Hi Penelope! Welcome to the forum!
My first thought is that they're being over watered. Then I read your post and saw that you said the top is dry. Hmmmm....
How is the drainage from the pot they're in? If it's not draining well, then when you poke your finger through it'll be soggy on the bottom, and dry on on top. Nasturtium have pretty extensive roots. They fill a pot really fast, especially if there are lots in one pot. So if the bottom of the soil is soggy, but top is dry, your plant will look very much like your pictures. You can remedy that by creating more drainage in the bottom of your container. They like a little bit of abuse!
If it's NOT soggy on the bottom, then you may not be watering them enough! They need about an inch per week.
They really don't like hot hot weather. I direct seed mine out in the garden. Here in SC, the temps get brutal. Mostly staying in the mid 90's, and the nasturtium DO NOT LIKE IT! I plant them out under the canopy of squash and zucchini plants. The squash plants get HUGE and offer shade during the brunt of the misery of our summers. The squash vine borers do their yearly damage and the weather starts cooling some by the time I have to pull the big canopy plants. That's when the nasturtium really start looking amazing. By fall they are big and lush and full of blooms and seeds! I give them away to my favorite local farm to table restaurant! They like the cool! They hate the hot, so heating them up like that isn't likely to make them very happy.
Most of your plant still looks good though, so it may just be the change in environment.
Lindsay
