As of right now, we are in a position definitively to state that, whatever the (few and cryptic) references mean when they say that mangels are susceptible to frost damage, they DON’T mean in the seedling stage. We have been planting mangels after last frost for years in fear of just that. This year, however, one of our younger farmers put in a bed of mangels in mid-April, a full month before avg. last frost for our area. Eccolo! Not only did the seeds germinate more quickly and consistently than they have in the past, but they weathered many frosts, several of them hard. Coming in so early they were well ahead of germinating weeds, so we were able to see the rows to wheel hoe between them while the weeds were still tiny. Later we did a preliminary thinning with the hoe, and yesterday when we did our final thinning (one or two plants per foot) the seedlings were mostly five or six inches tall and easily withstood the disturbance of losing their near neighbors. Altogether, a big, BIG improvement on a mid-May planting.
So now you know — plant your mangels early.