Some Greenhouse Observations

School cancelled today on account of the spring blizzard. This gives me a chance to closely observe what is happening with the greenhouse throught the day, in conditions similar to what I might see in January if I try to keep it going through the winter.

Last fall the greenhouse froze up in late October or early November. I should have logged that here. Oops!

Probably I could have planted in the greenhouse in early February, but I just didn't have the time or energy to clean out the mess I'd left there last fall. But last week I was home for spring break and once I caught up on my sleep I couldn't ignore the greenhouse any longer.

I put in tomatoes and basil, started from seed in January. I also planted 2 short rows (2ft) of lettuce, a short row of radishes, and 4 short rows of carrots. I also noticed that the Walmart was selling spinach and Romaine lettuce starts in packs of 9 for $3.50, so I couldn't resist planting a 6ft row of each.

Later in the week I observed that cold weather was in the forecast. I have gallon milk bottles filled with water around the perimeter of all the greenhouse beds and between each of the planted rows and these do help to moderate the temperature, but there were three consecutive days of cold predicted. I braved the first night of below freezing weather, but the second day was cold and cloudy and the water bottles didn't get enough insolation to warm up so I chickened out and set up a heater.

I found an old electric space heater in the garage. It has a 200, a 300, and a 500 watt setting and an adjustable thermostat. I had to rewire the line cord and plug and clean it up, but once I did, it worked fine. I set it up on March 20 and connected it through a Kill-A-Watt meter so I could see if it was going to bankrupt me.

I set the thermostat through the scientific process of going out to the greenhouse early in the morning, deciding that it was a bit chilly in there and turning up the thermostat until the heater just kicked on. I hung up an old thermometer on one of the tomato cages and made sporadic observations over the next few days, especially first thing in the morning while it was still dark. The thermometer usually read around 45, so I figured that was good enough and I left the whole thing alone until last night. The meter recorded 39.6 kWh as of last night, so the heater used about 6.6 kWh each day. At $.09 per kWh according to my electric bill, works out to about $0.60 a day.

Yesterday afternoon when I realized that we were going to have a real storm, I stopped at Walmart on the way home, and picked up a cheap recording thermometer. The one I bought came from Accurite. It records current temperature as well as min/max for indoors and outdoors. I chose this one because it had a water proof wired probe that is intended to be put through a window to record outside temperature.

I attached the main unit to one of the tomato towers in the greenhouse and buried the probe just beneath the soil at the base of that plant. I finished all of this last night around 9:00 P.M. Inside the greenhouse the small heater was running and the temperature at shoulder level was around 60F. The soil temperature was 55F.

Overnight, the outside temperature dropped to 4F. The low temperature recorded at shoulder level in the greenhouse was 39F. The low soil temperature was 55F.

This morning at 10:00, the inside temperature at shoulder level was 82F. The circulation fan and the heater were both running and the soil temperature remained 55F. The Kill-A-Watt meter read 53.9, so the whole set up required 14 kWh, or about $1.26 worth of electricity to keep it warm during the coldest night I would expect it to see normally.

I don't like the idea of using a lot of electricity to keep the greenhouse warm, but I expect that I will be able to take the heater out by the middle of April. If I can significantly extend the growing season by spending $30 on electricity, I think that is worth the cost.

In future years, I hope to reglaze the greenhouse with double or triple wall polycarbonate, and this will greatly improve the efficiency. But the PC is expensive stuff, I figure close to $1000 to reglaze the whole thing, and truthfully, it doesn't look as nice as the clear glass.

I'll check the temperature about hourly today. I'm interested to see if it gets warm enough inside to activate the solar roof vent, and I want to make sure that the heater is not kicking on when the solar vent is open.