You had some really hard questions, or at least ones that I have no good answer for.
Since you keep good notes, PLEASE keep posting what you figure out through trial and error, or write up tips and articles!
I think that the people who know best how to do these things tend not to have lots of words for how they do it. They "just know how" and have some system they follow, whether or not they even have a conscious list of rules they follow.
>> Now that some of the 72 pot seedlings have germinated do I keep them ALL in the tray, do I move out the small little plants, if so what do I do with them?
1. Once any seedlings in a tray emerge, remove the humidity dome from all of them and give them some drier air, and air movement if possible. Otherwise you'll become familiar with damping off.
In the future, you might plant similar seed sin the same tray, or cut the insert tray into 3, 4 or 5 "chunks". Sow each"chunk" with just one kind of seed, so they all germinate as close together as possible. Cover with plastic film or plastic bags, so you can unveil each "chunk" right after germination. Or you can remove the germinated "chunks" of trays from the humidity bags, and leave UN-emerged chunks under the plastic.
The plastic just serves to keep the soil surface moist without constant spraying. It maybe holds in a little warmth also. Extra humidity and warmth are good for germination, but SEEDLINGS like it drier and cooler.
I tend to cover with 18" wide Saran Wrap, since I have a roll. I cut mini-blind-slats in half or thirds the long way so I have skinny little stiff uprights to hold the plastic off the soil surface.
>> Exactly how long can seedlings stay in those seed trays?
Good question.
Whatever light, soil, air and temperatures you keep, those plus the plant species and variety determine how fast your seeds will become root-bound. If you know how long it took last year, you know about how long it will take next year. After 3 years, you'll have a good idea of the "plus or minus" ranges.
>> ... the roots on those got quite out of hand ... so I put them into bigger pots.
That's the answer: until just before they become too root-bound. If roots start coming out the bottoms of the cells, you are getting close. You probably have to pop some out to see HOW rootbound they are.
What do others think about when to pot seedlings up? I LIKE to pot up seedlings when they are really rootbound, with lots of white roots showing or even circling. The rootball is so solid that I could almost drive nails with it (exaggeration). There is NO WAY you can break any roots in a rootball like that! In fact, I can transplant these into the ground without bending over: make a trench and then drop the rootbound seedlings from 3-4 feet up. If I hit a rock, they might just bounce.
However, what I LIKE is bad for the seedlings. The roots can just barely recover from having been so rootbound that they circle the cell. Seedling growth went on "hold" as the roots approached 100% root-bound-ness.
At the other extreme is potting up almost as soon as any root tip touches the cell wall. The seedling never has any rootbound tress and the growth rate never checks. However, now it requires COORDINATION to transplant the roots without the root ball falling apart or breaking up. I GUESS it is OK to transplant roots that sprawl around sparsely and let the soil mix fall out.
My guess (still learning) is that the seedlings would like a chance to get their roots into MOST of the soil mix in the cell, but not sit there so long they have a death grip on every grain and fiber of the mix. Perhaps "some white roots showing at the surface of the root ball, but not very much circling".
"Some roots showing but not much circling" will hold the root ball together ENOUGH for tidy transplanting if you are AT ALL coordinated.
Of course, all this is "no answer", since "how long" totally depends on:
- how big the cells are (72 cells per tray? 50? 128? 36?),
- what growth conditions (e.g. soil aeration, light intensity and temperature)
- plant species and variety ("vigor")
You might prepare to pot up 10-20 seedlings as soon as some seedlings have 2-3 pairs of true leaves. Then pick the BIGGEST seedling from each variety. Try to re-pot that "early" seedling.
If it is not yet at all root-bound when you try to re-pot it, skip the rest of that variety for another 3-4 days. If the biggest seedling from one variety iIS root-bound enough to pot up, also re-pot any of that variety that are almost as big.
And record how many days it was from sowing to root-bound!
And what temperatures and soil mix and cell size you used.
>> How do you know when they can go out?
I have almost no idea, but where's what I go by:
For warmth-loving seedlings, outdoor soil temp matters.
They have to be big enough to survive the level of insects and slugs you have.
One bite will kill a tiny seedling, but be shrugged off by something with 20 leaves.
You probably need to harden them off anyway. If they wilt a lot their first day outside in shade, bring them back inside and try again a week or more later.
>> I am not quite sure exactly time frames on that part
That's where your notes will serve you well. You can't totally PREDICT since weather and growth rates vary every day. But past experience will suggest the date ranges, and then you can try to predict the weather, and choose whether you want to be optimistic and ambitious and early this year, or cautious.
If you start twice as many seeds as you need, you can put some out early and some out late. Fill the yard and hoe up the ones that are less than ideal, and replace those with other plants. Give lots of seedlings away, especially if you KNOW they will get planted, and some produce or seeds or favors might come back to you.
