HELP with encouraging substantial roots on seedlings

[Anthony_Toronto]

Not sure where I have gone awry. I usually try to have larger, more advanced 'seedlings' to at least try to take best advantage of my shorter growing season (I'm just north of Toronto). Several years back I would sprout seedlings, transplant into a small cell/pot when they had a few true leaves, then transplant again to a medium sized or larger pot (up to 8 inches or so). Plants were usually 8 weeks or so old, and 2 to 3 feet tall, with stems about as thick as a pinky finger at the base, and looked good. When it came time to set out, I remember grabbing the plants at the base of the stem and pulling them out of the pot, and having the entirety of the material in the 8-inch pot come out in a ball, all held together by the roots. That is what I am looking for.

However, for the last 5 or 6 several seasons, I have had tremendous difficulty getting any substantial root development. I germinate in seed starting mix, usually with good to great success. Once the plants have 2 (or more) sets of true leaves I transplant, sometimes into smaller cells (like the 4-cell packs that most annuals come in), sometimes to 3 or 4 inch pots, sometimes to larger pots. Sometimes I pot up once from the initial starter cells, sometimes twice, sometimes thrice. I use potting mix, sometimes (always, recently) using moisture control mix, which seems to make watering easier (absorbs well, rather than floating on the surface forever before absorbing). I generally let the pots dry out before re-watering, and water from the bottom when I can, and when I fertilize it is with something like 10-50-10 (varying over the years...sometimes a lot of fert, sometimes very little fert), all of which I understood to be things that would promote root growth, or in any event not suppress it. I've even tried doing some deliberate damage (as well as unintentional damage) to the tap root to encourage more fibrous roots. And every year in the last 5 or 6 years I have had the same result, no matter what I did or changed...crappy roots, with the main roots being maybe 3 inches wide in total (on a 3-foot tall thick great looking green leafy plant), with a few stringy roots that grow out to the side edges and bottom of the pot, but basically fall away when I unpot the plants while setting out in the garden. More or less watering, more or less fertilizer, smaller or bigger pots, more or fewer pot-ups...none of it seems to make any difference.

Don't get me wrong, my plants do ok in the garden eventually (usually), but don't do well with early stress in the garden (presumably because lack of roots means lack of ability to take in enough water/nutrients to withstand a little heat or dry), but I want to once again get seedlings with a a little more of a strong base of roots.

Sorry for the long post...does anyone have any suggestions that are any different from everything that I have already tried? On thing I can't recall is the medium that I used way way back in my pots...but I can't imagine that it was potting soil or there is no way that I would have been able to pull out the entire root ball from the pot by the stem, so it must have been some sort of lighter potting mix (definitely not moisture control though...maybe that is the culprit? controlling moisure so much that roots don't need to branch out? given how much I let the pots dry out between waterings I don't see how). I believe I made some inquiries on this a few years back, but not sure anything that I came away with had any change in results. I have seen older research on how roots change with potting up (branching out more/becoming more fibrous, vs. having a deep tap root for plants without many pot-ups, sowed into field from seedlings for example) so I know what I should be trying to achieve, but its not happening, even with up to 8-week old seedlings.

Thanks,
Anthony