I think you can plant them now. Smaller plants recover quicker to 'transplant shock'. They need to send down a long taproot as well. They won't dry out so fast either if planted in the ground.

Just beware of slugs - they do like lupins and the younger the better.

This website shows how to deter slugs with aluminum foil & various other methods. Take a look...

Six Environmentally Friendly Ways to Stop Slugs!

http://the-frugal-graduate.blogspot.com/2012/06/six-environmentally-friendly-ways-to.html

I have to agree with Arbuthnot, if you plant them out being so small the slugs and snails will quickly crop them off. I would pot them on into 9cm pots and then into one litre ones. When they are about 9 inches high, then I would consider planting them out. If it becomes too late in the season and you have somewhere to over-winter then they will sprout as a lot stronger and bigger plants next year.

Thank you so much, I did think it's a little late as my other Lupins are now gone over.

If the slugs are as bad as it sounds in the UK, I would wait also. They aren't quite so bad here in NY - something gobbles them up in very short order. Your Lupins do need to establish a good deep root system before winter sets in or you may not see them next year. They are short-lived perennials.

which Lupin are they? I grow an annual one called pixie and they grow direct in the soil. and yes slugs etc are a problem and the 'Emerald isle' will be perfect for them with all the rain you get over there :o)

Charnwood, why not cut the old flower stalks down to ground level and they should flower again; perhaps not quite as large as the first flowers but will still be worthwhile?

I also grow pixie, but I keep it in pots now having had the whole lot taken by slugs last year. I tend to agree with those who say keep them in pots a while longer. The bigger they are when you plant them in the ground, the better chance you have of getting flowers. If you plant them now...good luck, its a war out there! I’m ashamed to say This year I have used pellets. I’m so sick of having my money wasted. I can put a 1L pot grown plant in the ground and the next day...all gone. Its infuriating.

That sounds horrible! I had no idea the slug situation was this extreme in the UK. My lupins self-seed and come back year after year.

Oh, its bad. I reckon its because we are surrounded by monoculture. They can smell my garden for miles around! At night, sometimes I swear I can hear the sound of slithering!! ;)

And nothing gives me more pleasure than the sight of a big confused load of slime around a pelleet....death throes of a slug.