Very short echinacea

Hi,  I have got two echinaceas in my 'sunny' border (although the border only actually gets sun about 50% of the day).  They are supposed to be the same type of echinacea and the flowers look identical.  One is about 120cms high - like it is supposed to be - but the one the other side is only about 30-40cm.  The tall one is surrounded by lupins and the short one has got a lavender next to it and a large jasmine behind it with lupins on the other side.  I have fed them both equally this year with a diluted seaweed feed plus put chicken pellets around them both.  Any ideas why one is so stunted please?  Thanks 

You have answered your own question

If both plants are identical then in ideal and similar conditions they would grow identically

In a garden situation that is not the case,different levels of light, sun/shade and then different competition for water and nutrients.

That is what you have got-but once again are both plants identical?

I love and grow lots of echinaceas ...ESP the newer varieties. They are temperamental things and I have had experiment with them to get them to flower. Although prairie plants they need water and need to buid up good root system, I like to grow them in groups but always they differ in size and vigour. I had sundown like this but only one plant had proved reliable and the others v poor. are you growing older or new varieties? They do not like competition either. I now grow echinaceas in pots to make large plants cutting off flower spikes before planting out

Thanks gents - both of them are supposed to be Echinacea purpurea 'Magnus' although I am wondering whether I accidentally got sent one of these and one of the standard Echinaceas, which are supposed to be substantially taller. Not sure whether these are old or new varieties? Even the smaller one, though, is supposed to be 1m rather than 30cm.  The small one is by a large Jasmine which I actually thought was dead as it got fooled by the warm weather around Xmas time into thinking it was Spring, put on loads of leafs and then all of them got killed when we had the really cold weather in March.  Amazingly though it has come back strong and even flowered this year.  Perhaps it is taking a lot of the moisture and feed out of the soil leaving the Echinacea on that side starved, although the lupins and lavenders around it are all the same size as the ones on the other side of the border.  Or perhaps as Christopher says, it is just temperamental! 

I think your tall echinacea is tall because it may be struggling to get light if it is surrounded by lupins etc. is it a bit straggly? Magnus is a good older variety, supposed to be reliable etc. I grow a variety called tiki torch..single orange flowers and right now it is a fantastic sight next to blue salvia to one side and a ceratostigma on the other. I do think cutting it back mid season makes it produce a big, bushy, full of flower plant now and to late autumn. Other varieties are looking good too now but they are infuriating....or were. Back to your Magnus plants lunarz feed, water, mulch and remove spent flowers and stems ASAP and I THINK they will be fine