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If you have space I think it is very important you have a greenhouse. If SHTF or you get bad weather for weeks at a time this could affect your vegetables in a bad way.
Greenhouses produce a better yield as the temperatures and humidity are more stable. Your food actually LOVES greenhouses. I found a website that has 13 cheap DIY greenhouse plans for you to look at and build.
My favorite looking one is nice enough that it could double as a guest house. The cost is higher, but a building like this would make a beautiful asset to most properties, and it could be built out of recycled materials to save money.
Image Credit: offgridworld.com