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When to move seedlings out to a cold frame/greenhouse?

Watertown, WI(Zone 5a)

I'm running out of room in my seed starting area. Usually, I keep everything indoors until it's warm enough to harden off, but this year I find myself crunched for space and I'd like to put some things outdoors. Here's my question...

Last year, my husband bought me one of those mini-greenhouse setups with 3 shelves, wrapped in heavy plastic that you unzip to access. My impatiens are impatient--I planted them too early and they're blooming already and need to be moved out of flats and into their own pots. I also have smaller petunias and pansies that should be potted up. How soon is too soon to put these outdoors in the mini-greenhouse? We're getting daytime temps in the 40s now, but at night it's still going down into the 20s occasionally. Next week looks to be warmer, but it's the weather...unpredictable as always.


Thumbnail by KaylyRed
(Pam) Warren, CT(Zone 5b)

I have one of those things too, and wondering the same as you. Years ago when I had a cold frame, I put 1/2 gallon bottles filled with water to store up heat during the day and keep it warmer at night. Maybe that would work here too?

Last spring I didn't use it so early. I left the zipper undone (I'm not there during the week) and clipped a piece of frost blanket over the opening, which kept things from frying when the sun was out and they didn't seem to mind the night temps. This year I have an automatic opener and I'm wondering how I could adapt it. Maybe just detach the zippered door, mount it on a lightweight wood frame, attach it at the bottom so it would vent out the top where it gets really hot?

Ideas, anyone?


Galesburg, IL

It all depends on the temperatures in your greenhouse/coldframe. I use a min/max thermometer with memory to record temps for a few days inside my coldframe before I move plants into them. You need to make sure that the temps stay above freezing at night. I will also use blankets/tarps on top of the coldframe at night if the temp is going to get particularly cold for a night or two. Jugs of water are a great way to moderate the temps at night. I use old laundry detergent jugs that are dark blue in color (heat up more with sunlight) but you can use anything you have that will hold water. I have even used 5 gal buckets of water in my coldframe, The other consideration is sunlight, you don't want to move anything directly into the sun in a coldframe/greenhouse, you need to keep them in the shade for a few days to let them acclimate (especially shade lovers like impatiens). On my coldframe I use old slotted shutters to provide shade for several days. Of course, you can keep your greenhouse in the shade for the day, but then it won;t build up the heat needed to moderate temps at night. One thing you might try is to leave your greenhouse in the shade during the day, and then fill up jugs with hot tap water and put them with the plants at night. I have moved some daylily seedlings into my coldframe this morning, and will move more plants in the next couple of days. Like you, I have run out of space inside and am being forced to get things outside.

I would recommend getting a thermometer and experimenting with jugs of water for a day or two before you move your plants outside. The cost of a cheap thermometer greatly outweighs the cost of plants you could loose if your greenhouse freezes.

My coldframe is made of wood with plastic tops and sits on the ground on the south side of an old chicken house. I start moving plants out when temps at night are going to be above about 24 degrees. I can handle colder temps with a couple of blankets but I don't like having to deal with them.

Your other option is to move the plants out during the day and carrying them back inside a garage or other structure during the night if very cold temps are expected.

(Pam) Warren, CT(Zone 5b)

I forgot I used to use a min/max thermometer, it's been a few years since I gardened on this level. Thanks for the reminder!


(Pam) Warren, CT(Zone 5b)

BTW trc, how deep is your cold frame? I'm still considering that as an option too. Thanks!

Galesburg, IL

Here is a picture of my coldframe, you can see the blue jugs of water lined up in the back of it. The size is 3' x 6' with the back 22" tall and the front 12" tall. It is made of scrap 2x6 and 2x8 lumber. The top two covers are just 2x2 lumber with plastic stretched across and hinged in the back. The bottom is gravel for drainage and to keep the wood off of the ground. I kept the front at 12 inches so I could line up plants in the very front of it to shade young plants. I can comfortably fit 9 flats with some room for other things in pots. Unlike some who will use movable cold frames in their garden to extend their growing season, I use mine only for potted plants. I do use it in fall to protect some things in baskets from early frosts.

Thumbnail by trc65
Watertown, WI(Zone 5a)

trc, thanks so much for the advice. I think I'll invest in a thermometer and keep an eye on things over the weekend, just as you suggested, and consider moving things outside next week.

St. Louis County, MO(Zone 5a)

Kayly, If your night temps are falling too low, others in the greenhouse forum have used the little twinkle Christmas lights turned on just at night. They put them at the bottom and the heat rises. You might test that out, too.

Watertown, WI(Zone 5a)

Oh, good idea, Cathy! I hadn't thought of that. I'll give it a whirl. :)

Greeley, CO(Zone 5b)

I used one of those last year. I was able to put my tomatoes out at the end of March when our temps were in the upper 20's/low 30's. I kept it warm at night by using a shop light w/ a 100watt bulb in it. The light was clamped to the shelf on the last from the bottom so it would reflect down and not touch the ground. I then wrapped the whole thing in a fleece blanket to keep the warm air in. This kept the temps up above 40 degrees. I went through quit a bit to keep it toasty for the tomatoes, but it was worth it. I didn't have to keep that that up for very long and only on nights when it got way too cold. It also eliminated the need to move them when they needed to be hardened off since I just opened the cover. I hope that helps.

Helena, MT

Like you KaylyRed, I have to use a hoop house to transfer seedlings from my indoor seed starting setup. I use an electric chicken house heater which is thermostatically controlled to maintain night time temperatures around 50 degrees. My wife has a similar tiny heater in the bathroom she uses when showering which might work for your mini-greenhouse.

Watertown, WI(Zone 5a)

Thanks for all the suggestions! I did pick up a min/max thermometer and the greenhouse dipped down to 32 degrees (only 4 degrees warmer than the outside temp of 28 right now.) So...I think supplemental heating is in order. I've certainly had impatiens survive chilly nights outdoors, but I don't want to push it.

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