Frost hardiness in Leeks

This top picture is a modern variety Atlanta which so far is well able to cope with our colder than usual winter. Lows of -15c overnight
The older variety Mussleburgh grew well enough in the summer but was still behind Atlanta, but the big failing is the lack of frost hardiness in our coldest snaps. The outer leaves are showing signs of damage and on the larger plants the stem is starting to slime off. At best this slows down harvesting but at worst makes the plant unsaleable.