I have a very simple logic:
1. If a plant is already branching at lower nodes and it is starting to get taller than I want before moving it outside then I will "prune" it Veronica Flores style. If it's getting tall and there are no signs of growth at the nodes, then I don't prune it.
2. If I feel a plant is still too tall to withstand wind when I move it outdoors, then I will sink it all the way down the the bottom of a tall pot (bury half to 3/4 the height of the plant).
But generally, though, this is the reason why I don't start seeds until the last week in March - I can't plant out until almost mid-June and I want my plants to be as short and fat as possible when I do plant out. Planting in January or February is just asking for trouble, especially with aphids, in my experience.
During previous years, I have started in ordinary, "pre-fertilised" garden soil and not fertilised at all until moving outdoors. I ran 3500k flourescent tubes 16 hours per day. Cotyledons were smallish and usually with a length of 2-3cm from the stalk, turned yellow and fell off within less than 2 weeks. Branching usually did not occur and the plants just grew straight and tall.
This year, I have experienced a different result. I started in a mix of 10 parts "pre-fertilised" coir, 1 part perlite and 1 part vermiculite. I am also feeding a weak solution of FloraMicro and FloraMato with every watering (5ml each into 10 liters water, 10ml each after second true leaves appear). I am running 6400k LED lights 24 hours per day. Cotyledons have grown to be huge with a length up to 6-7cm from the main stalks and are still dark green and showing no signs of falling off even after 3 weeks or more for most plants. Branching has occured at _every_ node (even at the cotyledons themselves) on _every_ plant of every variety I have going except for a few pubes. I still have nearly 7 weeks until plant out, so let's see if they actually get too tall for my indoor space.
Edited by podz, 25 April 2019 - 05:53 AM.