I have been experimenting with drying field rushes before weaving with them. I dry them out for a week or so in a warmish room and then rehydrate them for a few minutes before using by soaking in the bath. It makes for better cordage, although it is still stiff and inflexible (not to mention hard on the fingers to make!)
For Brigid’s crosses it is much better to dry the rushes first, so that the crosses don’t become loose and fall apart when they have dried.