Tznius of Long, Loose Hair and Sheitlach
It is written in Shulchan Aruch, “Lo teilachna bnos Yisroel pru’os rosh b’shuk, achas penuyah v’achas eishes ish,” Jewish daughters should not go in the street with their hair uncovered, whether they are married or single. Does this mean that single girls should cover their hair? Some take it quite literally and assume that single girls should also cover their hair. However, the vast majority of poskim reject this interpretation, and the halachah follows their view (that single girls need not cover their hair). One interpretation is that the Shulchan Aruch is referring to currently single girls who were previously married, such as widows and divorcees. Indeed, that is the halachah: any woman who was married is required to cover her hair.
According to another interpretation mentioned by some poskim, the Shulchan Aruch is referring even to girls who were never married: they are not allowed to be pru’os rosh, but in this context pru’os rosh refers to hair that is loose; they have to have their hair tied back. While these poskim do not specify a particular length, it is common practice that hair which is somewhat longer than shoulder length should be tied back, at least partially. According to all opinions, if the loose hairstyle is untzniusdik (immodest) by being overly attention-getting, it is unacceptable. This includes exceptionally long, loose hair, as well as hairstyles that are fluffy, wild, and unkempt. The same applies to a sheitel: if it is long it should be tied back, and most certainly if it is exceptionally long, or gives a not tzniusdik impression. The Midrash tells us that Hakodosh Boruch Hu, the Aibershter, brought Chava to Adam HaRishon only after He had braided her hair. Some sefarim explain according to Kabbalah why it is important for women and girls with long hair to tie their hair back.
ראה ישעי׳ ג, טז וטפוף ובפירש״י שם. שו״ת שבות יעקב א, ג: אם שערותי׳ קלועים. תורה תמימה בראשית ב, כב. שם נשא, ה, יח. ולהעיר ממנהגי מהרי״ל נישואין יא. וראה בן יהוידע שבת צה, א. שם עירובין ק, ב. שם יומא מז, א. שם נדה מה, ב. וראה עין אליהו עירובין יח, א.