What spices go with rhubarb
Quick answer
Ginger, Turmeric, Green cardamom — in roughly that order of how well established each combination is.
Each is explained below, with the cuisines it belongs to and the point at which it stops working.
Why rhubarb behaves as it does
Rhubarb is dominated by malic and oxalic acid and has essentially no sweetness, which is why western recipes bury it in sugar and why that is not the only option. Ginger is the near-universal partner in British and northern European cooking, and it works because its heat reads against the acid rather than against a sweetness that is not there. Persian and Iraqi cooking treats rhubarb as a souring agent for lamb stews in the same slot tamarind or dried lime would occupy, seasoned with mint and turmeric. Both traditions are answering the same fact about the plant.
Classic
Long-established in at least one cuisine — the combinations you can rely on.
Ginger
ClassicThe near-universal British and northern European pairing, and it works because rhubarb has almost no sugar of its own. Ginger's heat reads against the acid rather than against a sweetness that is not there, which is why it succeeds where warm sweet spices mostly do not.
How: Stewed together; In a crumble
Good
Turmeric
GoodThe Persian and Iraqi treatment, where rhubarb is a souring agent for lamb in the slot tamarind or dried lime would otherwise fill. Turmeric is the base of that stew and rhubarb is the acid, which makes this a structural pairing rather than a flavour one.
How: In khoresh rivas, with lamb and mint
Green cardamom
GoodFloral against sharp, and a Scandinavian habit more than a British one. Cardamom suits rhubarb better than cinnamon does because it does not add warmth to something whose interest is its coldness.
How: In a compote or a cake
Cuts and forms
Forced, Field, Stewed, Roasted. These do not all take the same spicing — the notes above flag where the difference matters.