This spring with my introductory plant science class, we tested out milk jug greenhouses. We are still getting the data analysis completed, but I'm very pleased with the results. While I can't confirm that the results were statistically significant, I can tell you that the germination rates with our milk jug greenhouses were about equal to the germination rates in the commercial greenhouse. These cheap little greenhouses work!
The idea with the milk jug greenhouses is that they are a very cheap and easy way to get your seeds started earlier than you might be able to start them in the ground. I've seen some people make the milk jugs into a sort of plant cover to protect the plant. But by using the whole milk jug, if you have a cold snap, you can bring the whole thing inside. It is cheap, easy and effective in starting your own seeds and hopefully getting an extended growing season with your garden.
Here is how to do it.
Save a milk jug, clean it out. You won't need the cap so that can be recycled immediately.
Make drainage holes in the bottom. My husband used a knife but in class we were safer and used a drill. Four drainage holes worked great for our class project.
Cut the jug horizontally almost all the way around. Where the handle is, leave a couple inches un cut. The end result should be the top half of the jug being 'hinged' to the bottom half.
Add potting soil and start your seeds. We have started beans, peas, tomatoes, squash and cucumbers all with good success. For each we have just planted four seeds per jug.
Close the hinge and tape the jug to seal the cut. We have the best luck with duct tape, shipping tape was a bit too flimsy. Make sure the cap is removed and do not seal where the cap had been. The open cap allows for ventilation.
Set out in the sun and enjoy! We did water them as needed, but if you get enough rainfall you may not need to water much at all. If there is risk of frost or you have a cold snap, bring them inside. You could set them in front of a window and take the back out when the conditions improve.
In 2-4 weeks when your starts are looking great, un-tape the jug and transplant!
I was very pleased with how successful the germination was in the jugs when compared to a commerical greenhouse. It was very inexpensive to try out and you could reuse the milk jugs year after year. We will be repeating this experiment in class with different seeds next spring and I will be using this in my personal garden as well.
Note: This was not an idea I came up with, but one that I came across online. Whomever originated this idea is brilliant.
I'll update you when the data is complete!