I’ve seen several articles online this winter about the milk jug greenhouse — starting seeds in a milk jug outside before your last frost date for some winter seed starting. It all sounds so easy!
The idea is that you can start seeds in these heat-holding greenhouse stand-ins outside, getting your garden going while it’s still winter. The uncapped milk jug offers ventilation and built-up snow around the milk jugs act as insulation to keep the seedlings warm.
But does this work? Will the seeds germinate? And will the seedlings survive? It just didn’t seem to make sense. It gets pretty cold here and many seeds need temperatures in the 70s to germinate. I gave it a shot and here’s what I found.
My Milk Jug Greenhouse Experiment
To try it out, I cut a milk jug in half, placed four seed starting pellets planted with romanesco inside and taped it up with duct tape on February 11. I’m in USDA Zone 6a, so it was about three months before our last frost date. Romanesco is a cool-season crop. Should work, shouldn’t it?
I put the jug in a spot that gets some nice afternoon sun, but it was still quite weak in February and March. Just after I put the jug outside, we had a few pretty warm days. The seeds germinated! It was working!
Then the temperatures dropped quite low and we had no snow pack. During a bad cold snap, I brought the jug inside at night. We had a violent windstorm one night, and the jug was blown over.
A little more than a month later, I brought the jug inside to open it and see how the experiment was going. Those seedlings were crisp. I actually couldn’t tell if they burned or froze, but probably froze.
Winter Seed Starting Takeaways
It was just too difficult to see what was going on in the seed environment inside the jugs, since they were duct taped closed. Too dry? Too wet? I couldn’t tell. According to the internet, it looks like the minimum outdoor growing temperature for romanesco is around 40 to 45 degrees. I’m sure temps dropped below that in the milk jug.
If there had been a snow pack around the jugs, it might have helped. I wonder if using three or four inches of seed starting mix instead of the seed starting pellets would have offered a few more inches of insulation, too. Maybe a different crop would have worked better.
So the milk jug greenhouse method didn’t work for me this time. I probably won’t try this method again. It seemed a little too good to be true. I’ll stick with traditional seed starting methods.
Have you successfully tried the milk jug greenhouse? Let me know in the comments or on Twitter @howtonaturechat.