How to quickly and cheaply convert a vegetable garden and two polytunnels into zero maintenance garden space?

The organisation who owns the land hosting the community garden I’ve been looking after for the last few years will be denying access to the garden starting in January. It’s all rather complicated and upsetting but it’s a government organisation who own the land so there you go. The location is up north, near the coast. So it rarely freezes or snows but it can get windy and stormy.

Rather than just walk away and let weeds take over, I’m looking for ways to transform the space into a zero maintenance garden space for the benefit of the local wildlife. The garden area is mostly a big outdoor area partially shaded by an old oak tree and surrounded by hedges. It’s currently planted in potatoes so I’m sure they’ll just take over. There are some well established raspberries which have been spreading out so I’m sure they’ll continue their domination of those areas. There are a few small raised beds that I’ll be throwing some wildflower seeds into which will hopefully self-seed in the summer.

But the big puzzle is the polytunnels. I’ve already planted herb plants around the internal edges of the big polytunnel as the frequent rainfall soaks the ground around the edges of the polytunnel so those herbs will survive and probably do quite well. However, the centre of the polytunnels will dry out. Is there anything I can sow or do with that centre area of the polytunnel? Perhaps some extremely tough plants? I expect the polytunnel doors will be left open and the polytunnels themselves will be reduced to rags within a year or two.

Do you have any ideas on what I can do to create a wild zero maintenance garden space for the local flora and fauna?