Posted: 16/07/2017 at 17:12
Hi Adey, I've used those size plugs before from T&M and they are too small to risk planting out but are OK if you plant them in 9cm pots and grow them on until they are larger. Some will grow enough to plant in autumn but many would be best overwintered in their pots and planted in spring. The delphiniums in particular should be potted-on at least twice (9cm then 15cm) and protected from slugs, overwintered and planted in spring. Molluscs love them so you need to keep an eye on them until they are a foot or more tall. Good luck and as long as you give them plenty of TLC they should all flower next year.
Lots of perennial seeds can be sown now but the attrition rate over winter is normally pretty high so, again, TLC will be required.