Overnight temps for seedlings

[TerraCayda]

I'm contemplating whether to put seedlings out in the unheated greenhouse in mid Feb. Back in S. CA, temps like this was normal and about the same as here in S. AZ. The issue, I just found out, was that the cold temp hours are longer here than in S. CA. Tomatoes often overwintered in S. CA, and potted peppers were also fine off-ground even when the lows got high 30s. But here in S. AZ, as soon as low temps got around 40, the remaining tomato plants pretty much died, and potted peppers got badly damaged.
My unheated greenhouse protects babies from cold winds, but it doesn't keep them warm. I keep them inside tubs with a clear tarp on top, but do you think low 40s all night is still too cold? I'm aiming to take them outside to the greenhouse until cold temp hours are much shorter and plant them in late-Feb to mid-March. Unfortunately I can't get the old hourly record.

[MissS]

I have my seedlings out in overnight temps in the 40's with no ill effects. In fact many of us believe in the 'cold treatment' of seedlings. It slows down growth and helps to create nice sturdy thick stems. What I would be concerned about is using plastic over the top of your containers because it holds moisture in and does not allow air flow. Cold + damp = diseases. You want to keep them well ventilated to prevent any fungi taking over. I don't cover mine but if you feel that you need to cover them then use something like a bed sheet that allows some air in. I would even consider adding a fan on them to circulate the air.

[TerraCayda]

Thank you! I'll take out the bigger babies in a week or so. The average shows 30s is done in January, so from the second week on it shouldn't dip below 40 or it would be a brief moment of 38.
Yes, moisture will be bad news...........I'll need to open the tarp over the tub in the morning anyway as the daytime temps can be in the 70s or even 80s in Feb. It can easily be 78/45 in mid-Feb to cook double-tarped seedlings . I'm not TOO concerned about a fungus problem here. BONE DRY in the West. But I may change the tarp to a shade cloth----sun is pretty strong here, and it may burn them before I get to them.

I have my seedlings out in overnight temps in the 40's with no ill effects. In fact many of us believe in the 'cold treatment' of seedlings. It slows down growth and helps to create nice sturdy thick stems. What I would be concerned about is using plastic over the top of your containers because it holds moisture in and does not allow air flow. Cold + damp = diseases. You want to keep them well ventilated to prevent any fungi taking over. I don't cover mine but if you feel that you need to cover them then use something like a bed sheet that allows some air in. I would even consider adding a fan on them to circulate the air.

[PlainJane]

I wouldn’t worry about a few nights in low 40s. However, if it’s weeks on end of low 40s they may slow their growth.

[karstopography]

They might get a little purple looking leaves in low 40s, but the purple will go away once the temperature rises into the 60s.

[bower]

In my experiments here, I've found that seedlings exposed to mid to high 40's when they're already 6-8 weeks old are just fine, as long as daytime highs also bounce up to 60F or more.
I did have a couple of seedling volunteer tomatoes in pots with cabbages last winter, which germinated after a couple of sunny hot days and then hung on at the sprout to first leaf stage, seemingly not bothered even by temperatures close to freezing. They grew normally after it warmed up and I rescued them from below cabbage. But the thing is, soil temperature is more key than air temperature. If the soil doesn't freeze, they can take the cooler air, even a few degrees below freezing.
I don't recommend you treat your plants that way on purpose! My seedlings do get "cold treatment" under lights, but at temperatures over 50F, not into the 40's, which helps them to tolerate colder conditions after they've grown for some weeks.
If they look stressed by the temperature, coddle them a bit and they'll like it. Maybe they're not in a large volume of soil, so it cools quicker. Put some bottles of water around them and underneath them if you can, to moderate the temperature extremes. Or put them on top of a bigger container of dirt. That will keep their feet warmer at night. A little purple on the leaves or stems is just telling you they were cold and whatever genes they have for cold tolerance are being activated.
I do agree with MissS that air circulation will help, even if it's just during the day. If your tub keeps them warm in the daytime and a bit too humid, it promotes the leaves to grow really tender, and that means they are more vulnerable to cold stress. Drier air helps to toughen the leaves.
I've had young pepper plants that looked really stressed in a tub outdoors, so I laid some square juice bottles of water underneath them. They were fine after that, at the same temperatures.

[TerraCayda]

Never seen purple leaves! I probably would have panicked if you hadn't told me.
The daytime temp is going to be 70's and 80's in Feb with lows in the mid-40's with occasional surprise cold. I wouldn't put them in the ground for at least a month after taking them out to the greenhouse.

Now that most of my seedlings are in the 4" pots from 2", I'm anxious to get them out of the kitchen and the living room . The tub got a bit too heavy to transport to the warm patio everyday.

They might get a little purple looking leaves in low 40s, but the purple will go away once the temperature rises into the 60s.

[karstopography]

Never seen purple leaves! I probably would have panicked if you hadn't told me.
The daytime temp is going to be 70's and 80's in Feb with lows in the mid-40's with occasional surprise cold. I wouldn't put them in the ground for at least a month after taking them out to the greenhouse.

Now that most of my seedlings are in the 4" pots from 2", I'm anxious to get them out of the kitchen and the living room . The tub got a bit too heavy to transport to the warm patio everyday.

They might get a little purple looking leaves in low 40s, but the purple will go away once the temperature rises into the 60s.

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My tomato seedlings Monday and then this morning. Note how purple they are Monday and how less purple they are today.

[TerraCayda]

I've seen purple stems though. Never new purple happens from cold temps!

My tomato seedlings Monday and then this morning. Note how purple they are Monday and how less purple they are today.

[CtGrower]

I have my seedlings out in overnight temps in the 40's with no ill effects. In fact many of us believe in the 'cold treatment' of seedlings. It slows down growth and helps to create nice sturdy thick stems. What I would be concerned about is using plastic over the top of your containers because it holds moisture in and does not allow air flow. Cold + damp = diseases. You want to keep them well ventilated to prevent any fungi taking over. I don't cover mine but if you feel that you need to cover them then use something like a bed sheet that allows some air in. I would even consider adding a fan on them to circulate the air.

How small are the seedlings when you put them out in temps in the 40's? We have a new temporary greenhouse to try this year and.. I have no experience with it. Would you put small (1" tall) seedlings out in 40F, or would you wait until they are bigger (4") to pot them up in 4" pots before putting them in the colder temperatures?

[MissS]

I haven't put little guys out there, however, I often get self-sown seedlings and am surprised at the temperatures that they endure. My guess is that they are so little that the mulch in the beds protects them some. My plants will usually have two sets of leaves before they go outdoors.

Actually, I think that I did leave some little ones out last year. I forgot to bring them in and they were fine in the morning when I found them. So maybe not the best or most popular practice but I think they will survive.

[svalli]

Some people here use containers full of water as thermal mass to keep the temperature inside the unheated greenhouse dropping too cold during night. Could you do something like that?

[TerraCayda]

Actually I was roaming around the house to see what I can use for water containers. There was a reason I've accumulated 20 lb. cat litter plastic jugs!! I've seen others use 2-litre bottles to surround seedlings but never got serious about it.

Some people here use containers full of water as thermal mass to keep the temperature inside the unheated greenhouse dropping too cold during night. Could you do something like that?

[worth1]

By being outside every chance they get when the sun is weak you have less hardening off to deal with and you'll end up with better plants.
Leaving them out on the warm days to collect energy and bringing them in at night will allow the plants to grow over night.
It's amazing how much plants grow at night after collecting energy during the day.
I'm not taking about elongated growth either I'm talking regular healthy growth.

[Shule]

I agree with bower about the daily high temperatures being important, too (not just the low). If the daily high is too cold the plants really don't like it.

Given a warm enough high, the low forties (and even colder) for the low seems fine here as long as it doesn't frost (temperature is taken in the air; the ground can potentially be cold enough to frost when the weather says it's up to 41 degrees or so).

If you want tomatoes that handle the early season colder range well (both with regard to sprouting and growing), try Brandywine Pink. It's a great variety for getting an early start.

[TerraCayda]

High temps are not a problem year-round here (way too hot from June). Highs are in the 70's and 80's in Feb, as opposed to 60's and 70's in S. CA. Sun is pretty strong even in Jan, so last year after I moved to AZ, some seedlings got burned during my daily transporting them to a warm patio to harden off. What's unknown to me is the effect of long cold hours that I'm not familiar with, and everybody seems to be fine with young seedlings in the 40's overnight in 4" pots. Low temps remain mostly in the 40's in Feb but never freezes. Aiming to put them in the ground/pots inside the unheated greenhouse in late-Feb to early-March whereas I was planting in April-May in S. CA.

I grew Brandywine series many years ago. While the taste was fine, they got crossed off my list after finding more productive varieties.

I agree with bower about the daily high temperatures being important, too (not just the low). If the daily high is too cold the plants really don't like it.

Given a warm enough high, the low forties (and even colder) for the low seems fine here as long as it doesn't frost (temperature is taken in the air; the ground can potentially be cold enough to frost when the weather says it's up to 41 degrees or so).

If you want tomatoes that handle the early season colder range well (both with regard to sprouting and growing), try Brandywine Pink. It's a great variety for getting an early start.

[zeuspaul]

My concern would be damping off. When I placed tiny ones outside in cold temps it didn't work out well. But I assume it was a combination of cold and dampness in our early morning hours. Also not much support from the tiny stems even in a light breeze.

Inside a greenhouse in dry Arizona conditions you may have better success. But waiting until the stems are stronger may give better success.

[TerraCayda]

My concern would be damping off. When I placed tiny ones outside in cold temps it didn't work out well. But I assume it was a combination of cold and dampness in our early morning hours. Also not much support from the tiny stems even in a light breeze.

Inside a greenhouse in dry Arizona conditions you may have better success. But waiting until the stems are stronger may give better success.

I grew tomatoes in San Diego for decades. Damping off was a serious problem because of the humidity, and I used Actinovate as soon as they germinated back when it came in small pouches. Now in Tucson I don't even know how often to water anything as everything dries so quickly. I didn't have any damping off last year (first year here), but I did use leftover Actinovate hoping it's still alive. I lost a few this year; I think I actually drowned them.