I am raising this bed up about 15cm to the top of the sleeper. I would rather not lift and replant the periwinkle if possible. Can I top the bed up with soil and expect the periwinkle to be happy or do I need to dig it out and replant?
I've mulched right over them, but only an inch or so (2 to 3 cm) so they weren't completely buried. I think 15 cm would be a bit much for them to cope with. If I had to do that, I would try to lift up the long stems so they come up above the new surface level. But probably better to lift and replant at least a few clumps to be sure of not losing it all.
South East London
Perriwinkle is pretty hard to get rid of! Dig out a few plants and pot them temporarily . If you manage to kill the others, you can then replace them easily enough.
In London. Keen but lazy.