Joe - can't say I have had any greater success having been at this for much longer than you. I have not had good luck keeping things in pots and I made lots of mistakes in moving things from pots to the ground. I too have had significant losses trying to overwinter things in pots outside unless they were protected from winter wet - have to plunge the pots to minimize freeze thaw cycles and cover to ensure they don't get too much moisture.
There are some critical factors for seedlings survival - two mentioned already were temperature & moisture. An additional one to consider is aeration. I was talking with a long time lily grower and he was convinced that we smother some seedlings in the standard potting mixes as the mix consolidates after a short period of time. Most potting mixes are primarily peat based and as it breaks down it compresses and it becomes more difficult to control moisture. Is the affect of being too wet just preventing oxygen from getting to the roots? I did some more looking into this and the nursery trade uses of different potting mixes. What I found out was that the longer the plant was expected to stay in a pot, the higher the fraction of shredded wood/bark that was used. The wood/bark helps maintain the structure and porosity of the growing media in the pot.
Since learning more about this, I have started incorporating mulch into the soil mix where I am planning to grow things in pots. I grow a few tomato plants in pots and found it much easier to control the moisture for them when I add mulch to the mix. I can water every day and they don't get too wet as it drains very freely. The downside is I need to fertilize more as it would just get washed out with frequent watering.
Growing things in plastic pots doesn't help with aeration or moisture. I hadn't really thought about the merits of using terra cotta pots until I saw a recent discussion on this and that the pots are porous. I have never cared for terra cotta pots and avoided them as they were expensive and fragile but am not reconsidering they likely have value for more sensitive plants?
My biggest challenge with seedlings has been transplanting them to the garden and the losses I see there. I was looking at things from a farming background and thought that pulling seedlings apart and growing them spread out would be the best to ensure they have room. I have painfully learned that lilies prefer to be crowded and do best when they are. This changes everything about how I was approaching things and hoping this will improve my success rate.
The other challenge with transitioning from pot to garden is the difference in soil types. I use mostly potting soil for seedlings and then transition them to the garden where they in a sandy loam soil. The two types of growing media are different and depending on how the seedlings are planted can cause other issues. Bob Griesbach was a big proponent of using garden soil to start seedlings. He would fill rigid plastic flats with garden soil and plant his seeds right in the soil they will be in the rest of their lives (if they survive). He would start the seedlings in these trays in his greenhouse and leave them there all the way through the summer (where they saw really high temps - this was to weed out the weak ones that couldn't take heat) and plant them out in the fall. He would dig a hole that would receive the entire contents of the flat and drop the whole flat in the hole trying not to disturb things more than necessary. This works for robust growers like his own OT's, Aurelians, & Asiatics but not sure how well that would work for trying to grow some of the more delicate species?
So my current process is to start all seeds in 4" square plastic pots using Promix. I planted seeds very thick with the intent to have them a bit crowded in the pots. I germinate the seeds in plastic bags before moving them to the pots. I will plan to plant these out between now and the fall - some pots have grown so rapidly they I will likely need to plant out in the next month where other will remain under lights until fall when I will plant them out. I will be planting them as a clump and trying to minimize disturbing of the bulbs/roots as much as possible. This is for Asiatic/Aurelian/Trumpet crosses. For Orientals I am utilizing my refrigerator to allow me to give the plants a cold cycle - planning on 12 weeks cold and then a grow cycle under lights and then repeat until they are big enough to make the transition to the outside seedling bed. I will need to time this to ensure it links up to our growing season.
Keep us posted on your progress!
