I have started a couple different types of seeds, which include, tomatoes, celery, pepper, onion (walla walla), and some other things. I have no idea when to transplant these plants from my plug tray, which have 128 cells (each cell being only about a 1" by a 1", ~2" deep) to a larger tray with 36 cells. I am sure that each plant has its own time but what is a good rule of thumb, if you will, to know when to transplant these young plants? I am asking because I want to avoid breaking them because I tried moving them to soon, but not that they are getting to large and crowded.
What is the first day of planting in your area where frost is not going to happen again until fall?
I would up pot into your 36 cells (2")? then plan to up pot into 3 and 4" pots. That should give you enough time to acclimate and get them into the ground in the garden's beds. Not sure what your last frost date is...mine is Mother's Day plus a month for our elevation. Two years ago we had a freeze every single month during the summer. Sigh. Have sprinklers on a thermometer driven solenoid. When that temperature drops, the sprinklers come on. This is just before the sun comes up. Plants cells will freeze. What causes most of the damage is not the freezing but the fast thawing when the sun comes up. During that time, sprinklers will slow the thawing and prevent rupture of plant cells.
I love floating row cover/reemay. Works so well to extend the season. Just have to use it. I acclimate my starts before planting in the garden soil (my garden is covered by a green house but still need to acclimate starts before planting in garden soil, use reemay vigilantly) and then I cover the newly planted starts until growing vigorously. Or know that the temperatures are predictable, no freezes.
I've read differing figures but that cloth stops the temperature from descending too close to below 50 degrees F. If the air temperature is 30 degrees F this cloth will keep the temperature 10 to 20 degrees warmer beneath the cloth. Seeds do not like cold soil. Below 50 degrees F. But if the day temperatures are between 60 and 70 that will warm the soil and if that soil is double dug, fruffed up, has DOM incorporated, it will hold that heat. Far better than cold, non tilled, compacted, wet, clay soil with freezing temperatures that will kill baby foliage. I especially group my potted plants together in my big greenhouse during the fall and wrap them with reemay as groups. Potted plants have roots susceptible to too low temperatures. They are more fragile than the plants in the ground. Those roots are protected. But the top growth needs protection from a snap freeze...especially in the mornings just before the sun rises.
My rule of thumb for up potting is to watch for the roots coming out of the bottom drain hole. When you see those you up pot. Squeeze the bottom of the cell to push the plug of soil and plant out of the soil. Do not pull on plant, especially at this size. Always use only potting soil for starting plants and all plants that need to live in a pot.
You could also start acclimating them to the garden and provide a mini green house with milk or water plastic bottles. Still using the reemay on top of all IF there is any chance of freeze...32 degrees Fahrenheit. Or less during the mornings just before sunrise.
Hope this helps. What has been your experience in the past? I heartily recommend double dug raised beds with little trenches. Keep paths compacted, never rototill paths. Have you done that mason jar test to see how much clay is in your soil?
If the weather is too iffy, your plants can last longer in their little pots than you think. Always be more paranoid, so sad to raise starts to then plant them and have them killed by a freak freeze. Call your Cooperative extension service, they KNOW the answers to your questions for your soils and your micro climate...Wisconsin?