QUESTION (from the comments here): Can you post more about the origin of your new blog title? I looked it up and found some references that said when someone's "working on their back 40" or whatever, it meant they were up to no good. I'd never heard the term before you used it.
ANSWER: I could, but I like to be ambiguous whenever possible. Not sure if you've noticed that. It's a style choice.
Anyhow, to answer your question, my back 40 is an old expression, maybe from homesteading days ("40 acres and a mule"?), and it refers to the land behind the ranch house--the 40 acres you were supposed to make productive, but maybe you didn't.
Saying "I'm plowing the back 40" is telling someone that you're off doing something that you're supposed to be doing--while maybe you aren't. You're off somewhere out in the back 40, so who's to say what you're doing. (Saying someone is "off plowing the back 40" usually means s/he's having a tryst.)
Forty acres is a lot of land. Once upon a time, that was a reasonable amount of land for homesteading, but in the modern world it's excessive to most people. People wouldn't know what to do with 40 acres today, it would lay fallow, be wild, get overgrown. You can refer to the parts of your yard you don't get to as "your back 40".
I'm being ironic. I live in the city, not the country. I don't have 40 acres. (In reality, I don't even have 40 feet, it's more like 25.) I'm letting you know this is a blog about a small garden, and I'm being cheeky about it. In my garden, even though it's small, I want there to be some wild, some overgrown. Some stuff that I don't get to. I'm echoing that in my blog title.
I also blog about things I'm doing in my neighborhood, or in San Francisco, or in the Bay Area. Everything (almost everything) I blog about is going on quite close to home. So I'm using the expression metaphorically too. I'm saying to everyone, "All this is going on in my back 40."
Am I being productive? Is this what I should be doing with my time? Am I up to no good? That's for you to decide.
You can see why I don't want to have to explain things. I know 50 people come by every day to see the blog, and that's nice. But, really, I do this for me.