This would be why people keep Weather.com open in another window constantly- LOL!

For me, it starts when the day time temps are ~42/43 consistently. But here in NH, the 'complete changeover' is like menopause: it takes a LONG time. It might be 2-3 weeks of off, on, off, on every day & night depending on what the weather is doing. This is why I like milk jugs with hinges- cuz it's so easy to pop them on & off when the weather starts to change.

Once the night-time temps are consistently 45 or so, off mine come for good.

Once the container is done germinating.

When the first sprouts appear, take the lids off! If they are perennials, or hardy annuals that self-sow, they can take the frost.

What I try to keep in mind, is that nature drops seeds in the fall and they survive, even thrive, without the pampering of having a cover. Keep it simple, and as close to "natural" as possible.

when I see those first leaves unfurled - they go topless. the annuals I watch and it's it's going to be a cold night they get a sheet. but other than that they are on their own.

I'm with nancy_drew, but I will keep covers on with a little more venting to induce more germination. I also keep their orginal covers/tops and or sheets of plastic/tarps available when they are completely topless in case of heavy rain or hail for the frailer seedlings :)

Vera

I agree with keeping it 'close to nature,' but by the very fact that we are keeping them in a slightly warmer environment in the containers, doesn't it stand to reason that they are sprouting at an un-natural time (which would justify the on/off activity of tops)?
If you DSd the same seed right next to a WS container of same, would they sprout at the same time? Anyone ever done an experiment?

All I'm suggesting is that Ma Nature wouldn't send up seedlings possibly as early as we see them in jugs...would this be a correct statement? Ergo, taking tops off the minute the seeds sprout COULD be detrimental to seedlings (esp things like Zinnias where we push the envelope by sowing in jugs EARLIER than packet directions for DSing...thereby getting a head start on them, but only to the extent that we keep them protected until danger of frost is past)
Where's trudi...she's probably done the experiment.

Taking the tops off when they sprout, like myself and others have pointed out, is fine for perennials and hardy annuals that self-sow. These plants can take a frost. Do Zinnias fall into those catagories? I think perhaps not.

Right; I get that part- about stuff that would normally see winter through without batting an eye...I was addressing the generalization of the OP's question.
Gee, is it mid-winter already? I just love January at GW.

Most of my containers have plenty of headroom so I don't take the tops off unless it is very warm and there is a possiblity of seedlings frying. I may prop up the flap, lift the bag/plastic a bit, push up the top of the 2L for more venting but do not remove the tops permanently until temps are above freezing. Have been known to quickly replace tops or close flaps if a frost is forecast. Last year was doing that as it hailed to protect those fragile sprouts. Still had huge hail pieces in some jugs. Broke through the plastic or slipped in through the mouth.

Last year I was out there in a storm, tornado sirens blasting, being pelted by heavy rain, strong wind, and hail, flipping the tops closed.

I open mine after germination but now leave them attached at the hinge for just this kind of thing. Or, of course, frost for annuals.

Karen

We wsowers can be a foolish lot. After a couple of golf sized hail stones to my head I thought, "forget this" and was otta there. Glad you didn't struck by lightning or carried away by a tornado, Karen.

n-drew & others- Someone brought up a point on another thread, and now I'm curious: I have always had my containers on the south side of my house- full sun.

Does "global positioning" (LOL) make a difference as to whether you are having to even DO the whole covers on/off thing? Ie: is there less up & down of temps if your jugs are on a different side of your house (north, east or west) rather than the southern side?

I realize I'm over thinking this again. I've been a successful WSer for 4 seasons now, and maybe- quite possibly, it's this simple: I'm keeping them in their covered environments longer than they really NEED to be. I tend to keep it ALL under wraps- regardless of per, ann, bi, etc until the weather has stabilized (mid 40s @ night) long about 3rd week or so in April. Yes, they all survive, but I'm realizing now that what you're saying is I don't HAVE to be doing this on/off dance for everything- esp the per & reseeders, which can take the lower temps.

Ok, I think I'm finally getting it.

Last question...do you plant out as soon as the soil can be worked then, or do you still wait longer even though tops are off completely? Why wait if tops are off?

I live in a north facing condo. My cold frame probably has nowhere near the temperature fluctuation as it would in the full sun.

I don't plant mine out quite as quickly, I have had a slug problem in the past. I also wait until the whole container is done germinating... and this can be a while after the lid is off.

I do think that being on the north side of the building, in morning sun only, slows down my germination a bit... it doesn't get quite as warm in my cold frame. It's kind of a give and take... I probably don't have to water quite as often either.