I am a starter my self and in my case its the other way around. (Bahamas) to keep the temps within limits.

As far as I know tomatoes, peppers, cukes need at least 70F to germinate and grow, and dont let the temp drop below 67 during night or day. If you read on several site (seed companies) they show you the min and max temp on seeds.

otherwise start with cold crops like lettuce untill night temps come over 70 and/or add a heater on a thermostat.
Good luck :)

Would keeping the greenhouse at 50-55 at night be ok if it's warmer during the day? Would having them on mild bottom heat (keeping roots around 70) make it easier for the seedlings to tolerate night temps around 50?

Our greenhouses are all set (auto thermostats) for 45 degree air temps at night and 65 max in the daytime. That is for germinated crops. In my zone maintaining the daytime temps costs 2x the cost of heating. Yours may be more balanced.

Germination takes place in an enclosed heated hot box area where the soil temps are maintained with heat mats at 65-78 (depending on the crop). But one germinated all crops need to be immediately removed from heat mats to avoid cooking the young roots. So the answer to that part of your question is no.

Excess heat is more of a threat to young seedlings than is cooler temps as long (as it doesn't fall below 40) and heat can build up very fast in any greenhouse on a sunny day unless it is well vented. The air quickly becomes stagnant and is a prime breeding ground for fungus disease onset.

You can find out much more about this over on the Greenhouses forum and the Growing from Seed forum here.

Dave

Well, starting my tomatoes indoors in the winter, the house temp (and air temp around them) goes from 65F at night to 68F in the daytime. I'll put their tray outside to get some direct sun when the temp goes over 50F once they've sprouted. Never had any problems. Sure, they'll grow faster if it's warmer, but it isn't as if they won't grow. It's just a matter of deciding when you want to put them out, and how big you want them to be before putting them out. I start my toms in January, and put them out in early March, and they're 4-5 inches tall by then.

So I think you not need be overly particular about temperatures. Just don't kill 'em.

Word of caution for the peppers: pepper seedlings exposed to low temperatures will induce changes in the plants. I've heard this referred to as vernalization, but I'm not sure that is the proper term. The effect is that the plants will put out much more foliage/ top growth and will be much later to set fruit, pushing them back by a month or more to harvest. I can attest to the effect, I have an unheated hoophouse and have seen this happen. keep the plants over 50 and you should be OK, 55-60 would be better. What style/size of greenhouse are you planning?

That's really interesting about peppers, and the impact of low temperatures on them. The term "vernalization" though, I believe, refers to exposure to lower temperatures making plants more likely to flower and bear fruit. That is, it's considered a good thing.

In fact, tomatoes fruit better if, while seedlings, they are exposed to temperatures in the 50s. That's vernalization. In the tropics, tomatoes don't fruit as well, I understand, because they never get exposed to those temperatures.

Now, I do overwinter peppers, and during the winter they are exposed to temperatures down to (but not below) freezing, and I've never noticed an effect on fruit bearing the following year. But maybe this is just for seedlings?

Last year, the room I started my seedlings in got down below 50 degrees every night. The plants grew just fine, and were productive in the garden. The only pots I had on a heater were the seed starters. Once they started growing, they got down to about 45 degrees most nights.

I start tomato and pepper seedlings in an unheated greenhouse with heat mats. I'm not sure exactly what temperature it is right around the plants, but with the warm soil and a plastic dome cover over them, they do very well. The ambient air in the greenhouse gets into the 40s at night, but never below freezing. If I'm worried about things getting too cold, I put a frost blanket over the plastic dome.

During the day, things warm up fast. Leaving the plastic cover on is an efficient way to kill all the seedlings. Also the heat mats are on a thermostat so they turn themselves off.

Hope that helps!

Tomato and pepper seedlings are more flexible than more mature plants. I routinely let my greenhouse go down to 35 degrees at night, BUT the next day, I make sure they go back up over 100 degrees in bright sunshine. Low night temp slows growth significantly. High temps the next day reverses this effect. The way to look at this is number of hours at temperature below 65 must be countered by the same number of hours at a comparable temperature above 65. So if my plants go down to 35, that is 30 degrees below 65, then the next day, I make sure they get the same number of hours at 95 degrees. This is really easy to do, just watch the weather daily and adjust the ventilation to set the temperature in the greenhouse as needed. Bright sunshine can bring my greenhouse to 120 degrees when the outside temperature is only 40 degrees. So the range of temperatures you can work with is from 35F to 120F. Don't go above or below this range for healthy seedlings.

There is one warning however for capsicum chinense peppers. They are more sensitive to low temperatures than most of the other pepper species. I make a point to ensure C. Chinense don't go below 45 degrees. This is because the seedlings will stop growing and won't start again until they have about 2 weeks of temps above 75 degrees. Bell peppers and most cayenne/jalapeno types are not sensitive to cold in this way.

As mentioned above, letting tomato plants get below 45 degrees for short periods of time can increase production. Dig around on the net and you can find some articles from the 1930's that document this effect.

Boy, this is an old post, but filled with lots of inaccuracies.

i am a small commerical grower and produce 5-6k tomato and 20k pepper starts per year.

Nighttime temps during germination are NEVER below 70F. Daytime is 80-85F. Never, ever, are temps allowed above that. High temps lead to stretched out, weak, spindly plants.

Once germination is complete nightime temps are reduced to 65F and daytime to 75F. This WILL require fans and heat.

Too cold nights will result in tomatoes with catfaced and zippered first fruits, making them unsalable. Too cold nights will result in short, stocky, pepper plants that are very slow to recover agressive growth when transplanted.

The only acclimation should be about 5-7 days on a wagon or sheltered outside area before transplanting. This gets plants acclimated to higher light levels and windy conditions.

But what do I know. I’ve only been doing this 20 years.

Large greenhouses are much easier to control temps. A 30x100 will be much easier to control temps than a 16x50 or smaller. Thermal buffering is much greater on a larger structure.

I don't think anyone here is talking about the effect of cold on tomatoes and peppers while fruiting. We're talking about plants that are maturing, and whether those maturing plants can get stunted by cold. As noted, my pepper plants have withstood 40F nights, and they're doing dandy. Colder than that is admittedly not good.

Oh, sorry, I though this was another, more recent and relevant discussion.