Green Ice becomes green slush: My mini-greenhouse fails

My cheap mini-greenhouse after two winters’ use

The mini-greenhouse I made by cutting the bottom from a plastic storage box failed during recent frigid temperatures.

I picked lettuce from the mini-greenhouse every month during the 2011-2012 winter—one of the warmest on record here in central Maryland, where the lowest temperature recorded on our electronic thermometer was 18°F.  Frost often formed on the condensation on the inside of the box, but there was enough warmth from soil and sun for the lettuce in the box to live and grow.

Last harvest: Green Ice and Red Velvet lettuce

But not this winter.  When the forecast for the night of January 21 called for the mid-teens, I harvested most of the Green Ice and Red Velvet lettuce in the mini-greenhouse.  I left just three plants to be exposed to the lower temperatures.  Nighttime temperatures dipped to the mid-teens and lower that week.  Our thermometer registered 15°F  on the morning of January 22 and 11°F on January 23, and daytime temperatures for several days never climbed out of the 20s.

But temperatures recently changed and became springlike, topping out in the low 70s on January 30.  Ice drops on the inside of the mini-greenhouse became liquid again.  It was time to see how the Green Ice and Red Velvet had fared during the deep freeze.

All three plants were alive but frost damaged.  The crown of the Green Ice plant looked freezer burned, and the outer leaves looked, well, like green slush.  Outer leaves of the two small Red Velvet plants were limp.  Optimistic I proclaimed, “Alive!”  Pessimistic I replied, “But not edible.”

Jeanine S.’s even simpler mini-greenhouse,
bottom of a storage container turned upside down

Jeanine S., a Master Gardener in Harford County (Maryland), reports similar results with an even simpler mini-greenhouse, just the bottom of a storage container turned upside down over her lettuce.

“I harvested lettuce at Thanksgiving, Christmas, and again in January during Winter 2011-2012,” she explained. “This year I harvested through Christmas, but most of what was left after last week’s deep freeze has been badly ‘freezer burned.’ Some it may pull through, but it will probably taste bitter. My mini-greenhouse worked well with arugula, Butter Crunch, Deer’s Tongue, Red Sails, and Ithaca, as well as a ‘mixed’ variety.”

What have I learned from these mini-greenhouse experiments?

First, simple protection is enough to extend the harvest season of so-called “cool weather” crops, such as lettuce.  The mini-greenhouses provide enough protection for lettuce to survive down to 18°F, but somwhere between that temperature and 11°F, the freeze severely damages the lettuce.

Second, time and weather have taken their toll on the cheap, plastic storage box that I’ve used as the mini-greenhouse.  I’ve had to reinforce slowly cracking corners with duct tape.  I probably will try to use the box again next spring, but at some point I expect a corner to break off and I’ll add the mini-greenhouse to our recycling bin.  At $13.67, though, the mini-greenhouse didn’t break our budget—and provided us monthly lettuce harvests over nearly two winters.

Why don’t you create your own mini-greenhouse late next month as the weather begins to warm and plant an early crop of spring lettuce—or next October when early frost threatens?

Nothing ventured, nothing learned, nothing harvested.  Let the fun begin.

If you want to read more about my mini-greenhouse experiment, click on the links to some of my earlier postings:  Experiment begins, September 2011.  Early tips, January 2012. Cutting lettuce, March 2012. 

3 Comments on “Green Ice becomes green slush: My mini-greenhouse fails

  1. How about a mini hoop house covered in clear plastic over the mini green house when it goes below 20 or so? The double strategy works for Eliot Coleman in Maine using his winter harvest techniques. The main difference would be volume of warmed air.



  2. Yes, that certainly would improve chances, and it probably would be better than tossing an old blanket over the box on the coldest nights,as a friend has suggested. I've resisted such additions box so I could see just how low the temperatures had to go for the box to fail.


  3. Maybe if you put the box next to your house, the radiant heat will be just enough to keep it from going to slushy mush…
    By the way I am going to be growing green ice this year, how is the taste


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