By Cornishsally
Cornwall,
United Kingdom 
Taking dahlia cuttings. A friend very kindly sent me a Sarah Raven collection of eight dahlias & I've got them in pots in the spare room at the moment. The growing instructions say to pinch out the tips of the main shoot and to reduce the shoots to five. Please could someone tell me how tall they should be before I pinch them and reduce the shoots? I'd like to try growing the shoots as cuttings. As the photo shows, some are growing much faster than others, though all have good roots. Oh , and if the roots are coming through the bottom of the pots ( they are!) will it damage the plants if I have to break some of the roots to get them out of the pots when it comes to planting them in the garden. Though gawd knows where! I have no space!
28 Apr, 2012
Answers
Thank you so much for such a detailed and helpful reply. Do you think they'd be ok in my unheated greenhouse now (it's very ancient and has a gap where the door doesn't close properly)? I did put them in there when I first potted them up but it was so cold at night and they didn't seem to be doing anything that I decided to bring them indoors.It would be quite a business carrying them all out of the house everyday. The spare room is very light and sunny - I start all my seeds in there. (could this be why no-one comes to stay?!!)At what point should I plant them out? After they've been stopped or is it more to do with the temperature?
28 Apr, 2012
They will be OK in there. Temperatures of 50'f are sufficient for them to keep on growing steadily. As you have a mixed selection there will be a difference as to how they grow, but that one in the 2nd pot back row looks a bit weird:)
Soon as we get a bit of sun they'll start taking off.
Obviously, if frosts are forecast you have to be a bit careful but night temperatures above freezing they'll be fine.
Some of mine are in the coldframe already.
Planting out is done when no more frosts are forecast.
Mine usually go out end of May, but prior to that i plant them in their pots in their final positions anytime from mid May and just keep an eye on the forecast.Yours will be ready by then, some you will have stopped before planting, a few of them look as though it will be after planting. Doesn't matter either way, you stop them when they are ready.
28 Apr, 2012
I put mine outside during the day and just put them on a tray to carry back inside easy peasy!
29 Apr, 2012
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Those need at least another month before stopping, or pinching out the main shoot.
If you want to try cuttings, then what you have to do is take them from the tuber. Your top left pot has about 5 shoots growing. Use a sharp knife to remove all but one of those shoots and let that grow on and eventually plant out. You cut as near as possible to where the stem joins the tuber, just below a leaf joint. They look about the right size......about 3 " is ideal. Fresh shoots will appear from where you have taken the cuttings, but those can be pulled off later as the more shoots you have, the weaker the plant will be, and more difficult to control at flowering time.
Rooting shoots is possible but you won't have time this year.
I would be careful leaving them in your spare room too long. They will get very stringy. If you can, put them outside during the day, bring them in at night.
Time to stop is when they have 4 pairs of strong leaves.
When you stop, a "lateral", or shoot, should appear from each of those leaf joints, meaning you could have 8 laterals. Let them all grow, then choose the best 5 to grow on.
Each lateral will produce flowers.
Wouldn't worry about root damage, at this stage the roots are coming from the tubers, not the shoots that are growing.The tuber is providing all the plant needs for growth at this stage. They will produce their own root system as they grow in the garden.
28 Apr, 2012