, but the system is only sustainable as described by the net change of the individual's ecological footprint. Very little of this footprint is based on gardening - much more is based on fuel consumption. I would say only agricultural questions at the farm level would be appropriate for sustainable, due to a farm's net ecological change by accepting one farming method over another.
Again- I think healthy & ecological is great-- I just don't think that healthy and ecological necessarily constitutes sustainable.
The problem is that 95% of the questions people may ask about gardening on here will be better suited to the Gardening SE, but a large portion, probably a majority, of the likely readers are more likely to search for the info here on SL. SE site content is not mutually exclusive. If there were a way to get the SE servers to auto-duplicate questions on both sites based on tags...or maybe just a change to the search function? Clean data is important, but accessibility I think that would be the best solution.