Three pounds of fruit, an all-butter crust and one trick borrowed from jam making: this blueberry rhubarb pie cuts into clean wedges instead of collapsing into purple soup on the plate.
It took me two tries to get there. The first blueberry rhubarb pie tasted wonderful and ran everywhere, because raw rhubarb carries far more water than it looks like it does. The second one cooked half the blueberries down first, and that single change fixed the slice, the color and the flavor all at once.
- The filling is cooked and cooled before it ever goes into the crust
- Sharp rhubarb against sweet blueberries, so neither fruit takes over
- A thin layer of jam on the bottom crust, which is the anti-sog insurance
- Orange zest and nutmeg, the two additions that stop it tasting like jam
- Fresh or frozen rhubarb both work, so it is not only a spring dessert
- The whole pie freezes baked or unbaked and keeps for three months
A blueberry rhubarb pie comes down to water. Rhubarb releases a great deal of it, and every decision below is about getting that water out before it can turn your bottom crust to paste.
- What Rhubarb Is, and Which Stalks to Buy
- Ingredient Notes
- Why This Blueberry Rhubarb Pie Cooks Half the Fruit First
- How To Make Blueberry Rhubarb Pie
- How To Tell When the Pie Is Done
- Make It Ahead and Freeze It
- Storing Blueberry Rhubarb Pie
- Variations and Substitutions
- What to Serve with Blueberry Rhubarb Pie
- Blueberry Rhubarb Pie FAQ
What Rhubarb Is, and Which Stalks to Buy
Rhubarb is a vegetable that the United States legally reclassified as a fruit in 1947 for tariff purposes, which tells you most of what you need to know about how confusing it is. You eat the stalk. The leaves contain enough oxalic acid to be genuinely toxic and should always be cut off and thrown away.
Color is not ripeness. Deep red stalks and pale green ones are different varieties, not different stages. Red looks better in a blueberry rhubarb pie and green tastes exactly the same, so buy whichever is firm.
Firm is the only test. A stalk should snap rather than bend. Limp rhubarb has already lost water and will give you a stringy, hollow texture in the finished pie.
Thin beats thick. Stalks thicker than about an inch get fibrous. If yours are heavy and coarse, peel the outer strings off with a vegetable peeler.
Frozen works. Frozen rhubarb goes straight into the pan without thawing, and this is one of the few recipes where that costs you nothing, because the fruit gets cooked anyway. Add five minutes to the reduction.
If you have more rhubarb than pie, freeze the rest. The National Center for Home Food Preservation recommends heating rhubarb in boiling water for one minute and cooling it promptly before packing, which keeps the color and flavor from fading in the freezer.
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Ingredient Notes
A blueberry rhubarb pie has a short list and no unusual shopping. Exact amounts are in the recipe card below.
- Rhubarb: a pound and a half, sliced about half an inch thick. Thicker pieces stay too firm against the cooked berries.
- Blueberries: a pound and a half, and half of them get cooked down into a jam. Frozen berries work and want no thawing.
- Granulated sugar: a cup and a quarter, split between the saucepan and the bowl. Early rhubarb is sharper than late, so taste and go up to a cup and a half if it makes your jaw ache.
- Cornstarch: whisked into cold water first, then cooked into the hot berries. Never tipped in dry, which gives you lumps you cannot whisk out.
- Orange zest and juice: the one thing people cannot identify. It lifts the rhubarb and stops the whole thing reading as generic berry pie.
- Cinnamon and nutmeg: a quarter teaspoon and an eighth. Any more and you have made a spice pie.
- Blueberry jam: two tablespoons brushed over the raw bottom crust. It seals the pastry against the filling and is the least famous good idea in this recipe.
- Butter and shortening: butter for flavor, a little shortening for a crust that stays tender. All butter works if that is what you keep.
A deep 9.5-inch pie plate, glass or ceramic. Three pounds of fruit will overflow a shallow 9-inch tin, and glass lets you see whether the bottom has actually browned.
Why This Blueberry Rhubarb Pie Cooks Half the Fruit First
This is the part that separates a good slice from a puddle, and almost no blueberry rhubarb pie recipe bothers with it.
Half the blueberries go into a saucepan with sugar and cook for fifteen to twenty minutes until the volume has halved. What leaves is water. What is left behind is a thick, dark, intensely flavored jam that already contains all the sweetness the pie needs.
The cornstarch slurry then goes into that hot jam and cooks for a couple of minutes, so the starch is fully activated on the stove rather than gambling on the oven. Starch that has not reached a boil stays chalky, which is why so many fruit pies taste faintly of raw flour.
Only then does the rest go in: whole blueberries, raw rhubarb, zest, juice and spice. The rhubarb still cooks in the oven, but it is now suspended in something already thickened rather than swimming in the liquid it is about to release.
Cool it completely before it goes near the crust. Warm filling melts the butter in raw pastry on contact, and that is the actual cause of a soggy bottom far more often than baking time is.
How To Make Blueberry Rhubarb Pie
Make the dough first, because a blueberry rhubarb pie is really two waits: half an hour for the pastry to chill and about the same for the filling to cool.
- First, pulse the flour, salt and sugar in a food processor, add the diced cold butter and shortening and pulse to the size of peas, then drizzle in ice water until it just gathers. Split it into two discs and chill for 30 minutes.
- Next, cook half the blueberries with half a cup of the sugar over medium heat for 15 to 20 minutes, until reduced by half, then whisk in the cornstarch slurry and boil for two minutes until it is glossy and thick.
- Then scrape that into a large bowl with the remaining blueberries and sugar, the sliced rhubarb, orange zest, juice, salt, cinnamon and nutmeg, and leave it to cool to room temperature.
- After that, roll out the first disc, line the pie plate, brush the base and sides with blueberry jam and tip the cooled filling in. Roll the second disc, lay it over, trim to an inch of overhang, tuck and crimp.
- Finally, brush with egg wash, sprinkle with sugar, cut several vents and bake at 400°F for 20 minutes, then at 375°F for 55 to 70 minutes more, until the juices bubble thickly through the vents.
Cornstarch, Tapioca or Flour
All three thicken a blueberry rhubarb pie, and they do not behave the same way.
Cornstarch sets clear and glossy and has twice the thickening power of flour. It is used here because it can be cooked on the stove first, which is the whole point of the method above.
Tapioca gives the most reliable set in a raw fruit filling and is what most bakers reach for. Instant or minute tapioca needs about fifteen minutes of sitting in the fruit before baking so the granules hydrate.
Flour is the old answer and the weakest. It sets cloudy, mutes the color and needs roughly twice as much, which you can taste.
How To Tell When the Pie Is Done
Golden crust is not the test. A blueberry rhubarb pie can be perfectly browned on top and still be liquid underneath.
Watch the vents. You want the juices coming through them to be bubbling thickly and slowly, like lava, not running thin and fast. That usually happens somewhere between seventy five and ninety minutes of total baking, and it is worth going the extra ten minutes if you are unsure.
Check the bottom if the plate is glass. It should be dry and lightly browned, not pale and damp looking.
Start watching the edge at about forty minutes. If it is browning faster than the center, cover just the rim with a ring of foil or a pie shield and carry on.
Then leave it alone. A blueberry rhubarb pie cut warm will run, every time, because cornstarch does not finish setting until the filling drops below about 140°F. Four hours on a rack is the honest minimum, and an hour in the refrigerator after that gives the cleanest wedges of all.
Make It Ahead and Freeze It
Blueberry rhubarb pie is the rare dessert that genuinely prefers to be made the day before.
The dough can be made three days ahead and refrigerated, or frozen for three months. The filling can be cooked and cooled up to two days ahead and kept covered in the refrigerator.
A fully baked pie holds at room temperature overnight, and honestly slices better on day two than on the day it was baked.
To freeze it unbaked, assemble it in a metal plate, wrap it well and freeze it solid for up to three months. Bake it from frozen, without thawing, adding about twenty minutes and covering the edge early. Thawing it first is what produces a gray, waterlogged filling.
Storing Blueberry Rhubarb Pie
Cooled and covered, a blueberry rhubarb pie keeps better than most fruit desserts, and the crust is what decides where you put it.
- Room temperature: up to 2 days, loosely covered. The crust stays crispest here, which is the argument against the refrigerator.
- Refrigerator: up to 5 days, covered. The pastry softens slightly, and ten minutes in a 325°F oven brings it back.
- Freezer, baked: three months. Cool it completely, wrap it twice, and reheat from frozen at 350°F for about half an hour.
- Individual slices: freeze them on a tray first, then wrap. They reheat in fifteen minutes and are the reason to bother freezing at all.
- What not to do: do not cover a warm pie. The trapped steam condenses on the crust and softens it in an hour.
Variations and Substitutions
The cooked-down berry base is the part to keep. Everything else in a blueberry rhubarb pie is negotiable.
- Strawberry rhubarb: swap the blueberries for hulled strawberries and cut the sugar by a quarter cup, since strawberries bring more of their own.
- Lattice top: cut the second disc into strips and weave it. It bakes faster, so start checking at fifty minutes.
- Crumb top: a cup of flour, half a cup of brown sugar and six tablespoons of cold butter rubbed together, which turns it into a crumb pie.
- Ginger: a teaspoon of ground ginger or two tablespoons of chopped stem ginger in place of the nutmeg, which suits rhubarb better than cinnamon does.
- All-butter crust: replace the shortening with three more tablespoons of butter. Flakier, slightly harder to roll.
- Less sweet: drop to a cup of sugar total if your rhubarb is late-season and mild, and taste the cooled filling before it goes in.
What to Serve with Blueberry Rhubarb Pie
Blueberry rhubarb pie wants something cold and unsweetened beside it, because the filling is already doing the sweet part.
- Lightly whipped cream with no sugar in it at all, which is what the photographs above have on them.
- A scoop of something tart and frozen, such as maple lavender frozen yogurt, instead of vanilla ice cream.
- Sharp cheddar on the side, the New England way with fruit pie, which sounds odd and works.
- Strong black coffee, or a small glass of something bitter, on a table that also has bourbon pecan pumpkin cheesecake on it.
- Nothing at all, cold, standing at the refrigerator the next morning. This is the correct answer.
Here is what readers ask most often.
Blueberry Rhubarb Pie FAQ
Frequently Asked Questions
Can I use frozen rhubarb and frozen blueberries?
Yes, and you do not need to thaw either one. Frozen rhubarb goes straight into the cooled filling, and frozen blueberries go straight into the saucepan for the reduction, which just takes about five minutes longer to lose its water. Thawing is the mistake: thawed fruit sheds its liquid into a bowl before it ever reaches the pan, and you lose both the color and the body of the filling.
Why is my blueberry rhubarb pie runny?
Nearly always because it was cut too soon. Cornstarch does not finish setting until the filling cools below about 140°F, so a pie cut an hour out of the oven will run no matter how well it was made. Give it four hours on a rack, or bake it the day before. The other causes are underbaking, which you can spot from thin fast-running juices at the vents, and raw rhubarb that never had its water cooked out.
Do I have to peel rhubarb?
Usually not. Young thin stalks have tender skin that softens completely in the oven. Thick, coarse stalks over about an inch across are worth running a vegetable peeler down, because the outer strings stay tough and turn up as threads in the slice. Never eat the leaves, which contain enough oxalic acid to be toxic, and cut them off as soon as you get the stalks home.
More Questions About Baking with Rhubarb
Good to Know
How do I stop the bottom crust going soggy?
Three things, in order of effect. Cool the filling completely before it touches the pastry. Brush the raw base with two tablespoons of jam, which forms a seal. Bake on a sheet pan that has been preheating with the oven, so the base gets direct heat from below. A glass plate helps too, because you can see when the bottom has actually browned.
Can I make this without a food processor?
Yes. Rub the cold diced butter and shortening into the flour with your fingertips until the pieces are the size of peas, working quickly so nothing melts, then stir in the ice water with a fork. A box grater and frozen butter is faster still.
Is 400 degrees too hot for a fruit pie?
Not for the first twenty minutes. That initial blast sets the pastry and starts the base browning before the filling has time to soak in. Dropping to 375°F after that lets a long bake happen without the crimped edge burning, which is what a steady 400°F would do over an hour and a half.
How much rhubarb is a pound and a half?
Roughly five or six medium stalks, or about five cups once sliced half an inch thick. Rhubarb is sold by weight rather than by bunch, so weigh it if you can. A little over is fine; a little under and the blueberries take over.
Bake it the day before and you get the clean purple wedge rather than the delicious mess. For another fruit dessert that improves overnight, try my pear upside-down cake, or the shameless peanut butter honeycomb pie.
Blueberry Rhubarb Pie
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Ingredients
Blueberry Rhubarb Filling
- 1 1/2 pounds rhubarb trimmed and sliced into 1/2-inch pieces
- 1 1/2 pounds blueberries divided in half
- 1 1/4 cups granulated sugar divided, plus more for the top
- 1/4 cup cornstarch
- 1/4 cup cold water whisked with the cornstarch
- 1 tablespoon grated orange zest from 1 orange
- 1/4 teaspoon kosher salt
- 1 tablespoon fresh orange juice
- 1/4 teaspoon ground cinnamon
- 1/8 teaspoon freshly grated nutmeg
- 2 tablespoons blueberry jam for brushing the bottom crust
- 1 large egg beaten with a splash of water
All-Butter Pie Crust
- 12 tablespoons very cold unsalted butter 1 1/2 sticks, diced
- 1/3 cup very cold butter-flavored shortening or 3 more tablespoons butter
- 3 cups all-purpose flour
- 1 teaspoon kosher salt
- 2 tablespoons granulated sugar
- 6 to 8 tablespoons ice water about 1/2 cup
Equipment
- 9-inch pie pan
- Food Processor
- Saucepan
- Mixing bowl
Instructions
Make the Pastry
- First, dice the butter and shortening and return them to the refrigerator. Pulse the flour, salt and sugar in a food processor, add the cold fat and pulse until the pieces are the size of peas.
- Drizzle in the ice water with the machine running, stopping the moment the dough gathers. Turn it out onto a floured board, cut it in half, form two discs, wrap them and chill for at least 30 minutes.
Cook the Filling
- Next, mash half the blueberries with 1/2 cup of the sugar in a saucepan and cook over medium heat for 15 to 20 minutes, until reduced by half. Whisk the cornstarch into the cold water, pour it in, and boil for 2 minutes until glossy and thick.
- Then scrape it into a large bowl with the remaining blueberries and sugar, the sliced rhubarb, orange zest, salt, orange juice, cinnamon and nutmeg. Stir and leave it to cool to room temperature. Warm filling melts raw pastry on contact.
Assemble the Pie
- After that, heat the oven to 400 degrees F with a sheet pan on the middle rack. Roll the first disc out and line a deep 9.5-inch pie plate, then brush the base and sides with the blueberry jam and tip in the cooled filling.
- Roll the second disc, lay it over the top and trim to about an inch of overhang. Tuck the top edge under the bottom, crimp, brush with egg wash, sprinkle with sugar and cut several vents.
Bake and Cool
- Bake on the hot sheet pan at 400 degrees F for 20 minutes, then lower the oven to 375 degrees F and bake for 55 to 70 minutes more. Cover the crimped edge with a ring of foil once it is as brown as you want it.
- Finally, pull it out when the juices at the vents bubble thickly and slowly rather than running thin. Cool completely on a rack, at least 4 hours, and refrigerate for an hour before slicing if you want the cleanest wedges.