Sulphate of Ammonia

Sulphate of ammonia is a nitrogen-only fertiliser and won't be much help to plants which are suffering from the cold (unless you find a way of setting light to it!  )

Although nitrogen is the first nutrient to leach away in high rainfall it's not the only thing plants need - I'd suggest a dressing with a balanced fertiliser, or more fast-acting, a watering with something like Miracle Grow, Phostrogen, or comfrey tea to make sure they're not having to battle with starvation as well as hypothermia.