After a bit of research, if I had the light on for 30 days at 12 hours a day, it would be 360 hours a month at 400 watts which would be equivalent to around 160 kilowatt hours. Given that the electricity rate is around 0.11, it should equal around $17.60 a month. That's not all that much especially if I don't even plan on having it turned on whenever there is a warm stretch. For example, all of last winter would have probably never required any additional heating for the greenhouse.

Now I just need a 10 guage 200ft extension cord since my metal halide's cord can't stretch that far which seems to cost the most out of anything. It will probably cost the most just for the cord because I don't want to burn out the 16 guage regular cords that I have.


As far as heating, I think it really depends on the insulation, size of the greenhouse, and the overall need. It seems like if a little paraffin heater is capable of defrosting my greenhouse, then a 400 watt metal halide should be more than enough.

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Steve

Just talked to our lighting man.

Go to: http://www.pllight.com

You will need to know the wattage you intend to use, the voltage, the phase (I presume single phase for homeowners) and the type (sodium for supplemental to some exposure to natural daylight or metal halide if no exposure to natural daylight). If you email PL they will give you an accurate consumption rate.

The newest lights are placed amongst the plants not overhead and their heat loss gets factored into the ghouse's heating requirements. A big tomato g'house here just installed them - very expensive and required 3km of copper wiring.

johnw

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johnw coastal Nova Scotia

Solar heaters if you make them your self with out all the frivalous do-dads commercial ones have are not that expensive. The one I am makeing to use for my house water heater using copper pipeing will be around $200. If I used the same for my 60' green house I estimate it to come in under 500. I would use 55 gallon drums for the heat sink.
The vacume sealed tubes can be replace with a totally sealed frame with double glazing(glass). Oh and every thing out side needs to be well insulated.
Oh no pumps required the heat difference will do the water flow.
MarCat

I'm using single phase 110 volt, 400 watt metal halide 4200 white light, and it will likely be a size 10 extension cord that goes 200ft long. At 0.11 cents per kilowatt hour, I really can't imagine it costing any more than $20 a month if I used it every day for 12 hours which is very unlikely as it is meant as supplemental light as well as an emergency heating system. The one reason why I don't really need a large heating system is because my greenhouse is only about 436 cubic feet, and the heat sink will be at least 110 gallons. After installing all the insulation, the set up might not even get that much below freezing without an external source of energy, especially if I really load it up with tons of plants.

If I'm trying to help these potted plants + the bicolor out a bit through the winter, I think the biggest limiting factor might end up being the near freezing temperature of the soil which might kill the photosynthesis, even if the air temperature happens to be up to something like 60F. I could install heating cables, but I don't think they will do much if the starting temperature happens to be as low as 40F. The best thing I can do is have rotting manure in the greenhouse. Without using any excess heating, I am expecting the average winter variation of temperatures to be around 26F to 44F, so hopefully using the metal halide at night can keep the entire set up above freezing.

One of the threats of using these polytunnel greenhouses is that if there are 70mph winds when it is extremely cold outside, there's no guarantee that the frame will hold up since it is only aluminum. I had a greenhouse like this one crumble in the spring of 2011 when 70mph+ winds hit as the . That's why I plan to tie it down very well with tent stakes, and reinforce all the probable stress points in the frame to make it safe to get through winter.

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Just a rough guess but your greenhouse is about the same size as mine. With the gas thermostat at 5c and the electric back-up at 4c I still rack up a $500 bill for the year, maybe a bit less as with an October fill I frustratingly need another fill in early March though the sytem is usually shut off in early April. Last winter I only went through 1 tankful.

johnw

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johnw coastal Nova Scotia

John,

How warm do you usually keep the greenhouse over the winter? I'm guessing that having it range from 45-55F would be enough for pretty much all plants, especially the ones that require a cool cycle. I plan on insulating very well, and using a minimal amount of intervention so I don't mind letting my greenhouse hang around the 32-44F range all winter if it ends up being brutally cold, but with the 400 watt metal halide, I doubt it will get even close to freezing. I think it gives off more heat than my paraffin/ kerosene lamp. Of course it sometimes gets into the 60s in January which will make the greenhouse exceed 80F once in a while in the middle of winter, but that's when I won't use any intervention.

Another thing I need to decide is when to actually move all the plants in the greenhouse. I already know that when day time highs get into the 80s, it can rise up to 110F or more, and my current greenhouse has a door, but no air vents. When it cools down to the 45-65F range, then it might be safe to move in all the plants in. By then, temperature will likely become the limiting factor in photosynthesis.

In case anyone was wondering, this is the greenhouse model that I'm using.
http://www.ebay.com/itm/Outdoor-10x6-6x ... 43a928118d

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Steve - As mentioned the thermostats are set at 5c. We generally move frost tender stuff inside in late October and other stuff in by early December. By then we still open or crack the window or leave the door ajar by day to prevent over-heating on mild days.

We only have one window but things are fine with door and window shut until early February when the sun can heat up the greenhouse yet it's still too cold outside to leave the window opened. It gets even crazier in March when it can be sunny and bitterly cold one minute and cloudy or foggy the next and so it's luck of the draw when you leave for work. A preventative spray for botryitis is wise in late Nov. and again in mid January.

In my next life I will have automatic vents, even the wax cylinder types which might even crack the door for you. Probably placing a cinder block on the ground by the door to prevent a gust of wind from tearing the door wide open and/or severing the vent arm might be a wise decision.

So here managing the ghouse is not as bad as when the plants come out in April as some are too far developed to take even a slight brush with frost. As a result plants go in and out frequently.

johnw

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johnw coastal Nova Scotia

If your thermostat is only set at 5C, and you still need to use $500 or fuel, then we probably have a pretty big gap as far as the average winter temperatures despite having similar winter extremes. We tend to average -8C for lows, and 2C for highs for an average winter, but it has way warmer than that last winter.

I plan on positioning my door on the east side to prevent the gusts of wind from tearing open the door, and the entire setup will be tarped for a double layer insulation if we happen to get a few week stretch where temperatures remain below freezing. That's when the metal halide gets turned on.


Mine will probably go up in October as well depending on the weather. I've always taken down the greenhouse in either late February or the first week of March, but I think it would be fine to keep it up until April, or maybe even May as long as it doesn't get to hot in there. With insulation alone, it is about a zone 8 in my greenhouse, but on the coldest nights, I like to heat it up just a bit in order to prevent the pots from freezing.

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Steve - My guess is propane gas is much more expensive here too. I only went through 1 tank of gas last year and of course I forgot that includes heating the well-insulated main floor of the barn even though the halides help it greatly.

Here you can see the effect of the cold Labrador Current on March to late May weather here even though we are Z6a on the coast.

January
Daily Max -2c
Daily Min. -9.2c

February
Daily Max -2c
Daily Min. -8.8c

March
Daily Max +3.2c
Daily Min. -4.8c

April
Daily Max +8.1c
Daily Min. +.2c

May
Daily Max +13.6c
Daily Min. +4.7c

Spring is a very long event here as that current saves us from wild temp swings at that time of year even though most people curse it.

You will have to be right on top of things in March and April with your ventilating.

I always say the Zones don't say so much. Would it be possible to still be in Z6 if the temp at a constant -5c year round?

johnw - incredible rains and thunder last night for 2 hours - coming down at 50mm/hr. at one point.

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johnw coastal Nova Scotia

John,

Your average winter temperatures are only about 2C cooler than they are here so the difference shouldn't be that big. One advantage I see in using your propane heater is that you are providing your plants with additional CO2. The optimal level of CO2 is 1500ppm while the normal CO2 in the air is around 400ppm.

I know I over-stuff my greenhouse so the CO2 concentrations are probably well below optimal however I try to offset that as much as I can by having lots of compost, manure, and rotting plant materials inside of the greenhouse.


If the LT forecasters are right about this winter, the average temperatures should be well above normal which is good for the heating costs, but have you ever considered dropping the thermostat to around 2C instead of 5C? I've had it get down to nearly -10C inside the greenhouse before turning on the paraffin heater which successfully brought the temperature back to the -3C range, and even my potted bamboos looked fine. Unless you have tropical plants, you might not need the greenhouse to be above freezing by that much.

-10C happened when we had the record cold night of -22C, and this was with the single layer greenhouse. I believe that a double layer greenhouse could make a difference of a few degrees while the metal halide might even move the temperature above freezing if we ever have a night that gets that cold.

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Steve - +2c, given a margin of error, is too close to the freezing point to take a chance. I grow lots of hardy things in there but by mid Decemeber some like the bulbs are starting to grow and cannot take a freeze. South African and Chilean bulbs, vireya rhodos, cyclamen and the like simply cannot take frozen pots. Folks in the UK found out the hard way in the past two winters, the damage wrought freezing temps in greenhouses full of what were considered super hardy bulbs such as snowdrops which can take extreme lows in the ground but not in pots was depressingly common. I'm sure Markj can confirm the horrors his pals had, even among alpine plant growers.

I was at a nursery yesterday in the Annapolis Valley, dropped off a Sasa plamata and came home with a gratis Phyllostachys nigra to test. There was no cultivar name so it may be years before we can figure out which one it is. All the nigras had sent rhizomes out the drainage holes and through the plastic they were sitting on. The rhizomes were pinkish under the plastic with wonderful fresh pink roots which unfortunately dried out on the way home.

johnw

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johnw coastal Nova Scotia

Steve, I see figures like this on various gardening forums but when I look at the scientific literature a very different picture emerges. Here are some of the things I gathered from reading them.

1) CO2 is supplimented in green houses because of the air circulation is not enough to replenish the CO2 used up by the plants.

2) There is a point of diminishing return as CO2 levels are increased above about 340 ppm.

3) Increasing CO2 level increases the production of starches and sugars, but not nutrients.

4) At 1000 ppm we start getting into health effects on sensitive individuals.

5) Plant growth is limited by whatever is in shortest supply, very seldom is this CO2.

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My Bamboo List.

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There are actually product out on the market specifically for greenhouses in order to increase the CO2, but even if they do manage to raise the CO2 ppm to above 1000 which I highly doubt, I don't think the temperatures are warm enough over winter to make the CO2 useful. The sunlight is not all that strong in December, and the 400 watt metal halide is still very weak in comparison to full sun in the middle of the day. My camera's manual settings can judge the brightness very accurately. I haven't looked at many commercial greenhouses, but I've don't really hear about people using CO2 generators to speed up the growth of their plants.


I plan on installing the greenhouse as soon as I think it will benefit my plants. The forecast is still calling for fairly warm temperatures up until around October 7th when daytime highs start dropping into the 60s. Until then, it's still fine to set up the frame. I feel that the rain water and natural conditions are important to a certain extent. The big question is whether or not I should leave the greenhouse on longer and let all the bamboos shoot inside of the greenhouse before removing it.

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I finally decided to put up the greenhouse and I noticed that the temperatures in there rose almost right away. Now that the sun is down it is still a nice warm 61.7F in there vs the 53.2F outside. I have the greenhouse over the moso bicolor along with many of my taller potted bamboos. As far as buying the poly tunnels, one thing I've found out is not to get the ones with the 6 air vents because that really defeats the purpose of trying to have good insulation.

I'm expecting the temperatures to hang around 50-100F for the next couple months in there so everything inside should have a nice burst of growth. I haven't tried the metal halide in there yet, but I find it almost pointless to use when night time temperatures are nowhere close to freezing, and the daylight is still pretty strong.


One thing I really want to try is leaving all the plants in there through shooting season, and just try leaving the greenhouse on all summer long to see if the plants are OK with extreme heat + humidity at the expense of a 25% sunlight reduction.

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Steve - Surely you'll have to give good ventilation. One of the drawbacks under plastic is the proliferation of spider mites, mealy bugs and thrips. We have access to a warm g'house but have to get the plants out in late April just so the bug problem doesn't get to the point of no return. As you know I put the bareroot potted rhizomes in there for a month and got them out just in the nick of time,

You should have some spectacular results just with plastic on for the winter and early spring and then in late fall. Just be careful those vigorous shoots don't rorar through the roof when you turn your back!.be interesting to see your results against those in the garden.

johnw

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