#there's a popular fanon that Lucille's hair was cut off in the asylum and she resolved to never cut it again after that

[marzipanandminutiae]

Did the actresses in Crimson Peak wear wigs/hair extensions or was that their actual hair?

Wigs!

Jessica Chastain is a natural redhead and wears wigs for most of her non-ginger roles (interestingly, so is Hiddles, but he dyed his actual hair for the part). Mia Wasikowska, though she goes blonde a lot, naturally has brown hair. And the reason they didn't both just dye their hair is probably the same reason 99% of actresses in period dramas use wigs and hairpieces.

Namely, western women pre-1920s usually had much longer hair than their modern counterparts.

Lucille's hair is actually floor-length, though it's difficult to tell because you only see it down for a hot second and at least part of it is usually blocked from view. But her braid is almost down to her knees- and braiding takes up a significant amount of length.

(From a deleted scene. Lack of chemise notwithstanding...Very Yes.)

Edith's hair is slightly less Rapunzel-ish, but still at least to her waist:

For two actresses in modern Hollywood to both have such long hair- and unlayered, too -would be very unlikely. I imagine that's why they, like most women acting in period pieces, wore wigs

Jessica Chastain actually talks a bit about her "down" wig, which was apparently pretty uncomfortable, in this interview (caution: slight inaccuracies in a discussion of Elizabeth Bathory, though I wouldn't expect her to know better since everyone believes the "bathing in blood" myth).