Like John with his Ferrari and Dwight with his family, Peter and Mary Holmes use their garden to distract themselves from the coming end of the world. Although they won't live long enough to see their work come to fruition, they pour their hearts and souls into this tiny plot of land.
Of course, gardens have some inherent symbolism of their own. For obvious reasons, they represent growth and reproduction, two things that contrast sharply with nuclear annihilation. This symbolic meaning is amplified by the fact that Peter and Mary are a young married couple with a brand new baby girl. Because of this, their garden represents their dashed hopes for a life together.
What's more important, however, is what the garden means to Peter and Mary. It's actually pretty simple—working on the garden helps them forget that the world is about to end. Just think back to that morning when they "went on happily planning their garden for the next ten years," which finally alleviated the tension between them (4.36). Although they know deep down that they'll never see their garden grow, they'll gladly spend a few hours pretending this is not so.