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I know this is an old thread but that looks like sunburn. Best bet when you see this is to move the plant to a location that gets less sun. Also move the rocks away from the base in this case. If it's in the ground you can put a paper towel cardboard tube around the stalk where burn is until it's fully acclimated.

If not the plant can either fully burn and die or more likely crack and damage the stalk and start what's called air rooting. It also opens the plant to be more prone to disease if it cracks and opens.

Hope this helps.

The rocks that touch the stem got too hot and burned the stem. Take the rocks out and move it to a less sunny location

In my area it rained all day 2 days a go .my perennials perked up but are now turning yellow again! my annuals are doing great. I can’t figure it out ? Feed water or leave them alone?…