Experimental container garden - critique needed

TrpnBils(6B)May 18, 2014

Thinking ahead to next winter....

I'm a high school science teacher and one of my pet projects is my experimental design class. This year, we experimented on the growing conditions of avocado plants in the school's greenhouse and got some cool results, but we used a grant to cover the cost of propane to heat the greenhouse. Now, having used up that grant, I'm trying to come up with alternatives for heating projects next year since the agriculture department doesn't use the greenhouse for anything during those coldest months of the year. We will not be able to heat the greenhouse this coming year, so this is one of my ideas.... tell me why it won't work or give me any other kind of feedback you'd like!

First, we have three 2x4'x8" boxes used as planters from this year that we can reuse. I would like to set up a circulating pump to move heated water underneath the soil and then cover the bed either with low hoops or something similar to hold in heat.


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For some reason Photobucket's IMG link won't let the picture show up....try this link.

Here is a link that might be useful:

    Bookmark   May 18, 2014 at 10:17PM
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- Interesting idea but what about the cost of heating and circulating the water?

- One thing that I have read some people do is to put a good size compost pile inside a greenhouse, maybe directly under the planting area, and use the heat from the composting process.

There are compost-heat systems of various design and complexity, including Jean Pain's approach if you want to use heated water.

    Bookmark   May 19, 2014 at 1:29AM
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I thought about the costs and, although I don't have any hard numbers yet, I would think it would have to be cheaper.

The pumps run about 60w and a 200w heater (which would not be on constantly) should heat the volume I'm expecting to use. Last year it cost close to $4000 for propane just heating December - March.

    Bookmark   May 19, 2014 at 4:09PM
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Have you seen Caleb Warnock's book on winter gardening? He grows lots of plants in zone 6 through the winter and in the snow. I can foresee lots of experimental design possibilities in various cold frame set ups. Single pane windows vs double pane windows, using compost vs not using compost, types of compost, etcetera.

    Bookmark   May 19, 2014 at 5:34PM
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johns.coastal.patio(USDA 10b, Sunset 24)

I have no idea, but I am greatly amused by the people who calculate the BTU rating of resident chickens and rabbits.

... perhaps you would need too many chickens and rabbits.

    Bookmark   May 19, 2014 at 7:20PM
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I guess my question is whether that setup provides enough heat to keep the soil+air under the low hoops warm.

I took a look at listings for 200 watt aquarium water heaters, and it says that they are designed to keep up to an up to 55 gallon aquarium at a constant temperature like 76 degrees F.

Now, one presumes that the design is for the aquarium to be placed indoors in a more or less temperature controlled room during winter, not outdoors or in a unheated greenhouse. So, I wonder whether a 200W pump can really keep the air+soil+water at a constant, say, 60 degrees when the ambient temperature is near or below freezing. Insulation will be key, but there would appear to be a limit on how much you can insulate such a set up if you are still going to have some degree of circulation between the air within the low hoop setup and the air outside.

Edit: Here is a link to a aquarium heater size/wattage "guide." Seems 200 watt is needed to keep a 50 gallon tank at up to 18 degrees F above room temperature.

Here is a link that might be useful: Heater Guide

This post was edited by DHLCAL on Mon, May 19, 14 at 22:26

    Bookmark   May 19, 2014 at 9:56PM
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hmmm.... z6 is warm compared to my location... I'm betting you also get more sun than we do in the winter. Have you considered building an inexpensive solar furnace? This blog shows a DIY version using soda cans.... and notice that the article links to a much improved version.

EDIT: solar-powered furnace obviously can't be used alone, but might work in conjunction with your original idea.

Here is a link that might be useful: DIY solar furnace

This post was edited by party_music50 on Mon, May 19, 14 at 22:50

    Bookmark   May 19, 2014 at 10:45PM
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If you take a look at the bottom of that page there with the DIY Solar Furnace, there's a link to another one he made that had a 120-degree differential that's much better than the one on the main page. I've thought about doing these before, but I'm curious to see how big of an area they could actually heat that way. In other words, would it make enough of a difference to have it vent directly into the greenhouse, or should I build a separate "mini greenhouse" with the grow beds in it within the main greenhouse and then vent to there. FYI the greenhouse isn't that huge...it's about 1000 sqft. I'm also wondering if the heater could be used INSIDE the greenhouse itself, or if it would have to be mounted externally...I'm sure there would be a difference, but whether or not it's enough to warrant having to figure out how to bring the vent hose inside is something I'm not sure about.

Also, with regards to the aquarium heater - I used to work at a zoo in which we had a 75gallon aquarium heated for aquatic snakes. The area the heater was in was only about 50 degrees in the winter, but the aquarium heated to 84 degrees with no problem on a 300w heater (we actually had a malfunction one time in which the water got up to the mid 90's for a day in the middle of winter because it looked like a sauna when I opened up the lid!). I know the greenhouse probably wouldn't be that warm in the winter without supplemental heat, but it might be worth a shot, especially if I can insulate the holding tank...which would also cut down on the amount of time the heater would need to be on. I like the idea of using it in conjunction with the solar heater idea.

    Bookmark   May 20, 2014 at 9:57AM
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nil13(z21 L.A., CA (Mt. Washington))

You are going to need the same amount of energy to heat the static volume of the greenhouse regardless of the source. It doesn't matter if it is electricity or propane (efficiency of the heaters excluded of course), so pick the cheapest source.

So what to do? Change the volume or the heat loss or both. I think you have hit upon that by having a greenhouse within a greenhouse. It shrinks the volume and sets up an insulative space like an air filled double wall hoop house. I would find a way to heat the cold frame with propane as it's probably cheaper than electric. And insulate each night to prevent heat loss.

    Bookmark   May 20, 2014 at 12:18PM
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We did something similar to that this year with the avocado project where we wrapped two of the three 2x4'x8" planters in a clear plastic hood that stood about 24" high minus a couple of small vent holes. It worked extremely well, although I'd probably build the hood as a removable piece next year to make watering easier. Either way, we could definitely set up the "house within a house" for this experiment.

    Bookmark   May 20, 2014 at 1:03PM
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