Hi! am based in london, and have just got some redwood seedlings germinating. Any advice on humidity needs as they grow? is it worth keeping them inside whilst they're germinating / growing or leaving them outside with seed tray lid on?
There are 3 redwood species which the name "redwood" can refer too. They are all related, but have slightly different horticultural needs. All 3 will likely tolerate UK outdoor humidity with no problem.
Sequoia sempervirens - this is the coast redwood. It has foliage somewhat reminiscent of yew foliage, perhaps a bit softer and finer. This tree needs a potting media that never gets completely dry. The habitat is lower elevation, may get cold enough to get some snow in winter but the ground never freezes. They appreciate full sun, though can survive with some shade.
Sequoiadendron gigantea - this is the giant redwood of the Sierra Nevada mountains. It is more cold tolerant than the coast redwood. The foliage is somewhat similar to a scale leaf juniper or a Cupressus type cypress. The giant redwood tolerate a dry, more mineral soil. They do like water. All 3 redwood, especially as larger trees absorb a significant amount of their water needs through the heavy night fog, absorbing the condensing fog directly through their foliage. The Sierra Nevada giant redwood does prefer full sun, and is shade intolerant. This redwood comes from the lowest humidity habitat of the 3 species. It is not a desert habitat, it is a montane climate.
Misting foliage of all 3 redwoods is helpful especially on hot, low humidity days.
The third species is Metasequoia glyptostroboides - the dawn redwood. These come from river floodplain habitats in China. This species probably comes from the wettest, most humid habitat of the 3. The floodplains are very similar to the habitat of the North American bald cypress species Taxodium. They can grow with their roots flooded for several months every year. These are deciduous, dropping foliage in autumn. They are also very, very cold hardy. Surviving -30 C in winter with pots simply set on the ground for winter and loosely mulched (my personal experience). Superficially this redwood resembles bald cypress more than the other two species.
No matter which species of redwood you have, germinating seedlings should be outdoors, in full sun, or at least a half day of direct sun. All three will tolerate low humidity during the day. All three need moisture, or a potting media that is not allowed to dry out while they are seedlings.
Indoors the light will be insufficient for good growth. Indoors growth will be weak and slender, with long internodes (spacing) between branchlets and needles.
So if your seedlings are not outside, I would get them out as soon as possible.
All three of these trees are used in parks near me in Luton should be OK in a pot. I only have Dawn redwood & bald cypress in pots they grow well unprotected. The coast redwood and giant redwood I would protect from hard frost if its in a pot