Is it better to start seedlings in little party cups and then put in them in say 3 gal containers? Or could they go straight into the big ones.
From what I've seen, both methods work. I've asked a similar question before, and if I recall correctly, most people said to start in a party cup and then move it to a 3-5 gallon pot. I've seen more advanced growers start out in large pots, but the smaller cups are just an easier way to keep track of the plant. Personal preference, I'd say.
To add a bit, if you put small plants in big pots, just realize how easy it will be to overwater. You can look at 2-3 weeks for a small plant to use all the water in a big pot.
Main reason is because most people use different soils with seedlings & older plants. For example, you don't want to throw a seedling into Fox Farm Ocean Forest because thats too much nutrients for a baby. Usually starting off in a smaller pot with Fox Farm Light Warrior is your best bet. Or you can start your seeds in Rapid Rooter w/ humidity dome and transplant them after a week or a little longer.
To add, starting in the smaller party cups also promotes a nice root system to be transplanted in to the bigger pots. Roots are encouraged to occupy all the space and use up all the "food" in the 16/32 oz. party cup opposed to growing every which way and wasting "food" in the larger pot. I also think for new growers it helps in avoiding over watering. GL
Different soils? Naw, it's because it's easier to control the watering in a smaller cup. If you have a little sprout in a big 5 gal bucket it takes a lot of water to soak all that soil through for just a couple of inches of roots, and then that big bucket can be wet at the bottom but dry at the top where the roots are. Yes it can be done, but generally easier to manage the watering -- specifically the watering of the soil that the roots are reaching -- in a small pot.
Would starting a seed in ocean forest soil be alright then? Or should I go with pure earthworm castings or ewc/mushroom compost mix? Approximately how long does it take for the roots to fill out the small cups? Also I have heard that some sativas are too finicky to transplant....any truth to this?
You should be OK. For the starter pot you may want to mix that with extra perlite. How long roots take to fill out a small pot depends on many factors, not the least of which is that your small pot may be a different size than mine, but say about 3 weeks from when it pops the soil. As for strains too finicky to transplant -- can't say there are none, but I'm not aware of any and I'd have to say that is mighty finicky. Done right, transplanting shouldn't faze the plant much at all.
i've always read about FFOF burning seedlings at first, but they get over it. I've personally used MG African Violet, and its worked well for me. #2 - too many variables in that one with rooting, but prolly by week 2-3 or their 3rd leaf set...someone could prolly chime in on that one though.