Please help to identify this plant

This plant started growing in my garden this spring & since I didn't know what it was I decided to leave it til I could identify it. (I have added some wonderful plants to my garden this way!).

However it is now 5' tall and although I know I've seen it before (maybe wild?) can't seem to find it on any sites I've checked, & I don't have wildflower ident. book. Yellow flowers have 4 petals and seem to last only 1 day. It has just started to bloom, and flowers remind me of evening primrose.

thanks,

 

Take more pix when it finally blooms.

Oenothera biennis

 

It does look like an Oenothera.

 

Hi,

Here are 2 more pics taken 5 minutes ago showing the blossoms. I checked for pics and descriptions of Oenothera and O.biennis but couldn't find any reference to a plant this big - it is FIVE FEET TALL and about 4 feet around it has many branches which all culminate in these flower spikes. Thanks for your help,

 

It's Oenothera biennis - under garden conditions, wild plants typically get a lot bigger and more robust than their counterparts that don't have the benefit of amended soil, extra water, fertilizer, etc..

 

Here is a wild specimen that I photographed last year that is probably about 4 feet in height, in very poor soil conditions:

 

Thanks everyone for your replies.

I can see now that it is indeed O.biennis, both my neighbour and myself have the cultivated versions in our gardens, but this one is just so much bigger - like it's on steroids or something! Also, I have never seen one that branched out like this one is doing.

Love this site - just joined 2 days ago, but I will definitely visit freqently. I'm going to garden design next..... see you there I hope

Cate

 

The cultivated one in your garden is probably a form of Oenothera fruticosa, a completely different species.