I have a two tiered raised bed that happens to be five feet long and four feet wide, each tier being two feet wide on its own. I usually plant tomatoes in the upper level and lettuce in the lower level.
Last year I found someone selling an old window that fit perfectly over one of my 2×5 raised beds and I bought it for $10. Instant cold bed!
Switching things up for 2014, I planted cucumbers, onion, lettuce and spinach in the upper tier in late April and covered the whole lot with the two-pane glass window.
It’s been cool on and off since then but we’ve also had some fairly warm days when the temperature in the improvised cold frame gets rather balmy, encouraging those seeds to germinate long before they might elsewhere in the garden. The result? Hopefully I’ll be eating my lettuce 2-3 weeks before my neighbors.