In Brief: Does Ashwagandha Prevent Premature Greying of Hair?
The scientific evidence reporting the significance of ashwagandha in reducing premature greying of hair is lacking. Several studies showed that ashwagandha possessed antioxidant properties. It is claimed that ashwagandha may reduce free radicals that cause premature greying. Also, few studies suggested that it may increase the production of hair melanin and thereby may play a role in controlling hair greying.
Hair greying is an age-related feature that is associated with a gradual loss of pigment, melanin. It may also occur due to the depletion of melanocyte stem cells in the hair bulge. 
Sometimes premature greying can occur without any underlying mechanisms. There is evidence that oxidative stress may play a role in hair greying.
Ashwagandha possesses potent antioxidant properties that help in protecting from cellular damage caused due to free radicals. It, therefore, may reverse premature aging.
A study showed that the major component of ashwagandha, withanone showed anti-aging properties by protecting against oxidative damage.
Another study showed that ashwagandha increased the levels of natural antioxidants. By improving the activity of free radical scavenging enzymes, it may reduce oxidative damage.
Few studies have shown that ashwagandha stimulates the production of hair melanin.
A review article cited a study where 3g of ashwagandha root powder given daily to healthy males for one year showed a significant increase in hemoglobin, RBC, and hair melanin.
In the same review, another study showed that 0.5 g of ashwagandha administered in the dose of 2 tabs three times a day with milk to volunteers for one year had an increase in hair melanin compared to the control.
Some studies showed that ashwagandha also helped with reducing hair loss and promoting hair growth activity.
A case study of 57-year women with a chief complaint of scalp hair loss and burning sensation showed that over eight months of a standardized preparation of ashwagandha reduced scalp hair loss and previously elevated adrenal steroid metabolites.
Another in vivo study showed that ashwagandha formulation showed hair growth activity in Wistar rats and was well supported by the follicular study.
As there are only limited laboratory studies and experimental evidence on the effect of ashwagandha on premature greying, more investigations are warranted in the future.