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I'm a 2nd year gardener and am just now building a small 6x9' greenhouse onto the side of a shed. I've got some indoor starts that are beginning to take up a lot of space (my okra is ~9" tall!), and I'm anxious to start using the greenhouse. But I really don't know anything about a greenhouse environment and I'm concerned about killing all my starts - up til now they've been spoiled in my diy indoor greenhouse.

My question is how to avoid killing plants when I move them out of the house? I'm particularly curious about:

  • What are the lowest temperatures that I can safely keep plants at?
  • How much wiggle room do things like draped row-cover give me?
  • Do the plants need to be hardened off as part of the transition?
  • How can I learn which plants are more/less vulnerable to lower temperatures?
  • Is there any good reading on this topic, online or otherwise?

I live in northern Vermont and our last frost days is around mid-May. I don't have any heat source or electricity in the greenhouse - though I could run an extension cord out there if it's really necessary. This isn't a particularly insulated greenhouse; it's airtight, but there's a lot of single-pane glass.

  • Insulated containers, such as foam cups, may help protect at least the roots from major temperature changes. My watermelons germinated in a few days in them, notwithstanding it still got to freezing temperatures at night. I didn't try watermelons in anything else, though. Keep in mind that plants need ventilation. An airtight environment isn't ideal, although it may be needed once in a while. – Shule Apr 29 '16 at 7:20
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Plants grown warm, indoors, will need hardening off to transition to an outdoor greenhouse that's unheated.

One aspect you may not have considered with a "simple greenhouse" is cooking the plants on a bright, sunny day - an unventilated greenhouse can become a solar oven. There are unpowered wax-based automatic vents, or else you need a person on the job and not to forget in either direction.

For the most part if night-time temperatures in the greenhouse stay above freezing you'll be good, though 45-50 F is a better place to shoot for. It very much depends on the plants - peas, carrots spinach are fine out in the frost and freeze (i.e., the open garden, right now), tomatoes (eggplants, peppers) not so much, basil less than tomatoes, tomatillos are the most delicate darlings I have personal experience of (all wiped out on a night when even the basil was fine.) I would suspect but don't know that okra would be in the very tender end of things and a real challenge to grow in Northern Vermont.

For the fall or next year you might consider adding a layer of greenhouse film (plastic) unless you have some objection to it that keeps you on glass. The double layer will add a great deal of insulation and greenhouse film (as opposed to generic plastic that breaks down in sunlight) will easily last 4 or even 6 years.

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    Keep in mind that frosts occur at 36° F. or lower (since it's colder on the ground than where they take the temperature). I always leave my smaller vents open unless it might frost or freeze, although I don't always close them when it might frost or freeze, because plants seem to germinate better with ventilation, and the 27-36° F. temperatures usually don't hurt them (they do sometimes, though, probably if it's windy). If it's over 80° F. I open the big window of the greenhouse, too, otherwise the plants wilt. I use a Strong Camel 6'x5'x3' greenhouse, in full sun with plants germinated in it. – Shule Apr 29 '16 at 6:58
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Adding to @Ecnerwal excellent answer -

Draped row covers will provide a little additional support by trapping in more heat - maybe 1-3 degrees c - the key would be to take them off during the day to help heat the soil beneath them.

If you look at what hardiness zones plants can be grown in and when you can plant them, that should give you an indication as to how frost tolerant they are.

If you are planning on running a heater into it - and you value safety over plants, you want to connect it to an RCD adaptor (these check if electricity is shorting to ground and trip, and are a very valuable safety protection - the cost being that if things get a bit wet they will trip the fuse). If you are doing this, I'd turn the thermostat on it low - you probably don't need it to get to more then 5 degrees centigrade in the hothouse to provide adequate frost protection, and the cost of raising it above 10 centigrade will be very expensive.

If you are looking for better insulation by adding additional plastic, there needs to be an air gap between the plastic and glass for it to be effective. Having it on the inside will help the plastic last longer (shield it from the UV which breaks it down). Possibly an easier to apply alternative (for Winter only) would, apparently, be bubble wrap.

Tomatos are more slightly more cold-tolerant then eggplants (and, I believe Capsicums/peppers). For eggplants (only place I have real plant knowledge) the ideal minimum is 16 degrees centigrade, but if the plants are hardened off, 11-12 is OK, and they will survive an occasional temp to 3 or 4 centigrade (as long as they don't get hit with frost)

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As a resource you can consider Eliot Coleman's The Winter Harvest Handbook: Year Round Vegetable Production Using Deep-Organic Techniques and Unheated Greenhouses which discusses growing in unheated greenhouses, and the use of floating row covers to provide extra protection against sub zero temperatures. This was a method pioneered by Prof Emmert in the 1950s.

He also discusses which winter vegetables do best with this method.

You can download a sample from Amazon.

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