The Greenhouse by Night Is not the Greenhouse of Daylight Hours

Most everybody who has a greenhouse or is contemplating one has a plan for freezing nights: electric or propane heaters, water barrels and solar mass for heat mediation and a back-up plan if power fails or gas supplies run out.

A frequent question on garden forums is how to heat one's greenhouse using sunlight. Unless there's a grid of solar collection panels and a bank of batteries for energy storage and superlative insulation, it isn't going to happen during freezing temperatures. The best affordable practices use supplemental heat and conservation of energy.

What is going to happen on sunny days is heat buildup in the greenhouse. We can store a little heat using containers of water. Brick, stone and concrete in the floor absorbs heat that radiates back when the sun goes down. Even clay pots of soil hold some heat.

At noon here, the outside temperature was 46º and the greenhouse was 78.º
I did the obvious thing and opened the doors.

If you want to dig further into the science of heating and cooling a greenhouse, you could search for words like enthalpy, convection, mass transfer, phase change materials and radiation. I tend to garden indoors with a minimum of sophistication.

I've experimented a little with water jugs in front of the heaters -- we use 2 electric heaters set on low. (Electric space heaters generally all use the same amount of wattage, 1500 watts when placed on high, and 750 watts when using the space heater on low. We use separate circuits for each heater. Running them on low prolongs the life of the wiring.) 

My theory is that if the fans blow directly on gallon jugs of water, some energy exchange goes on there instead of all the heat rising directly to the ceiling. If the water heats up a bit, then it is more slowly released, delaying the time the heaters will come on again. I have not been inspired to go out there and time all this, it's still a theory unproven but it makes me happy to think that it might mediate the heat loss just a little. I put several jugs about 3 feet in front of the heaters.

I also undertook a sunlight trial with capping some jugs, leaving the tops of others open and covering a few with a black trash bag. Interesting to me is that the water does not heat up appreciably except where the sun shines directly on the jug but the air space in the top of the jug does get very warm. I may try half full jugs and other air spaces to see how that works, considering air as a fluid.

Resources:

Ventilation Maintenance by way of Rutgers University

Greenhouse Heat Info from Auburn University

Low Tech Tips for Greenhouse Cooling 

Humidity and Temperature via U Mass 

Do what works and uses the least energy. I have not addressed using free or inexpensive materials like bubble wrap to insulate north walls and polyester fleece to protect sensitive individual plants.

A Hot Greenhouse in July

This is the first summer we've left plants in the greenhouse. Installing a fogger with three nozzles has made the inside more tolerable for some plants, especially tropicals.

Eggplants like hot weather.

Tomato Plant grown from a sucker, blooming.

Cool nights around 65-70 degrees are necessary

to set tomato fruit. 

Staghorn fern. Bottom left are amaryllis from seeds.

Bromeliads are outside in shade for the summer.

This rose is a cutting stuck back in the spring. What fun to bring in a bloom!

As I am typing this post, the outside temperature is 95 degrees F with humidity of 16%. In the greenhouse it's 93F with humidity of 43%.

I start the fogger mornings when the humidity falls to about 40%. I turn it off when the sun gets so low in the west that the greenhouse is in shade.

We run the exhaust fan from early morning until well after dark when the plant leaves have dried.

Plants are placed where they are under the mist, mostly on the floor so that heat rises above them.

On a cloudy day, I turn the fogger on and off according to what seems reasonable.

I look forward to seeing how long we can grow eggplants, tomatoes and peppers when cold weather comes. At some point midwinter, it will not be reasonable to try to keep nighttime temperatures higher than required to keep certain ornamental plants alive. I think that having a way to better regulate humidity will make a difference in winter. Sunny winter days can get pretty hot and dry inside the greenhouse.

Linking to Flaunt Your Flowers at Tootsie Time Tootsie wrote about her greenhouse today, too.

Greenhouse Growing in Subtropical Heat

Can I grow year around in a greenhouse? Triple digit summer temps and occasional freezing winter nights require careful planning. We've made it to July and the Greenhouse still has viable plants.

From lower left: Esperanza that just never got planted out, a

pepper that came back in to experiment with temperatures and

water. Three tomato plants rooted from suckers; a half dozen

amaryllis seedlings, and various others bits.

The difference is the Fogging Nozzles we put in after we installed an exhaust fan.

A closer look at amaryllis seedlings, four in a tray and

two larger in pots toward the back, seeds from my

Christmas amaryllis. I floated them in water until

they formed roots, then potted them.

The purple stuff is alternanthera self seeded, and the chartreuse is

alternanthera seedlings that haven't found a home. There

were a multitude of green cuttings and there are still red, too,

looking for homes as I think of it.

There's a bit of ivy that our Church Youth gave out on

Mother's Day. The bigger, paler ivy leaves are new.

Canna that was accidentally dug up.

Gerbera Daisy seedlings,

seeds from an earlier blossom

Rose cutting that has taken off.

I failed to label it, will wait for bloom.

I paid two dollars for this staghorn back

in late winter. It has grown well.

The jugs hold water for quick grabbing in

the heat of the day for plants outside.

Eventually everything will probably go outside, but they've thrived with abundant watering and mediation of temperatures.

Everything is bunched up on the floor under

where the nozzles spray on hot days.

Sometimes it is 10 degrees cooler on the floor.

Note the 'patch' where the dog tore the blue plastic.

That will have a different skirt when winter comes.

The shelves hold pots that await winter.

I made lists of cuttings to take in late summer and even wrote down things that I will not repeat this year. I can hardly wait to see what difference having the fogger makes on sunny winter days when it heats up inside. Keeping the humidity up in winter can be a problem. I think we've solved it.

Bloom Day in the Greenhouse, March

Oh, OH! I forgot to put the link to Tootsietime so you could join in the linky fun for Fertilizer Friday. Here it is: Flaunt Your Flowers at Tootsie's.

March in the Greenhouse is about to be moving day in the greenhouse, except for a few of the most tender plants. Gerbera Daisies have been outside for a while, prefering cooler temperatures.



New in the greenhouse is a fogging system built from scrap parts: some copper tubing we had, mist heads used for some other purpose at our old house, a short hose from a hose reel and some purchased fittings to put it all together.

Fogging has a twofold purpose: cooling and humidifying. Cooling is in conjuntion with the fan we put up last week.

Five rooted Marieseii varigata Hydrangeas in one gallon pots over the winter went into the ground today. The biggest number of cuttings waiting now are Pentas in all colors.

Seedlings are coming along:

Tomatoes, peppers and eggplants. One

eggplant has not yet shown himself.

Parsley and chives. Pentas blooms fell into the parsley. Seedling chives are

threadlike. Other seeds planted are flowers, not yet ready for prime time.

Cycad pups have moved outside after spending the winter in the greenhouse.

Two have new growth. I planted out the one that had a single frond and not yet any

new growth. It has good roots.

Flowers and text are from the garden of Nell Jean blogged on Dotty Plants Journal in warm, humid Southwest Georgia.

Big Wind: Ventilating Fan for the Greenhouse

We've had an unusually warm winter. Roof vents on the greenhouse and a portable fan were hardly enough to keep little plants I'm holding from suffering in the heat. He-who-mows installed a vent fan in the end of the greenhouse.

Louvers in place before fan is placed.

We used a fan and shutters that were no longer used for their original purpose. We made a cardboard template to cut the polycarbonate wall and caulked the outside when the louvers were in place. 

 Something to crow about: steel angle reinforments to help strengthen the walls and hold the fan.

Another view of the bracing fastened to the aluminum GH frame.

The fan frame is fastened to the steel bracing.

A broad view. All plants are still not back in place.

All this done, I had to move back in one of my little heaters. Tonight and tomorrow night we expect temps as low as 35F degrees. Daytimes we will need the fan when the sun comes out.

If you need information on Greenhouse Heating, Cooling and Ventilation I recommend sites provided by Extension Services in a state that discusses the zone in which you garden. Their literature addresses figuring capacity, rates of air movement, and other factors to take into consideration with equations for doing so.

Flowers and text are from the garden of Nell Jean blogged on Dotty Plants Journal in hot, humid Southwest Georgia.