Wasilla Alaska Garden Adventures

Wasilla Alaska Garden Adventures - learning about gardening up north.

Sunday, April 17, 2011

Greenhouse disaster - another learning experience.

Gardening in Alaska is an interesting adventure... but pushing the limits of nature is fraught with challenges. We have plenty of daylight here in Wasilla for plant growth by late March with 12 hours of sun on the spring equinox and over 20 hours of sun on the summer solstice. Even with our abundance of sunlight by late March we still have 2 months of subfreezing nights to deal with. Most serious gardeners opt for putting up some sort of a greenhouse to take advantage of the sun and it's associated solar heating property. Adding heat to a greenhouse, in some manner, is still necessary whenever the sun does not shine, primarily at night, but also on some cloudy days. I opted for adding a 10,000 BTU direct vent Empire brand wall heater model DV-210-7SG and the unit has been working fine so far.

This is the smallest unit that Empire makes and fit within my budget. This is all a learning process for me and every project that I undertake is an experiment. I am learning that my greenhouse is quick to warm up whenever there is direct sun - but it is also just as quick to cool down when the sun sets at night. As I learned with my simple hoop greenhouse - a single layer of plastic film has no insulating value. There is some advantage to blocking the wind - but not on a cold, clear, still night. Double pane polycarbonate panels would be preferred - but the price is outside my budget for now. I used my spare gas grill 20 lb propane tank to provide the fuel source and last week was the first time I used the heater. I was anxious to move my tomato seedlings outside as they had outgrown the plant rack and desperately needed more growing space. I was interested to learn what my fuel consumption would be so I could do simple cost analysis for starting plants early.

I tried to estimate my potential fuel consumption: A 20 lb LP tank holds 4 Gals of fuel. One Gal of LP contains approximately  91,000 BTU of potential energy. My heater produces 10,000 BTU per hour. 91,000 BTU X 4 Gals = 364,000 BTU divided by 10,000 BTU per hour = 36.4 hours of heat with a full tank of gas. The  question for me was: How many hours would my heater run each night and how fast would I use up my fuel?

The first 2 nights I used a small electric room heater as an additional heat source and for a failure backup. My temperature monitor showed more than adequate heat and was able to keep the greenhouse 20 degrees warmer then outdoors. The overnight low was recorded at 27 degrees F for 4 hours and the greenhouse never dropped below 45 F. The third night I turned the electric heater off and with an outside low of 22 degrees F for 4 hours the propane heater was able to keep the greenhouse 10 degrees warmer with a low of 33 degrees. This is just above freezing - and some plants showed the cold stress. On Friday I checked the gage on my tank and it was reading in the low range - but not quite empty yet. I believed I could get by for one more night - and my fatal mistake was not connecting the electric backup heater.

At some point early on Saturday morning my tank ran out of fuel with an overnight low temperature of 19 degrees F and was below freezing for 11 hours. Not good for tomato plants. As you can expect the result was total disaster.

Lessons learned: Always have an adequate fuel supply and a backup system in case of failure. A low temperature alarm is a possible solution whenever I am home - but I work the night shift on 3 nights each week. When home I could get up and connect the backup heater whenever the alarm is activated. Electric heaters are expensive to run and I want to only use one as a last resort. Maybe some sort of a failure thermostat could activate the electric heater only when the propane heater fails.

I immediately planted replacement tomatoes on Saturday but it may be too late for this year. I am always able to buy replacement plants - but probably not the varieties that I wanted to try.

As always - you live and learn from your mistakes... but I will never give up. Gardening in Alaska is an adventure!


  1. I was wondering how the global bucket system worked for you last year. I look at the links in your post last year and thought it was a great idea. I am thinking of trying it. Thanks for the great blog.

  2. Roy - write to me direct if you want more info on grow-buckets. I will post an update here soon as I am making some changes. jlister at alaska dot com

  3. Hey buddy,
    I like to see your preparation to make greenhouse. Hope you will do your best.