I have multi sown leeks, onions and lettuce growing in my greenhouse which I sowed on 17th February.
How do I decide when is the right time to plant them out? Do I wait until the roots have filled the modules?
It may be soon depending on extra warmth, module size and compost used, but no rush in view of the weather.
Yes roots should be holding the compost together.
For leeks you are risking bolting by sowing this early, I recommend early April.
This video shows planting size and you don’t want to plant smaller than the ones I show, also it was a warm spring.