Coast redwoods are extremely insect and fungus resistant, hence the old growth wood's longevity (I know of an 80yr old bridge that is still sound, trunks can live 2250yrs). They have only 28 insects and 4 mites that can digest their wood, compared to over 300 species that can consume oak (can't remember which quercus it was compared to). However, the tannins which provide this protection increase in concentration exponentially until the tree is relatively mature at 300yrs.
In nature, the vast majority of seedlings never make the canopy, as they are consumed by elk or are out competed by reiterations (suckers) from established old trees' root systems. Reiterations of preexisting trees represent 98% of the trunks in an old growth redwood forest. A tree gets really established after 300yrs or so in old growth, and this is often the tallest the tree gets before breaking off when it reaches above the canopy, stimulating branching and thickening. That genetic individual redwood tree could live indefinitely (its latin name sempervirens = everliving tree). However, if the mythical Greek Titans teach us anything in how they were destroyed by their offspring, immortal beings need to be very judicious in their reproduction. So redwood seedlings generally only find the conditions to survive where a large tree has fallen. They either root on soil on a nurse log or on fallen giant's root ball reveals bare mineral soil so the deep forest duff cannot keep the seedling from reaching water and nutrients, and simultaneously it needs an almost miraculous fungal inoculation of that soil within a few weeks. The fungi necessary is also associated with red alder and doug fir, which are the primary and secondary succession species in establishing old growth redwood ecosystems, and you may be able to find a plantation of that more easily than it would be to find redwood duff. I remember seeing NW species being grown in Norwegian plantations (near Arendal). I also remember seeing sitka spruce, and would bet it also has some beneficial crossover fungus in its soil as a common companion to redwoods, but wonder if this has been transported in order to grow those trees healthily. I'd try making a compost tea from some of these common companion trees if possible, but I would not do any strong fertilization. Potting up proactively to avoid root bind is also important according to a friend in the park's restoration dept. They need a good amount of water, but also cannot tolerate wet feet for more than 24-36hrs. They really like a daily misting, as in large enough old growth stands they produce visible fog any time it gets hot (i.e. above 68F). This saves them the trouble of transporting that water (upto 500gal/day, equaling 4000lbs) up 300+ft. How's that for too much information!?