Where are you planning to keep them Donny? How much they grow and so need potting on will depend to a large extent on their growing conditions. More info please.

I've done this with delphinium plugs in the past. I potted them into a 9cm pot in the October and in early March when they started into to decent growth and roots were just visible out the bottom of the pot I moved them into a half litre pot. I then hardened them off for a couple of weeks in April. Outside during the day and back into the greenhouse/ cold frame over night. they were then planted out late april. Watch out for slugs at this time of year.

welcome to GoY and ask as many questions as you want. we enjoy trying to help. :o)

Greenhouse and coldframe are the thing though - just wondering if Donny is new to gardening whether he has either or is hoping to raise them in the house.

Thanks, Steragram. I intend planting them out in a border in front of a west-facing fence. The soil is quite crumbly and I will be applying some horse manure early next year.

Thank you also, Seaburngirl, that's excellent information. They should look great amongst the hollyhocks that I intend to plant out in the next few days!

yes if you don't have either then a cool garage/shed unheated porch/conservatory unheated spare room will all do as well. but if they have leaves [they may totally die back] they will need some light.

That's what was worrying me - without plenty of good light but in a warm room they would grow thin and straggly, and become weak. It should be fairly obvious when they need moving to bigger pots. Watch out for slugs when you finally plant them out! They can disappear overnight.

Thanks again, Steragram and Seaburngirl!

At the moment I am intending to grow the delphis outdoors in a border, but the possibility of growing them indoors rather appeals to me as a future 'project.'

The plugs that I bought online a couple of weeks ago are now about 5cms tall and looking quite healthy in the conservatory, where they will remain until next March/April. As recommended I will then transfer them to a half litre pot.

As a matter of interest, why can't the plugs be grown immediately in the larger pots? Is it because younger plugs fail to mature in this size of container.

Thanks, again, for your help in this matter. ?

the compost holds onto the water well in a pot so it gets very cold and wet and the roots don't flourish as well. in the open ground the soil drains better and holds less water so there is more oxygen for them.

Thanks, Seaburngirl. Can the plugs be planted directly into the larger pot, rather than start them off in smaller ones?

its better to move them up pot sizes as excess compost will be cold and wet. chose a pot that has about 2-3" all around the sides of the plug as the biggest one to use first off.
then check them for moisture, don't let them dry out and don't be tempted to stand them in a tray of water as they will rot off.

I enjoy potting on, I find it therapeutic.

Thanks, again, Seaburngirl. That makes sense........even to my non-gardening mind!

Donny growing them to maturity in the house would be quite a challenge, especially if you succeeded in getting them to flower, as they can be at least four feet tall then...Probably not a very good idea!