Saskatoon pie and crumble, however, pose two problems. First, the berries are relatively low-moisture, with pronounced pips and skins. When you cook them down with sugar they don’t ooze moisture like most other fruits, so they don’t produce cohesive pastry fillings without the addition of water, which simply dilutes the flavour of the berries. Second, they are low-acid when ripe, and on their own don’t make well-balanced fillings.
Rhubarb solves both of these problems. When cooked down, most rhubarb varieties are fluid, and help make saskatoons into a cohesive pastry filling. Rhubarb is also crazy tart, balancing the sweetness of the saskatoons.
A felicitous match.