@floral_uk z.8/9 SW UK It did not flower yet.

After investigating, it looks like the drip irrigation stopped working. So I need to manually water heavily for a while.

Unfortunately, this plant continues to head towards death. Maybe it dried out completely at some point so my intervention to water more was too late?

Fasciated plants are often weak and short lived ime. And I still think you may have been over-watering in the first place. I'm not surprised it's dying.

All the other Echium pininana in that planter were showing water stress and started to grow after I corrected the water problem. But it does look like the fasciated plant was extremely weak.

Could be heat stress more than watering. Echium pininana hates dry hot air and sudden temp spikes. In my garden in Kent, they wilted fast around 30°C even with moist soil. Try shading it a bit during peak hours and mist the foliage early morning.

@Elena Zzzz It's interesting you said that, because the two plants that appear to have died are the two with the most direct sun exposure. They did perk up if I overwatered them on a hot day, but there was no way I was going to remember to do that often enough.

But to @floral_uk z.8/9 SW UK's point, the fasciated plant did seem genetically weaker.

The plant to the left of the fasciated plant has become an enormous monster. It's starting to look like a hedge and hasn't even gone into the flowering phase yet. It might well end up getting to six feet tall before it starts to flower. Next Spring is going to be a big show.

@Elena Zzzz I wonder if your observation on wilting in dry heat might be an argument for putting some boulders at the base of a large Echium, on the sun side? That should create much cooler shaded conditions for the roots on the sun side.