When to plant out rhubarb?

I've grown four rhubarb plants from seed that I started this spring, I now have four strapping plants in 2 litre containers in the greenhouse. Does anyone know when would be the best time to put these in the ground? 

Late autumn or early spring. Prep the bed well with lots of nice rich organic material.

Okay. Bear in mind that, grown from seed, they're usually a year behind planted crowns.

Rhubarb crowns (bought) go into a well prepared bed in March and any flowers taken off then, you are looking to the following year for the stems.
From seed they will take a lot longer to harvest stems and I would put them in now.
We used plenty of manure or good compost and bone meal as well, they are hungry plants. A bit of shelter is good say near a wall or fence, they want to be around three feet apart.
We had long rows of the plants so forced some each year by putting straw round the crown and a bucket over the top, it was usually the first garden fruit we had hence the many rhubarb recipes and the annual glut of rhubarb and custard with school meals, I loved it a lot did not.

Frank.

Just seen the last two replies, many thanks Italophil and Frank, I've just taken on a half plot allotment so I'll follow your advice and plant them there now (once I've cleared the weeds!) with plenty of organic matter added to the soil, not much shade available though, maybe I'll plant one in the garden in a semi shaded spot and see which grows better.

Unless it gets blindingly hot, I'd go for the full sun. It needs plenty of sun.