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Old December 17, 2009   #1
Join Date: Dec 2009
Location: Oakland MS
Posts: 228
Default Unheated greenhouses?

Seems I remember as a kid, us having a homemade greenhouse constructed of heavy duty plastic. I remember we started plants in it, but I dont remember it being heated. Everything I've been reading on here talks about heated greenhouses. I'm just wanting a place to start my seedlings in, as my space here in the house is limited. I live in zone 5, IL. Would a greenhouse need to be heated for starting seedlings?
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Old December 17, 2009   #2
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Location: Slovenia, Europe zone 7b
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If you dont't have a cheap heat source, heated greenhouse must be made of insulating material.
I bought a plastic greenhouse last year for starting seedlings, luckily I started with lettuce and onions, because when temps were just bellow freezing, soil in pots was solid frozen. I tried with electric heater only to find out it must be constantly turned on.
But day temps inside were at least 5C/10F higher than outside even on cloudy days, so lettuce, chard, parsley and onions grew realy well. No chance tomatoes and peppers could survive nights .
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Old December 17, 2009   #3
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Location: Mounds, Oklahoma
Posts: 257

Unheated greenhouses are usually referred to as "Hoop Houses" The below link is a source for information. Steve Upson is researching these and just got back from Israel where they are experts in hoop house technology

Hoop House Survey Provides Glimpse of Fledgling Industry
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Old December 17, 2009   #4
Join Date: Apr 2008
Location: California
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Three ideas.

One, look at for ideas on starting seeds outdoors in minigreenhouses. The idea is to plant the seeds whenever you want, put the containers outdoors, and when the seeds are ready to sprout, they will. I used a bubblewrapped wire cage laid on the ground to start my tomatoes this year (no bottom heat), but then we don't get sustained freezing temperatures in early spring here. The main issues for me were venting on warm days, making the structure sturdy enough to withstand rainstorms, keeping the rain out, and keeping snails and slugs out.

Two, solar greenhouses are usually unheated. Orient them to face south and provide some thermal mass to collect heat on sunny days and release it at night. A couple resources I've come across lately (but haven't gotten my hands on yet) are
THE WINTER HARVEST HANDBOOK: Year-Round Vegetable Production Using
Deep-Organic Techniques and Unheated Greenhouses by Eliot Coleman (this is his newest book; he has several others on growing year-round in cold climates -- he lives in Maine)

SOLAR GARDENING: Growing Vegetables Year-Round the American Intensive Way .
By Leandre & Gretchen V. Poisson
(both published by Chelsea Green)

Three, which I think these two books address, in cold climates you need to protect the plants inside an unheated greenhouse, with frost blankets (heavy row cover), insulated panels, cloches, etc.
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Old December 17, 2009   #5
Join Date: Dec 2009
Location: Oakland MS
Posts: 228

Thanks guys, sounds like I have some reading and researching to do.

I do do some wintersowing, last year was my first year and I had so-so results, with a little more than 50% germination versus nearly everything germinating inside. But I really like to have my maters a bit bigger than wintersowing get them by plant out time..... now wintersowing flowers, that I like, even with the lower germination (for me last year, anyways).
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Old December 22, 2009   #6
Medbury Gardens
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Location: Medbury, New Zealand
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This unheated tunnelhouse has been one of the best things ive done when it comes to greatly extending my growing season,it was a lot of work to set up but well worth it,could be done in tiltslab or poured concrete which would be faster.
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Old July 16, 2010   #7
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Join Date: Jul 2010
Location: France
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Well first of all I am not sure what you winter temperatures are like, but I will give you an idea of what I have done here in France.
Seeds are sown in a tray in the house, as they need at least 60 to 67 degrees F to germinate.
Once they are up they do really need a minimum of 60, but my plants had mostly nights of 40f. This could cause poor setting on the first few trusses, but if you look at my blog my plants were ok with good setting.
Some night it went below 32 f and as long as you keep the plants free of frost they will survive.
There are ways to do this in a polytunnel as we call them. It is too exspensive to heat them the heat loss is too great. So line the lower part at least with bubble wrap. Also you can buy a mini plastic seeding green house and place the plants in this inside the polytunnel. Inside the mini greenhouse you can light a little nightlight candle on the very coldest of nights, again there is a picture on my blog from a friend that did this. Temperatures got to about 20f outside, but the plants still survived.

I built my own polytunnel, there is a fun video on my blog, but it will give you an idea. Also on my favorite links there is a good link showing details of another way to build a polytunnel.

Hope this helps.
Mr Tomato King Blog
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Old October 23, 2010   #8
Join Date: Oct 2010
Location: OH
Posts: 29

i lived in Zone 5 until this year. Didn't move, but has been 35 years since Cincy had temps in the minus 20s or lower.

Anyway, I start my toms in to-go plastic containers from a restaurant, usually around the middle of March. It takes them 3-6 days to sprout and within about two weeks, most of them have reached the 3" height level with a couple sets of true leaves. I transplant them into nursery cell flats, either the 18 or 36 per 1020 flat variety. By then, it is early April so they go into the greenhouse. It has a 13-mil fiberglass reinforced top and the typical AC coating. The GH faces south and has a 1200 watt electrical heater - just in case.

Because it is a hill-jack GH, the front is simply reclaimed house windows (but with plastic on the inside to act as an extra layer of insulation without significantly cutting down on light transmission). As long as the forecasted temps are above 45, I don't worry about turning on the heater. Even when it is, I can set the thermostat to the lowest setting.

Cool temps, IMO do not hurt seedlings - if anything, they make them stronger and contribute to a bushier plant. Warm temps, especially in combo with a moist soil and plants that are fertilized, will lead to leggy seedlings that take a lot more care to prepare for transplant.


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