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I have been moving my plants outside during the day and inside at night. I'm going to be away for a few days, and I need the plants to get sunlight (although it has been cloudy and raining lately). I thought of creating a DIY green house with plastic bags (Type 4). In here, it's said that type 4 is suitable for plants and gardening. And in here they talk about plastics to use for greenhouse, but I think they are going too specific in the subject and at large-scale.

When I put them outside, they get daylight, but no direct sunlight. With a light measurer I can see there is quite a difference by having them inside right next to the window, and outside right next to the window (way more light received outside).

This is the plastic bag greenhouse I made. Everything is sealed. I was planning on putting some sticks so it would be a bit higher as well, and rain would just fall, like a "roof". The plants are small tomato and red peper plants, which germinated and sprouted 3 leaves, so I replanted them there because I can't move them anywhere else right now.

Plastic Greenhouse

In here it says that it works well for humidity retention, and frost prevention. On this site, they asked here the best type of plastic, but neither the question nor answer mention a PLASTICBAG greenhouse. On this site as well, they used plastic cups and I have also used them when germinating. They worked well, but it was not as cold as it is now during the day. Same here for types of plastic sheets. They mention a thickness of 3.5 or 4mm at least.

TL;DR:

Can regular plastic bags (like for freezing food) be used as a plastic greenhouse to not only protect from frost, but also to retain HEAT? (as it is getting colder during the day, and plants need more warmth as well as humidity). These plastic bags don't even have 1mm in thickness I guess.

EDIT, more info:

Average temperature these days: 8ºC daytime, 4ºC Nighttime. Quite windy (from 2 to 10km/h) average.

Plants: Red Pepper plants, from supermarket, extracted some seeds. Tomato plants, bought from plants store. Marmande VR Tomato Holand Type. Harvest from 5 months.

Very early fruitful variety. Valued and grown for its early harvest. Low bush. Pale red, good sized, broand and rugged fruits.

Plants:

Tomatos and red peppers

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    What are your night time temperatures and what kind of plants; annuals, vegetables. Tropicals?
    – kevinskio
    Dec 4, 2022 at 14:54
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    The extent to which they provide frost protection is rather limited if there isn't a heat source. Likewise, for retaining heat, there has to be heat to retain. Get too cold and it just freezes in the greenhouse anyway, if there's no active heating. Passive heating is difficult to get much of on a small scale, because there's relatively more surface (losing heat) to relatively less interior volume (retaining heat.) And the other issue you have to cross if "leaving for a few days" is having the plants cook when the sun comes out, if there's no automatic venting.
    – Ecnerwal
    Dec 4, 2022 at 15:14
  • Added in the edit! Thanks! @kevinskio
    – M.K
    Dec 4, 2022 at 15:15
  • But they do not get direct sunlight! It's just the reflection of the sunlight on the building in front. I assume that due to the daily cold temperature, the "cooking" would not happen, but now you make me wonder! @Ecnerwal
    – M.K
    Dec 4, 2022 at 15:16

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