Anyone have any idea what is causing this? I’ve looked it up and found that at least for younger saplings. Some Browning can be normal in early spring, but none of the images had nearly this much Brown. There is barely any green left among the needles.
Does it change anything if it only went brown after it started warming up outside and was green all through winter?
That’s what really indicates it was winter kill.
Winter kill is often caused by damage to the water transporting section of the plant (xylem). Think of it as a straw that connects the roots to the leaves. If you make hole in the straw, crush it, etc the water stops flowing. This is what likely happened here.
The majority of the water transporting parts of the stem were damaged. When the weather warmed up, and the plant came out of dormancy, the plant needed lots more water. It’s damaged system couldn’t keep up with the demand and tissues died.
Winter kill is most common because of rapid temperature changes. So a mild winter followed by a sudden polar blast that drops the temps by 50F or more will do it.
Do you know if redwood sapplings can survive in a low light environment like a garage over winter? What would be my best bet of getting one to make it?
Your best bet is to transplant it correctly in the ground this spring and let the plant get established this summer. Plants in pots are exposed much to more extreme temperature changes than if they are in the ground.
You can also cover them with some straw if the weather looks like it’s going for a sudden deep freeze and the trees are not covered in snow.