What size pots @bluehamster1968 ? Can you see roots coming out of the bottom, or are they fairly young plants ? If you could give the varieties and also whereabouts you are in the UK, that will help with advice
They're fairly young(they were only delivered less than a week ago,and they were "plug plants").I don't know the size of pots they're in,but originally the pots were used for full size peonies,so they're at least 6 inches tall.They are the "ice cream collection" from Thompson Morgan,and they now have at least 4 inches of compost between the roots and the bottom of the pots.I'm in Suffolk.
You need to put the plug plants in a pot a little bit bigger than the plug and keep potting them on until they fill a pot about 5” diameter, that will probably take you to next April then you can plant them out. Make sure you untangle all the root ball of plug plant and spread the roots out when you first pot them up.
Gardening on the wild, windy west side of Dartmoor.
Personally, from your description @bluehamster1968 , l'd be inclined to keep them in a coldframe over Winter (if you have one). If they're not bursting out of the pots l wouldn't feel comfortable planting them out. It may be that they're not quite ready for the big wide world just yet.
I think they're the wider leaf type of Penstemon (as opposed to something like "Garnet"), so they may not be quite as tough. As ever, so much depends on the type of Winter that we get, and l know your part of the country is generally on the dry side, but a Beast from the East might not help. Other forum members might say differently though
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If you could give the varieties and also whereabouts you are in the UK, that will help with advice
I think they're the wider leaf type of Penstemon (as opposed to something like "Garnet"), so they may not be quite as tough.
As ever, so much depends on the type of Winter that we get, and l know your part of the country is generally on the dry side, but a Beast from the East might not help.
Other forum members might say differently though