Beginner Gardening: Why are my larkspur (delphinium) becoming chlorotic?, 3 by crayondoom

crayondoom wrote:
I planted these a couple months ago and they were doing well. They filled out a bit, went into full bloom and seeded, and now they are yellowing. One stem turned yellow and then browned, almost if it had been directly sprayed with something. They have been getting plenty of rain and have well-drained soil with a slow release fertilizer mixed in. All the other new plants around them are performing nicely. What is going on with these?