Adwoa and Effia are naturally drawn to each other. Both being African women married to British soldiers, they have a fair amount in common and very quickly establish a close bond of friendship. Sharing a cultural background means that the two women can speak to, and confide in each other, freely.
Though roughly the same age, Adwoa has more experience of life than Effie. She married a British soldier before Effie, so she’s in a position to offer her some sage advice on how to deal with her current situation. One very important piece of advice she gives Effia is to become pregnant to stay at the Castle. And she offers to assist Effia in getting pregnant by giving her a root to put under the marital bed as she and her husband James make love. She also says that it’s vital that James doesn’t see the root.
The implication is that the root somehow has magical properties, and that those properties will be neutralized if James discovers the root’s existence before he makes love to Effia. Adwoa’s advice shows, not just that she’s willing to act as a mentor to Effia, but that even though she’s married to an Englishman she still adheres to certain traditional tribal practices.