If its been out all winter, that's your explanation, unless there's no drainage in the pot and its waterlogged, but that usually means you get soggy yellowing of the leaves. I have 3 phormiums in pots, all look quite rough at the end of winter, so in spring I cut out any damaged leaves.

The only other possibility is they've been too dry for too long - I know the weather's wet and cold, but once a pot dries out, sufficient rain often doesn't get into the pot, so check that too, especially if its happening on all of the leaves.
Otherwise, post a photograph of the plant please.

You made me go out to the garden and check. I have 2 Tamarisk, first is 1 y/o and some of the more exposed branches are brown, but not as brown as they were a few months ago and certainly looking greener - I did a feed a few weeks ago. The second was bought bare root last Oct. The buds on that are red-brown. They do flower in late spring - could the brown tips be the flowering parts about to come out? Otherwise I think it may be water (or lack of) as Bamboo has said. Tamarisk are seaside plants so are tough as.

Gosh, ignore my silly reply. I saw Tenax and thought Tamarisk!

I was a bit confused,but it did occur to me towards the end of your answer what you'd done, Kildermorie... done it myself a couple of times!