What could be wrong with growing in natural fibers? They break down over time, adding organic matter to feed the seedling's roots, and seem so much "greener" than conventional plastic pots. But we're 6 weeks in, and my tomatoes and peppers are still tiny - we're talking less than 2" tall - all of them, that is, except for one. My daughter started a tomato from seed (from the same packet I used) over a week after I started my seeds, and now hers is about 6" tall and full of beautiful, healthy leaves. I was struck by the difference between her plant and mine, and realized the only difference was the container material; she chose to reuse a black plastic 4" pot from a nursery plant.
Why would the plants in the natural fiber containers be so stunted, so much smaller than the plant in the plastic container? Because cardboard and peat wick moisture away from the soil, and the roots of these plants then dry out far faster than if they were in plastic pots. I watered them once a day, but apparently I would need to water them more often to counteract the wicking action of the peat (and that's more time than this mama can devote to organisms with cell walls!). This includes the seedlings I kept in my little greenhouse outside, which does a pretty good job of containing moisture and creating a humid environment. Even there, my plants needed watering about 8 times more often, if not more, than her plant.
So while natural fibers are...well, more natural, I think from here out I will be sticking to plastic for seed starting containers. In this situation, plastic produced more growth in seedlings, required less watering, and could be used over and over, as the plastic planters are still usable after the seedling is transplanted. My cardboard products will instead go straight to the compost. Do not pass Go, do not collect $200.
“I haven't failed, I've found 10,000 ways that don't work” - Thomas Edison