Acanthus Mollis

I planted an Acanthus Mollis at the back of a raised bed 3 years ago. The soil is free draining but good structure and the border gets morning and early afternoon sun. The plant seems healthy with lots of leaves, yet it has never flowered and there are no signs of emerging flower bracts this year. Last year I was told it could be slugs so I pelleted the centre of the plant this spring. I would love it to flower, please can anyone help. 

Mine have never flowered and I believe it's beacuse they're too happy in my fertile loam soil.

Not far away there are several plants growing in a dry, sunny spot in very poor, thin  soil against a brick gate post and wall at the top of a drive and they flower their socks off every year.  I reckon they need stress to flower so they can produce seed as an insurance policy for survival.

Thanks for replies, and I'm happy that at least Lorelei has flowers as they are so striking. Have wondered about restricting the root system if only to prevent it from invading but after 3 years it might be too late. Have been reading through many gardening books to see if giving them a hard time might work but no-one mentioned it.  A.T. says they take their time to settle in. Will ask the experts at Tatton Show (can't wait love it) and post their view. 

I'm a big fan of these plants even though they are extremely invasive, but don't worry, I'll explain how and it won't sound quite so bad.

Like all plants that can be propagated by roots, that's its main method of invasion. it doesn't run underground like Bindweed but rather, once you have it, you can't get rid of it. Any piece of root left will grow into a new plant much like Oriental poppies, so trying to move or remove the plant will result in a war you will lose. (I'm still trying to get rid of a piece of root I planted ten years back in a border, then changed my mind and every now and then, a small clump of leaves emerge in defiance.)

This plant also likes to set seed, so removing the flower spikes once they go over is a good idea even though they do offer great architecture. I usually do this in autumn when I've had my fill of the spikes. Otherwise just weed around the plant a lot. The seeds are pea size, don't travel too far and as long as you pull them up early they are easy to remove.

As for flowering, I have two established clumps in my garden that flower every year in sandy neutral soil that is probably not especially fertile and I don't often feed them because they never show signs of needing a feed. Morning and afternoon sun are best, they need sun to flower but don't like the hottest part of the day and will wilt no matter how much you water them, but will perk right back up as soon as the sun passes.