Spray the plants daily with a mild fertiliser solution. I use terrestrial fertiliser like miracle grow with a little aquarium micronutrients mixed in. (terrestrial plants need more of the macros, and miracle gro is full of them. Make up miracle grow 1/10th Strength in a spray bottle, and add a couple drops of flourish Comprehensive. Should be plenty. You don't want puddles to form in the substrate, but you do want it really wet. Mist it thoroughly every day (or every other day) and keep it sealed up.
If the plants seem to die, don't get discouraged. Just keep doing the same thing. (maybe take a sample out and place in another tank just in case as a 'backup') My hairgrass and crypts all dissolved completely at first, and several days later sprouted back from the roots. One of my echinodorus is even trying to figure out whats going on. (the leaves died, but nearly 2 weeks later I see some tiny leaves growing out of the rosette)