Nasturtiums

Question

There are a few yellow leaves on the plants. They are in window boxes in sun.

Answer 1

It is common for some leaves to yellow on a Nasturtium. They are fast growers and the plant often shades itself. Just pinch off the leaves.
If it is more then the occasional yellow leaf, check your watering.
Too much or too little can cause yellowing leaves.

https://www.gardeningknowhow.com/ornamental/flowers/nasturtium/growing-nasturtiums.htm