I have horsetail plants in planter boxes outside my southern windows in Seattle. They did great for a while, but over the last year, they haven't looked so good. As you can see in the first photo, most of them have gotten pretty yellow. Last Spring/Summer, I barely got any new growth. This Spring, I was excited to see lots of new stalks coming up. But as you can see in the second photo, the new stalks have started to turn white and die :( They are in regular potting soil which I keep moist.

I can't find anything online to help me diagnose the problem because every source just talks about how resilient they are, how they grow like a weed in pretty much any soil, in varying light conditions and moisture levels. The only difficulty I've read about people having with Horsetail is how to contain it. I'm not a plant novice and I have lots of healthy indoor and outdoor plants. I'm perplexed by this typically super hardy plant. Any insight would be much appreciated.