sweet williams

can anybody tell whether in can divide sweet williams and if so how and when

Not really a plant suitable  for dividing-you are better of starting new plants off now from seed for flowering in 12 months.

As sotongeoff says, they are usually treated as biennial.  However, I remember my mother having them in her garden, and not planting freshly each year, so I have experimented.  This year, I have a wonderful display, and they are last year's plants.  I suspect that two years (or three at a stretch) is the most you can get out of them, so I shall renew the plants next time.  But I don't think they would be suitable for dividing.

i planted some from seed this year.. and still they are not very strong loooking at all..

Gardeningfantic - if you set seed this year, then you can look forward to blooms next year.  So be patient, nurture you little plants, even if they seem a bit on the weak side, and, fingers crossed, you'll be well rewarded this time next year.

Perhaps I confused things by referring to my "last year's plants" - by that I meant they had been sown in 2010, bloomed pretty well in 2011 and are going strong now in 2012.  But I wouldn't expect much from them next year.

I bought some sweet williams when they were in flower - four years ago. I dead-head them and feed them - and they are just as lovely again this year!!

All I can say is that I accquired a bare root cutting last year, stuck it in the ground, this year it's goiing to flower. They are tough plants

Perhaps we've been taken in by the "biennial" description.  Mine are absolutely wonderful this year - far better than last year.  I suppose that eventually they do wear themselves out - but these are just a picture alongside my lupins and verbascums. 

@Ross Gravett:  sorry - seem to have rather got away from the original question you posed.  It seems to me that these plants do have more than the traditional one year's flowering in them, so your question about dividing them perhaps needs some consideration.

Not having done it, I can only theorise.  But the good rule of thumb is to remember that plants are at their weakest when in flower if you are wanting to divide/move them.  So I think I would wait until they had finished flowering (and dead-heading them will give many weeks of blooms) and then give them a bit of a feed.  Come the autumn, when the soil is still warm, dig them up, divide and re-plant, watering in generously, but making sure that the roots have good drainage under them.  (Add grit if your soil is heavy). 

It must be worth a try - after all, what have you got to lose?