Summer Cabbage Shoots turning Brown

A friend has sown some summer cabbage "Primo " and pricked out into 2 x 2 inch cells to grow on and placed outside down in sunny Plympton Devon
They are only 1 inch tall but the leaves are turning brown at the tips
? Any suggestions friends
Everyone is just trying to be Happy.....So lets help Them.
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(I've started to harvest my first 'Primo' but then I did start them off in March! Smug smug!)