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Yellow leaves can be caused by a number of things. Typically in houseplants, it is either underwatering or overwatering. Water when the top of the soil feels dry, but to do let the soil dry out all the way. It may also need repotting if it's becoming rootbound, though they normally like being somewhat crowded. That said, if roots are too constricted, this would make it more difficult for the plant to take up water or nutrients. You can check the roots to see if this is an issue or not. Root pruning can help: https://www.gardeningknowhow.com/houseplants/hpgen/prune-roots.htm

This article will have some other causes for yellow leaves: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

You will need to have measurements of the window boxes to determine the amount of potting soil.
Your local garden center should be able to help you calculate the amount.

https://www.gardeningknowhow.com/garden-how-to/info/plants-per-square-foot.htm

Here is a link that will help you.

https://www.gardeningknowhow.com/houseplants/ivy/ivy-plant-propagation.htm

This kind of ivy can be used in a terrarium. You don't need to include moss in the soil, but you do want to make sure to have a layer of gravel for drainage at the bottom, below the soil. Fish tank gravel works well for this. It will help with drainage.

I would be leaning towards boiling water. It will kill any vegetation it touches, but not what it does not touch and you can control where the stream of water goes.

Sometimes Ivy struggles indoors when the humidity is to low.
Misting the plant and placing it on a tray of pebbles with water can help.
Trim away the dead material.

Here is a link to refresh you on the care requirements.

https://www.gardeningknowhow.com/houseplants/ivy/indoor-ivy-care.htm