What exactly is a sustainable garden? Well basically a sustainable garden is the one you have the resources to produce year after year with no outside inputs other than those you can count on as being available for at least the very long term. Five plus years minimum is my rule of thumb. One reason I switched basic garden types over the years was the quest to find the perfect sustainable garden for my location.
A couple of basic rules I finally adapted include things like hardware. For instance if you like to use a ground cloth to control weeds this would not be sustainable as the ground fabric would not be available in a grid down situation. The same line of thinking would apply to any temporary input that would need to be replaced every year or three. Like seeds of course, special chemical inputs like fertilizer or other amendments if you use them.
One reason I include very few Squash varieties in my garden is the resistance and numbers of the Squash bug invasions. Typically if I do plant Squash it is as a late season crop or maybe if I am feeling ambitious one or two plants at most early. I have never found a suitable and adequate method to control those demon bugs once they get started except by eliminating Squash plants period. Which is a shame cause thinly sliced yellow Squash and Zucchini lightly fried in butter is a Summer time snack I often will get a craving for. Triple that if my son is around as he loves it more than I do.
I have toyed with the idea of having a separate Squash bed off by it's self but I have not found a distance that does not allow the demon bugs to easily migrate and those nasty things will migrate to about any plant they can find once their preferred victim is dead. If you do not have Squash to attract them however they usually do not show up to begin with.
Basically in your quest to find what type of Garden is truly sustainable for you it is going to be trial and error in a lot of ways. All I can really do is explain what and why I reached the results I have reached and why I abandoned ways or types of gardens I tried in the past.
Bindweeds, Morning Glory, Rhizome grasses. These were the reasons I abandoned the Raised Beds and Eden type gardens. The Eden types were the absolute worst at controlling these weeds and once they were established the garden spot was useless for years unless you wanted to dig up all the wood chips as tilling them in made things worse. Raised Beds are a bit easier as you can dis-assemble them to remove the rhizomes from time to time.
I should add that my original Eden type garden is still useful to this day however despite being abandoned for 15 years or more. The Chickens LOVE IT!!! I often use the area that is contained with railroad ties to also stage round bales on. The wood chips broke down into a find dry dirt that still gets a lot of rhizome weeds in it but I also keep it mowed if open to control them. The chickens spend every day roaming in and out of hay bales and dirt bathing in this dirt, digging shallow holes to lay in etc. They think it is the biggest chicken play and bath yard in the world. I can go out to feed the chickens old bread or whatever and call em and the entire flock will come running from this area like a pack of ravenous dogs.
What everyone calls "The Ruth Stout Method" today, which I have been practicing for years myself and never heard of her before last year maybe, can be adapted against these rhizome weeds easier but will still require a period where you cannot use the area until the weeds are controlled. The best way I have found to control these weeds in what I am going to call a "Deep Mulch Garden" is by fencing off an area in the middle of a highly grazed pasture. Small Ruminants will graze almost all Rhizome weeds (except Thistle and Buffalo Burr) to the ground and eventually kill it off. By isolating a garden area inside a highly grazed pasture most of these Rhizome weeds are destroyed and unable to invade the garden itself. The constant grazing will kill these weeds off at the source eventually, especially in drought and extremely dry years. Occasionally one of these rhizome weed types can get started inside the fenced garden area and require a bit of work to remove but generally speaking the small ruminants will keep these weeds controlled with little to no managing personally. Sheep and/or Goats both seem to love eating these weed types.
Deep Mulch Gardening - I eventually moved to this overall method but only because I have an unlimited supply of hay even if I have to scythe it down by hand every year. It will always be there. Short of a war waging over my fields anyway. I guess I could add Wheat to the issue if I wanted to use Straw as the mulch but this is a good example of using what I have easily available and sustainable. Whatever is available is what ya want to use Hay, Straw, Leaves etc. as you will more than likely be unable to buy it in a grid down situation.
Seeds - I rarely buy seeds anymore. Or starter plants. I have over the years saved what works and all of it has been cross bred some I am sure. It has required me to make some sacrifices especially in the variety area. I no longer plant different types of Beans for instance as they would cross breed of course. Cucumbers and Watermelon are the same along with a few others. Interestingly enough Tomatoes are something I have little to no problems with cross breeding with unless I add in some type of cherry variety or plant them very close to each other. Even 20 or 30 feet with a trellis barrier of Beans seems to stop cross breeding with Tomatoes although I think the official distance is 100 feet or more.
Basically to have a sustainable garden you need to find what works and keep it pure or risk ruining or losing it. Kinda a heretical view for modern day Multi-Cult freaks that think diversity is a strength or something but there it is. Your only option, which might be a good idea for very long term, is to have a second area for experimenting with new varieties.
Ground or fabric covers - I do use these from time to time when I open up a new area or have to control a particularly hardy weed type. I also use a ground cover for walk paths. Since fabric covers and light plastic are not sustainable, even if the fabric says it will last 20 years or more wear and tear will kill it fast regardless. What I have found was a trick I actually learned 40 or more years ago from an old gardener who lived next to my grandparents. Old roofing sheets. I have a stack of these old ugly sheets that is almost 4 foot high when all together and have been using them for years. Some are filled with nail sized holes but that does not hurt any I just cover them with mulch. It will take a lifetime or longer for these things to break down and the old style galvanization types seems to have no ill effect for the soil either that I have witnessed. No weeds can penetrate them of course and they even work well early on when rains turn your garden to mud you cannot walk on.
Honestly I consider these old roofing sheets as the heart of my sustainable gardening infrastructure. Every second or third year I rotate my set up by removing these sheets adding compost and barn/fertilizer then setting everything back up starting with the main access pathways of roofing sheets in the new configuration. After the mulch layer you cannot even tell those sheets are in the Garden but it has saved me so much work over the years and money they are now essential to the entire process.
Another after the fact add in - If you cannot find very old roofing sheets then you are probably out of luck as I imagine only the very old steel ones will be sturdy enough to use for years like I have. I was blessed in that my 100+ year old barn had been re-roofed a few times and someone never hauled off the old sheets. About half the sheets I use now I actually found buried and dug them out myself. Still usable. I cannot imagine modern day sheet roofing to be sturdy enough for my gardening anyway.
Flexibility - Some of these methods just don't work well for some type of plants. I will say I had never considered using the mulch as a hill material for potatoes until I was introduced to it this year. Now that I am trying it of course I had to do it with out any ground cover besides the mulch. I have also not found a good method for growing Carrots without weeds and problems except in a raised bed type situation.
I am currently designing a carrot only moveable raised bed type thing that can double as a garden fence. The idea is I can then place the carrot beds around the garden and reduce my chicken invasions a bit. The chickens don't do a lot of damage but sometimes annoy me digging through my mulch.
The last issue that comes to mind, although I am sure I missed a bunch, is Corn. Basically I no longer include corn in my garden. What I do is choose an area each year and cover it with round bales although anything will work. Basically I kill the grass/weeds over a year and then till that area up in the Spring (sometimes plow it first) and just plant a corn plot. This year's plot is destroyed of course due to the drought. I could have saved it I think but just didn't bother but that is just me. If I wanted the extra work I could incorporate corn into the garden proper but usually the space issue makes it much easier to just rotate it around to a different location out somewhere and let it go.
I should also address fertilizer. I have found the mulch I use each year breaks down and basically provides it's own composted fertilizer of course. If I have a mineral deficiency in my soil I have yet to discover it. I do compost on the side but end up never really putting any of it on the garden except in whatever I use to start seedlings. I have never analyzed my soil, hell I don't even know what PH my garden is from year to year. I tried checking it myself years back but I am not even sure I was doing it right. Using a deep mulch pretty much takes care of soil amending. Occasionally I will clean out the sheep part of the barn and instead of taking the manure spreader out to the fields I will dump some on the garden instead to add Sheep Manure to the whole mix.
As I say alot of what I do will be different for you as your circumstances will vary according to location.
Keep Prepping Everyone!!!!