How and when to use a cold frame?

Mount Lebanon, PA


I received a cold frame for Christmas and am not sure how/when to begin to use it. I recently started some lettuce indoors and had fairly good success with it under the grow lights, but with my new seedlings to be started for my vegetable garden, I'd like to move some things out when possible to make room !

Our weather is still fairly cold and erratic lately (it's 65 degrees today but will be in the teens tomorrow...) but I'd like to get some of the colder weather plants outside. I know I need to harden them off, but do I do a typical hardening off as I would without the cold frame (a few hours out in the cold frame and then bring in?) Or do I just put them out in the frame on a mild day and let the frame do it's job? Can they remain in pots in the cold frame? How long should I wait for the seedlings to be moved outdoors? I've read that they should be about 4 in high... Lots of questions from a beginner :)

Everett, WA(Zone 8a)

Two things I just learned may be very obvious to you, and should have been to me, but I forgot them.

Don't just pick a handy spot and throw up some plastic . Cold frames don't work in the shade! Watch what spots GET SUN in these months. I think morning sun is most important for warmth, but late afternoon sun might be needed to help them get through the night.

I know every article about cold frames says this, but please be smarter than was and REMEMBER it: face south. Or southeast or southwest. But having a northern exposure won't do you any good at all.

So, like real estate, the three most important things are location, location and location:

location: face south!
location: avoid shade!
location: latitude - nothing you can do about this, but the low sun angle we have Up North reduces our light and heat gathering ability, and it makes shade a big problem.

I can summarize the above post in one word: DUHH!

I built a tent over a raised bed that I really wanted to warm up sooner. Wanting and wishing don't help at all!

Everett, WA(Zone 8a)

>> I know I need to harden them off, but do I do a typical hardening off as I would without the cold frame (a few hours out in the cold frame and then bring in?) Or do I just put them out in the frame on a mild day and let the frame do it's job?

If your plants are big before the weather is warm enough to harden them off "naked", use the cold frame to hold them until the weather warms up. Gradually leave the lid open wider and more often to harden them gradually within the frame.

That's like a two-stage hardening off: first they become hard enough to live in the sheltered frame. You might have to move into the frame for a few days, then pull them back indoors for one cold night, then goi bac k into the frame. Hello, insects!

Then, when you move them out of the frame, pick a mild or cloudy day to harden them the rest of the way. And if another cold night comes, you might have to pile them back into the frame for one night.

I was planning to use mine as a holding area for when my one light shelf overflowed.

>> Can they remain in pots in the cold frame?

Yes, and probably even in propagation trays, until they start to get root-bound.

Mount Lebanon, PA

So, if the plants get big before the weather is warm, I can move to the cold frame and leave them in there all day/night (cracking the lid when it's warm) assuming that the temperatures are mild? Or do I also gradually move them in and out of the cold frame?

Everett, WA(Zone 8a)

I think it depends on how effective the cold frame is. For example, if you have plenty of sun and no wind, and the cold frame has no leaks and good insulation, it should be like a safe, warm greenhouse that will keep plants warm enough all night. Just don't cook 'em!

But if the cold frame has leaks (like mine) or is in the shade (like mine), or you have a lot of wind any ANY leaks, they might not like the chills at night.

Do you have any recording thermometer? I found a cheap battery-powered one that remembers high and low temps. That would tell you how cold (or hot) it got while you weren't watching.

My plan was to not let my seedlings get too accustomed to "warm" for a long time. I can almost expect some late, unexpected cold snap. Or rather, there are always long warm spells that occur BEFORE spring is "really" here. I'm afraid that my cold frame might protect them too much in early warm spells, but soften them too much to survive a last chill.

I haven't actually used a successful cold frame yet, so I really hope that someone more experienced will chime in!

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Galesburg, IL

Before you move any plants outside, get a min/max thermometer and record the temps that you get in the cold frame compared to ambient temperatures. This will be important to make sure you aren't going to fry the plants during the day or freeze them at night.

An important consideration when moving the plants into the cold frame for the first several days is making sure the sun doesn't fry them. When I start moving plants into the cold frame, I will keep them shaded (in the frame) for about a week before letting them into the full sun.

Watch your moisture levels very carefully as well. They will need much more watering than in your house. It depends on how large your containers are, sun intensity and temps. On hot days with larger plants I will water up to three times a day in the cold frame.

Placing the cold frame against a building will help the cold frame maintain temps through cold nights. You can also place sheets, blankets, cardboard or anything else you have over the cold frame at night to hold in heat. Another tip to holding in heat is to use milk cartons filled with water in the cold frame. They will heat up during the day and release their heat at night. Using these methods, I have kept plants in my cold frame for days when outside temps never raised above freezing (as long as the days were sunny!).

When I move plants to the cold frame, I don't move them in and out of the house. It's too much of a pain to do that. You can keep them warm enough using the above mentioned methods.

I don't know exactly what your season is like in PA, but I'm in zone 5 and I won't usually start moving plants to the cold frame until about March 1 with a last frost date of April 15 around here.

Another tip - Before you first start using the cold frame, keep it closed for about a couple of weeks so you can start heating up/thawing the soil before moving plants outside.

Everett, WA(Zone 8a)

>> They will need much more watering than in your house.


I thought they would need less watering in the closed space. I thought I must have killed those trays last year by forgetting to water them for a LONG time. Maybe it wasn't all that long.

I also thought they were not getting very warm in that (shady) spot. Maybe the sun hit it during a part of the day I had not observed.

Pueblo, CO(Zone 5b)

I set mine up in the kids out-grown sandbox against a south-facing brick wall. I did this in hopes that the sand and brick would help maintain an even temp through the night. When I water, I soak the sand too - in hopes that it will raise the humidity a little so I won't have to water as often. Even so, seed starting mix in a small container is going to dry out fast.

Mount Lebanon, PA

Thanks for all the tips! I'm so impatient- I can't wait for spring!!

Enterprise, AL(Zone 8b)

I think as a gereral rule you can add one month to your growing season at the start and one month at the end with a good cold frame. So I think for starting out, it would be sort of like being in one zone warmer, so use that schedule as a guide for the cold frame.

Holly Springs, NC(Zone 7b)

I'm gone for work from 7am-6pm. I'm worried that a cold frame isn't a good choice for me because it is still really cold when I leave in the mornings. I've seen these hydraulic greenhouse window openers that can be adjusted to open the top of the cold frame in response to internal heat. Anyone have experience using one? I'm thinking about trying one during the weekend without any plants in the frame and then testing the temps. Any other suggestions for types of frames that may be more practical for my situation. I'm in Zone 7, so I'm not facing frost in the mornings (usually). I need to move some camellia, gardenia, and fig starts off my light shelves because they are hogging up space!

(Pam) Warren, CT(Zone 5b)

A solar-powered automatic opener is a godsend when you can't be there to monitor changes in temps and sun. I used one years ago as a weekend gardener in a conventional cold frame.

Now I use a mini-greenhouse stand with shelves. I leave the zipper undone and hang a piece of Reemay over the opening (inside the plastic cover), attached with clothespins top and bottom so it doesn't blow wide open. That has worked well for the last 2 years. I set the plants out on a Friday while I'm there so I can really pay attention to what's going on, and by the time I leave on Monday they are adjusted enough.

I do use a mini-maxi thermometer to keep track of the temps. Here's a record from last year's journal of what was going on when I first put things out. I had to leave that afternoon, but came back the following afternoon and everything was fine.

4/17, Tuesday: 55 last night, 70 at 2 pm.
I woke up in the night and heard the wind howling, so of course I was worried about the plants I left out. But no, today was another gorgeous day. It's supposed to drop down to 34 again tonight, so I setup the Mini GH and managed to get all the tomatoes and snaps inside. Left the door rolled up, put remay over the opening with clothes pins.

After that, night temps went to the low and mid 40's for two weeks, then we had two weeks of below freezing in early May, which killed the buds on many flowering trees. Everything in the stand survived. By then I had put a couple of jugs of water on the bottom shelf to help even out the temps, and it never went below 35 in there. Days were of course warmer, but still quite cold. Btw, I use trays filled with water and capillary matting on raised platforms to prevent drying out while I'm not there. I also sprinkle slug bait generously around the bottom of the stand.

Hope this helps!


Amazing what just a little protection can do!

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(Pam) Warren, CT(Zone 5b)

Just found another journal entry:

5/5 CT. Last week 23/68 (MGH 40/70) 3/4" rain.

Two different mini-maxi thermometers. One outdoor sensor is in the shade on the north side. One is inside the stand, which is on a south facing wall, opposite tall trees and shrubs that cast intermittent shade throughout the day.

Victoria, Australia

A cold frame is a simple structure that utilizes solar energy and insulation to create a microclimate within your garden.You also know how easy they are to make and use. Although we have a greenhouse on our farm, space is always limited, so we rely heavily, especially in spring and fall, on our cold frames to overwinter plants, extend the growing season, start seeds, and harden off plants.

For whatever purpose you want to use a cold frame, you need to keep in mind a few basic factors. First, some plants fare better in cold frames than others, with low-growing, cool-season plants being the best suited. Second, the type of cold frame you use dictates how much protection you can offer your plants. In all cases, the main conditions you need to monitor and control are temperature, sunlight, moisture, and wind exposure.

Casper, WY(Zone 4a)

I don't know about anyone else but I couldn't garden without a coldframe. My first one was made of railroad ties. I worked great. I moved from NE to WY bought a house and the first thing I did was to build a coldframe. It is the size of a door with a frame that holds shade material.

My coldframes was never intended to keep plants from freezing. I used to start many perennials in the spring, to be planted in the coldframe until Aug when they were large enough to plant in the garden. In Aug I would sow more and they would spend the winter in the coldframe. It tested the hardy from the non hardy in my zone 4. It also gave me a measure of control over small seedlings before they were large enough to plant in the garden.

The coldframe was placed against my fence so that the fence could hold the top open. I used the shading when the seedlings were first planted. It also shaded from the hot westen sun when it was open.

1] Below is my first built coldframe against the fence. It is the only one that has a shade cover.
2] Purchased coldframe with daylily seedlings.
3] Another purchased coldframe with Iris seedlings.

I have a 4th that is attached to the 3 the on in the 3rd photos. My soil in all coldframes have been amended since I have clay soil.

Now I use the coldframes mainly for iris and daylily seedlings from my crosses.

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Lincoln, NH(Zone 4a)

could you give me detailed information on using your coldframe for iris seedlings? I crossed several irises and have 10 seed pods that are ripening. I have never grown iris from seeds, have always bought rhizomes so I am totally a beginner. Any information about germination, time to do so, when to put them in the coldframe would be realty appreciated. Do I soak the seeds then refrigerate for ? weeks before planting them? I did buy 2 coldframes last week.

Casper, WY(Zone 4a)

check the thread you started. I answered you there.

Below are some of my iris seedlings blooming at age 14 months. They are the same that are shown in the 3rd photo above.

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