If you adore the classic combination of sweet and tart found in fruit bars, then these Rhubarb Squares are an absolute must-try! Often known as Rhubarb Crumble Bars or Rhubarb Oatmeal Bars, they feature a luscious rhubarb compote filling perfectly balanced between vibrant sweetness and tangy zest. This delightful filling is beautifully cradled between a buttery, irresistible oatmeal base and a crunchy crumble topping. They are not only a fantastic way to utilize an abundance of fresh spring rhubarb but also make for an exquisite dessert or a satisfying snack any time of year.

While I make sure to freeze fresh rhubarb to enjoy its unique flavor throughout the year, nothing quite compares to the excitement of its arrival in spring. These incredibly addictive Rhubarb Oatmeal Bars were inspired by my beloved Date Squares, which also serve as a wonderful treat after a delightful spring meal. For a complete springtime feast, consider pairing these with my refreshing Spring Salad with Fiddleheads & Blackberry Vinaigrette. Prepare to declare this the best rhubarb dessert you’ve ever tasted!

🍒 Reader Review: A Five-Star Favorite!

“Hi, Terry. I just made this recipe. (July 7, 2024) It’s a keeper. Just the way I’d want an oatmeal rhubarb square to taste.!” ⭐⭐⭐⭐⭐

🥘 Essential Ingredient Notes for Perfect Rhubarb Squares

Crafting these delectable rhubarb oatmeal bars requires a few key ingredients. Here’s a detailed look at what you’ll need and why each component is important:

• Quick Rolled Oats: I specifically opt for quick rolled oats (sometimes labeled “quick oats”) because their finer texture cooks faster and blends beautifully into the crumble, providing a tender yet slightly chewy consistency. This is crucial for the perfect bar texture. **Do NOT use instant oats**—they are too processed and will result in a mushy, undesirable texture.
• Unsalted Butter: Using unsalted butter gives you complete control over the salt content in your baking. Different brands of salted butter can vary significantly in their saltiness, so starting with unsalted butter allows for precise seasoning to achieve that ideal flavor balance in your rhubarb squares.
• Fresh or Frozen Rhubarb: You can successfully use either fresh or frozen rhubarb for this recipe. If using frozen rhubarb, it’s vital to defrost it completely first and then drain any excess liquid thoroughly. Rhubarb releases a lot of water when thawing, and failing to drain it will make your filling watery and prevent it from setting properly. Always measure your rhubarb *after* it has been defrosted and drained.
• Cornstarch: This is a critical ingredient for the filling. Cornstarch acts as a thickening agent, ensuring that your rhubarb compote is beautifully set and doesn’t run out of the bars when cut. It thickens the filling effectively without altering the delicious tart-sweet flavor of the rhubarb.
• Red Food Coloring (Optional): While purely aesthetic, adding a touch of red food coloring to your rhubarb compote can make a significant difference in its visual appeal. Rhubarb often loses some of its vibrant pink color during cooking and can sometimes appear a bit brownish. A small amount of coloring ensures your filling has that appetizing, characteristic rosy hue. I personally prefer Americolor super red for its concentrated pigment, meaning you need very little to achieve a beautiful color.

*A full list of ingredients and quantities can be found in the recipe card below.

📖 Variations & Creative Substitutions

This rhubarb square recipe is wonderfully adaptable! Feel free to experiment with these variations to customize them to your taste:

• Citrus Zest Alternatives: While lemon zest adds a bright, tangy counterpoint to the rhubarb, you can easily substitute 2 teaspoons of fresh lemon juice for the zest if preferred. Alternatively, try using orange zest for a slightly sweeter, more aromatic citrus note that pairs beautifully with rhubarb.
• Oats – Old-Fashioned vs. Quick: If you only have old-fashioned or regular rolled oats on hand, don’t worry! You can still achieve a similar texture to quick oats by giving them a few quick pulses in your food processor. This breaks them down slightly, making them integrate better into the crumble while retaining some chewiness. Avoid instant oats entirely, as their fine texture can lead to a gummy result.
• Adjusting Sweetness: Rhubarb’s tartness can vary, as can personal preference for sweetness. Some readers might find the suggested amount of sugar a bit much. My family and I prefer a balanced sweetness, and we find this recipe perfect, but you can always start by adding a quarter or a third less sugar to the filling. Once the rhubarb is cooked, taste it and add more sugar gradually until it reaches your desired level of sweetness.
• Add Nuts for Crunch: For an extra layer of texture and flavor, consider adding ½ cup of chopped walnuts or pecans to the oatmeal crumble mixture. The toasted nuts will complement the rhubarb beautifully.
• Spice It Up: A pinch of ground ginger or cardamom can elevate the flavor profile of the rhubarb filling, adding warmth and complexity. Start with ¼ teaspoon and adjust to your liking.
• Fruity Boost: If you want to enhance the color of your rhubarb filling naturally and add another layer of fruitiness, stir in ½ cup of chopped fresh strawberries or raspberries during the last few minutes of cooking the rhubarb.

🔪 Step-by-Step: How to Make Irresistible Rhubarb Squares

1. Step 1: Prepare the Filling. Begin by washing and cutting your fresh or defrosted rhubarb into uniform ½-inch pieces. In a medium saucepan, combine the chopped rhubarb with 1 ½ cups of sugar and the lemon zest. Place the saucepan on the stovetop over medium heat and cook gently until the rhubarb becomes tender and starts to break down, which typically takes about 15 minutes. Stir occasionally to prevent sticking.

2. Step 2: Thicken the Filling. In a separate small bowl, combine the remaining ½ cup of sugar with the cornstarch. Whisk these dry ingredients together thoroughly. Then, stir this mixture into the cooked rhubarb in the saucepan. The cornstarch will help to thicken the compote, preventing it from being too runny in your bars.

3. Step 3: Finish Cooking and Cool. Continue cooking the rhubarb mixture over medium heat for an additional 5-10 minutes, stirring constantly, until the rhubarb has completely broken down into a thick, jam-like consistency, as shown in the image. Once desired consistency is reached, remove from heat and allow the filling to cool completely before assembly. Cooling is crucial for the filling to set properly.

4. Step 4: Enhance Color (Optional). If you prefer a more vibrant pink color for your rhubarb filling, add a drop or two of red food coloring at this stage. Stir it in thoroughly until you achieve your desired hue.

5. Step 5: Prepare Base & Topping. Preheat your oven to 375 degrees F (190 degrees C). Lightly spray a 9×13 inch baking pan with non-stick cooking spray. In a large mixing bowl, combine the flour, baking soda, brown sugar, cinnamon, and quick oats. Stir these dry ingredients well. Next, cut in the very cold unsalted butter using a pastry cutter. Work quickly until the mixture resembles coarse crumbs. (Alternatively, see my top tip below for an easier method!).

6. Step 6: Form the Base. Take half of the oatmeal crumb mixture and firmly press it evenly into the bottom of your prepared 9×13 inch baking pan. Ensure it forms a compact and even layer, which will be the sturdy base for your bars.

7. Step 7: Add the Rhubarb Filling. Carefully spoon the cooled rhubarb mixture over the pressed oatmeal crust. Use a rubber spatula to spread the filling evenly from edge to edge, ensuring a consistent layer in your squares.

8. Step 8: Apply the Crumble Topping. Generously sprinkle the remaining oatmeal crumb mixture over the rhubarb layer. Distribute it evenly to create that signature golden, crunchy topping that defines these delicious bars.

9. Step 9: Bake, Cool, and Serve. Bake the squares in your preheated oven for 35-40 minutes, or until the top is beautifully golden brown and the filling is bubbly. Once baked, remove from the oven and allow them to cool completely in the pan on a wire rack. This is a crucial step! Only after they are completely cooled, cut them into squares. For an extra touch of indulgence, garnish with a dollop of fresh whipped cream and a sprinkle of cinnamon, if desired.

Hint: It’s absolutely crucial to wait until the rhubarb squares are completely cooled before attempting to cut them. If you cut them while still warm, the filling will be too soft and the bars will likely fall apart, losing their shape and structure. To speed up the cooling process, you can carefully place the pan in the refrigerator or even the freezer for a short period until firm.

✅ Expert Recipe Tips for the Best Rhubarb Squares

Achieve bakery-quality rhubarb squares every time with these valuable tips:

• Choose the Right Pan: Always use a glass baking pan or a coated non-reactive pan for best results. Rhubarb is quite acidic, and cooking it in reactive metals like aluminum, copper, or iron can cause a metallic taste, discolor the rhubarb (turning it brownish), and potentially damage your pan. A glass or ceramic dish ensures pure flavor and even baking.
• Quick Oats for Optimal Texture: For the perfect balance of chewiness and a slightly crisp crumble, quick oats are ideal. Their finer cut integrates beautifully into the crust and topping. If you only have old-fashioned rolled oats, a quick pulse in a food processor (just a few times, don’t over-process into flour) will break them down enough to mimic quick oats. Steer clear of instant oats, as they tend to become mushy and won’t give you the desired crumbly texture.
• Frozen Butter for Easy Crumb: The key to a wonderfully crumbly and tender base and topping is to keep your butter as cold as possible. Grating frozen butter directly into your dry ingredients is a game-changer! It distributes the butter more evenly and efficiently, reducing the need to overmix and ensuring a light, flaky texture. This method is far easier and less messy than trying to cut in cold butter cubes with a pastry cutter.
• Cornstarch is Non-Negotiable: Do not skip the cornstarch in the rhubarb filling. Its role is crucial for thickening the compote, preventing it from becoming a runny mess. Without it, your bars won’t hold their shape, and the filling will seep into the crust, making them soggy.
• Patience is Key: Cool Completely: I cannot stress this enough – let your rhubarb bars cool *completely* before attempting to slice them. The filling needs time to set fully, and cutting into warm bars will almost guarantee they fall apart. For speedier cooling, transfer the pan to the refrigerator or even the freezer for a short while.
• Tailor the Sweetness: Rhubarb’s tartness can vary depending on the variety and ripeness. It’s always a good idea to taste your cooked rhubarb filling (before adding it to the crust) and adjust the sugar if necessary. If your rhubarb is particularly tart, you might want to add an extra tablespoon or two of sugar.
• Natural Color Enhancement: If you prefer to avoid food coloring but still desire a more vibrant red hue for your rhubarb filling, try adding a handful of chopped fresh strawberries or raspberries to the rhubarb as it cooks. These berries will naturally boost the color and add a lovely complementary fruit flavor.
• Prepare Ahead for Convenience: These rhubarb squares are an excellent make-ahead dessert! You can bake them a day in advance and store them covered in the refrigerator overnight. This allows the flavors to meld beautifully and the bars to set perfectly, resulting in an even better texture.

🌡️ Storage Solutions for Your Rhubarb Squares

These delightful rhubarb oatmeal bars keep exceptionally well, ensuring you can enjoy them for days to come! Store them in an airtight container in the refrigerator for up to 4 days (though they’re so delicious, they might not last that long!). For longer storage, you can freeze them in a sealed container for up to 3 months. Thaw them in the refrigerator overnight or at room temperature before serving for the best texture.

👪 Serving Size & Customization

This recipe for rhubarb bars yields 16 generous square servings. However, if you’re serving them as a smaller bite or for a larger gathering, you can easily cut each square in half diagonally to create 32 charming triangular servings. This is perfect for potlucks or afternoon tea!

For your convenience, you can also half, double, or even triple the recipe directly in the recipe card below by simply clicking on the serving number and selecting your desired yield. The ingredient quantities will automatically adjust. Keep in mind that halving the recipe will require a 9-inch square pan, while doubling it will necessitate an additional 9×13 inch baking pan.

💡 Top Tip for Perfect Crumb Topping

For effortlessly perfect crumbly texture every time, keep your butter frozen (or at least very cold) and grate it into your dry ingredients. This method makes it significantly easier to incorporate the butter evenly and quickly, resulting in a consistent crumb mixture without the messy struggle of a pastry cutter or your hands. It’s a game-changer!

❔ Recipe FAQs: Your Rhubarb Square Questions Answered

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