What causes fret sprout in winter?

Fret sprout is what happens when your fretboard shrinks and your frets don't. Suddenly those fret ends that used to sit flush against the edge of the neck are poking out slightly, catching your hand as you slide up and down the neck. It's annoying, it's common, and it's almost always a humidity problem, not a manufacturing defect.

Why winter specifically

Winter heating dries out indoor air, often dropping well below the 40–60% relative humidity range wood wants to live in. Your fretboard — usually a fairly thin, exposed piece of wood — loses moisture and shrinks slightly across its width. The frets, being metal, don't shrink at all. The result: fret ends that used to be flush now stick out past the edge of the wood.

Is it permanent?

Not necessarily. If the shrinkage is moderate and you get the guitar back into a healthy humidity range, the wood can regain some of that lost moisture and the sprout can improve, sometimes significantly. If it's been dry for a long time, or the wood has shrunk enough to leave a permanent gap, the fret ends may need to be dressed (filed flush) to fix it properly.

The actual fix, long term

Humidify. A case humidifier during dry months, kept in the 40–60% range, prevents this from happening in the first place. If it's already sprouted, get the guitar humidified back to a healthy range first — sometimes that alone solves it. If sharp fret ends remain after that, a fret dress handles the rest.

Sharp frets aren't just annoying, they're diagnostic. If you're seeing sprout, your guitar is telling you it's too dry, and other parts of the instrument are feeling that stress too, even if they're not as obvious about it.