Q.lily of the valley
Why do my lily of the valley plants turn brown on the edges during the summer? They multiply and have lovely flowers in the spring, but when the weather turns warm, the leaves turn brown on the edges.

They may need more moisture during the hottest part of summer. They prefer a moist soil.
Here is a link to refresh you on the care requirements.
https://www.gardeningknowhow.com/ornamental/bulbs/lily-of-the-valley/growing-lily-of-the-valley.htm