This ties in nicely with growing a garden. A greenhouse allows you to get a jump on spring and start seedlings ahead of the last frost, assuming the nuclear winters don't freeze us all to death first. Greenhouses can be built with simple supplies and designs or constructed as master-built houses incorporated into $20 million estates. A greenhouse gives you more control over the environment your plants are grown in, and it's a nice backup if resurrected dinosaurs take out your outside garden. Greenhouses are also easier to defend, which is good when you have to repel the sun-hating, virus-carrying monster-human hybrids.