cooling greenhouse

gwoods34January 31, 2014

Late last summer I finished a 15'x16'x10' high, wood framed, suntuf polyclad gh specifically for fruit trees. I live in Idaho; the wind & -30f winters make fruit trees, except apples, impossible. By October I had planted 2 peach, 1 apricot, 1 nectarine & a cherry tree in it. On 3 sides the bottom 4 ft is a removable panel. My intention was to remove the panels after frost concerns had passed each spring and then close it up for the winter. I was going use a kerosene heater to keep the chill off during the blossoming season & then only when there was a danger of below -10f freezes during the winter months.

My problem is I had no idea how hot the gh was going to get; even in the winter. It was 22f outside the other day and it was 68f in the gh. I soon realized that when it's 95 outside it'll be 120f plus inside. I have a 20" exhaust fan & an auto opening wall louver which won't even cool it off much in the winter.

I've got to get something figured out & built or the trees I've planted will be cooked by spring time.

I'm really interested about the thermal mass & heat sink & fogging systems I'm reading about but would really like some advise. I know I have to correct/add to the ventilation I have now but I'd also like to build in some sort of heat sink system. Has anyone in this forum made available readable plans for small greenhouses.

please; I have to get busy.

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fruitnut Z7 4500ft SW TX

I've been growing fruit in a greenhouse in west Texas for the past 10 years. Our summers at 4500ft are similar to yours in say Lewiston and and warmer than Boise. You also probably have lower humidity so an evaporative cooling system would work great. This involves a wet wall on one end of the GH and exhaust fans on the other end. The advantage of this system is no rain on the fruit thus less rot and splitting.

If you don't mind rain on the fruit in summer or live in a dry area, as much of ID is in summer, then you need roll up sides and large vents on the ends. Basically have an open house, covered only in bird netting or 30% shade cloth in summer. Then close up as need to hold off freezes.

For most fruits you need humidity as low as possible all summer. So factor that into your plans. No spraying water inside in summer to cool things off.

There is much more I could cover. Read the writeup referenced below. Then if necessary email me thru Gardenweb.

Here is a link that might be useful: greenhouse fruit production in west Texas

    Bookmark   January 31, 2014 at 8:53PM
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Cooling the greenhouse- in the winter

Since you mention problems cooling the greenhouse, even in the winter, the solution for that is simply increased ventilation, as you noted in your post. Thermal mass/heat sinks/fogging systems, etc. can be used, but at least for the winter cooling, you just need more venting area. If it was 22 F outside and 68 F inside, and assuming you do not want to induce flowering in January, simply increasing the top and bottom vent area will solve that problem. As a general rule for passive ventilation, 20% of the floor area should be vent-opened, and 10% of the floor area should be bottom/side vents.
For your 240 sq. ft. GH, that would be about 50 sq. ft. of top vent, and 25 sq. ft. of bottom vent.

If you want an even simpler solution, add shade cloth to the dormant fruit tree greenhouse during the winter, which will reduce the inside temperatures significantly, depending on the shade factor you choose.

For the summer, this amount of passive ventilation area will also make cooling somewhat less of a challenge. Your 20 in. exhaust fan will compensate a little for the lack of passive vent area, but evaporative cooling (foggers), shading,etc. will probably be needed as well. The need for access of pollinating insects into your greenhouse during the flowering period, while excluding harmful insects, is an additional concern.

    Bookmark   January 31, 2014 at 9:25PM
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sand_mueller(z 7a, oklahoma)

I built this structure out of used window glass. Every piece of glass in it is either hinged or slides open. When open next summer I may slip sheets of newspaper between the either flipped over hinged or the sliding panes to alternate direct sun and shade. The idea is to avoid the heavy summer shading that stops plant growth.

    Bookmark   February 5, 2014 at 12:27PM
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sand_mueller(z 7a, oklahoma)

This is the original part of the greenhouse. The large door opens with a caster on the ground. Vents are on the bottom and top for convection venting. The taller and bigger the structure the better it ventilates. This is why greenhouses before electricity were usually tall and big (conservatories.) Altogether I have 600 ft. sq. and never use a fan, Each year I shade later than anyone else around here.

    Bookmark   February 5, 2014 at 12:33PM
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Nice pics, sand_mueller

To the OP, I am not a fruit grower, but I'm wondering, will the high winter temps inside the greenhouse impact the chill hour requirements for your peaches? I would think you would need to control temperature carefully so as to keep the trees from flowering too early.

    Bookmark   February 6, 2014 at 9:54PM
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thank you everyone for the responses to my question; I think I've got it worked out now.

    Bookmark   February 8, 2014 at 10:55PM
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Cole, I am a little concerned about chill time but a lot of areas that grow peaches have much warmer winters than we do and I've checked with a couple nurseries and they say it won't be a problem.

I've decided if the gh temps disrupt the tree's dormancy I'll just have to chalk this year up to the learning curve and have the gh climate corrected by the next winter.

    Bookmark   February 9, 2014 at 3:31PM
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fruitnut Z7 4500ft SW TX


The nurseries have no idea what's going on in your greenhouse and probably don't even know what it takes to chill various fruits. Sure it's no problem outside in ID so no one gives it a thought. But outside and in a greenhouse are way different.

You need a ~1,200 hrs in the 30s and 40s to chill most apple, peach, and apricot. This year I went 69 straight days between 37 and 57 to get 1270 chill hrs. But my nights are warm enough, ave 30F, that I can heat to ~40F so as to maximize chilling all night long.

I think the right approach in your climate is to just heat to avoid damaging temperatures in mid winter. I'd set a goal of maintaining 0F as a minimum. If it goes a little colder that's OK. But all those hrs between zero and 32F give exactly zero chilling hrs. So you need to maintain 30s and 40s on sunny winter days. That will give some chilling in mid winter. In fall and spring you might need to follow my example and heat to 40F to maximize chilling at night. Then hold daytime below 60F because above 60F is actually negative chilling. The meter is running backwards and you're losing chilling. Below 32 at night and 60+ by day is near zero chilling, could even be negative chilling.

There are lots of ways to fail growing fruit in a greenhouse. Pay attention to chilling because that's one way to fail.

This post was edited by fruitnut on Sun, Feb 9, 14 at 17:27

    Bookmark   February 9, 2014 at 5:26PM
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Thanks, you just taught me a ton more than I knew; I didn't know it could be too cold to count as chill hrs. I'll keep a copy of your note for future reference.

    Bookmark   February 9, 2014 at 6:05PM
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fruitnut Z7 4500ft SW TX


After all the talk of potential pitfalls remember this, my greenhouse fruit is by far the best I've grown. I've had orchards since 1971. The fruit was never quit good enough to make me happy. The greenhouse changed all that and with far fewer issues like birds, deer, hail, wind, rain, freezes etc etc. It was 24F outside this year in May. I'd already been eating apricots for a month from the greenhouse. Then in June a massive hailstorm. Didn't phase the greenhouse or fruit inside.

So good luck and keep my email in case you have more questions.

    Bookmark   February 9, 2014 at 9:04PM
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Can bees get into the greenhouse for pollination? That is probably going to be the largest problem you'll encounter. Bees are very important unless you learn the fine art of pollination ;) Good luck!


    Bookmark   February 14, 2014 at 10:32AM
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Since I last posted I've built 2 large vents. They're still manually operated. I've figured out how to install the automatic vent openers but I wonder if they'll ever be closed except on below zero nights and during spring frost concerns.

I've also installed a swamp cooler and rewired it to plug into a thermostat controlled outlet. Saturday I started another large low vent and have ordered a bunch of aluminet shade cloth.

All that and still I have a question. On almost everyone's list of cooling tools is foggers. My question after watching you-tube videos and reading everything I could is: has anyone ever tried putting the "mist maker" ultrasonic fogger (3 head) in the water reservoir of a swamp cooler letting the cooler fan pull the fog off the water and blow it into the gh?

Seems like something that obvious has probably been tried already but did it work? Probably silly, but still, I'd love to hear any thoughts about this idea.


    Bookmark   February 24, 2014 at 1:40AM
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fruitnut Z7 4500ft SW TX


To answer your question, the fogger would add little or nothing to an evaporative cooling system. Reason is the evap cooler is already doing what the fogger is supposed to do. Both cool the air by evaporating water. Once is about all that will help.

If you need more cooling than your evaporative cooler provides you need to move more air thru the cooler or provide more shade. One air exchange per minute works well thru my evap cooler.

Light in my greenhouse thru two layers of woven poly is 40-60% depending on sun angle etc. That's enough. Don't go over 40% shade cloth. If you need more light consider Extenday reflective fabric on the ground. This will reflect some light back into the canopy.

    Bookmark   February 25, 2014 at 12:29PM
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