Low Sugar Blackberry Rhubarb Jam

May 25, 2016(updated on August 30, 2021)

three jars of low sugar blackberry rhubarb jam on a bench

Lately, Trader Joe’s had been selling 12 ounce containers of blackberries for right around $3.50. For those of you who live in the Pacific Northwest, it might seem crazy to pay for good money for blackberries since come August, they’re going to be everywhere. But for those of us who live in less blackberry-rich environs, this is a very good price.

blackberries, rhubarb, and sugar in a pot before being cooked into jam

As a result, I’ve been making lots of preserves with blackberries. I did a batch scented with lavender, and another batch with cinnamon and nutmeg, like my mom always makes. I also did this low sugar blackberry rhubarb jam, encouraged by an email from a reader who asked if I’d ever done such a combination).

I really love how it turned out. Tangy from the rhubarb, rich from the berries, and just sweet enough with a relatively small amount of sugar. If you wanted to make this jam with honey rather than with sugar, reduce the amount to 1 cup and save 1/3 a cup to add at the end with the pectin.

a close up of two jars of low sugar blackberry rhubarb jam

Oh, and if you don’t want to spring for blackberries now, but can get them at a better price later in the season, you can still make this jam. Just chop up a pound of rhubarb now, put it in a ziptop bag, and tuck it into the freezer until August. Frozen rhubarb behaves beautifully in jams.

Finally, if this jam doesn’t float your boat, I’ve got a couple other blackberry recipes in the archives. Perhaps my classic Blackberry Jam or this Blackberry Apricot Jam will float your boat!

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Low Sugar Blackberry Rhubarb Jam


  • 1 pound of rhubarb finely sliced
  • 1 pound 8 ounces blackberries (or two 12-ounce packages)
  • 1 1/2 cups granulated sugar divided
  • 2 teaspoons calcium water part of the Pomona's Pectin system
  • 2 teaspoons Pomona's Pectin


  • Prepare a canning pot and enough jars to hold between 4 and 5 half pints (yields vary!).
  • Combine the rhubarb, blackberries, 1 cup sugar, and the calcium water in a low, wide, non-reactive pot. Stir vigorously to combine, with the intention of breaking up the berries. Let the fruit sit with the sugar for a few minutes, so that the sugar dissolves and you get a little syrupy liquid in the bottom of the pot.
  • Set the pot over high heat and bring to a boil. Reduce the heat to medium high and cook until the berries break down, the rhubarb is on the verge of dissolving, and the liquid thickens.
  • Whisk the pectin powder into the reserved sugar. Add the pectin-spiked sugar to the cooking fruit in two or three batches, stirring to integrate between each addition.
  • Cook for an additional minute or two, until you see visible signs of thickening.
  • Remove the jam from the heat and funnel it into prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 10 minutes.


You could also make this with honey, if you prefer. Use 1 cup of honey and reserve 1/3 a cup for the end to use as the pectin carrier.

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25 thoughts on "Low Sugar Blackberry Rhubarb Jam"

  • I also made jam, but this is a much healthier alternative. I need to try this soon. Thank you for sharing!

  • I did this combination last year and loved it. Roughly the same recipe as yours. I didn’t use any pectin, but did include lemon juice, and it set up fine. Maybe I got lucky.

  • I just turned my first batch of rhubarb into syrup (cocktails!!) but I think I need to put some in the freezer in anticipation of blackberry season!!

  • If one didn’t want to use the Pomona system could you use one box of the low sugar pectin?

    1. You could try it. I wouldn’t use the whole box, though. I didn’t test it with that pectin, so I can’t say for sure how much you’d need.

  • This is a beauty, Marisa. Thank you! And for those of us who will have access to an abundance of free berries in a couple of months, the tip to freeze the rhubarb now is spot on.

  • Rhubarb-blackberry! Yum. Would mean funneling some of the rhubarb away from the ongoing batch of rhubarb – whatever fruit we have oatmeal squares that my children seem to require, which could be tricky, but I can try! So, 2 questions – could i use maple sugar (which we make ourselves) rather than granulated, or will that mess things up? And secondly, could I just use maple syrup? My mother-in-law used maple sugar in her rhubarb custard pie last week, and it was amazing!

    1. You could certainly use maple sugar without making any changes. If you wanted to use maple syrup, I’d make sure to add a tablespoon or two of bottled lemon juice.

  • I am a Pacific Northwest girl and you are right – it would never occur to me to pay for blackberries:) I love the idea of freezing rhubarb and making this in August. Thanks for the delicious idea!

  • I just made this and it might be my favorite jam ever. I’m allergic to all stone fruit, and this came really close to tasting like the cherry jam I used to love and can no longer eat. I will be spending this weekend using up last year’s blackberries to make enough of this to last until this year’s blackberries are ripe. Thank you!!

  • How did I miss this post?? With all the rhubarb in my garden right now, I will definitely be freezing some for later to try this recipe out. Sounds yummy!

  • I live in the Northwest and am waiting for those berries to ripen!! I have rhubarb in the freezer, do I thaw it or add it in frozen? Also, I have a set of those jars but have been afraid to can with them, do you have any tips for them?

    1. Use it frozen. And there’s no trick to canning in these jars. Just let the sit in the canner for a few more minutes after the processing time is up to cool more gradually. Just pull it off the hot burner and let it sit.

  • Thanks for all your great recipes. I’ve made quite a few over the past couple of years. This year we’ve had such weird weather in the PNW that our thornless blackberries in our backyard are beginning to ripen. Very early. And since we still have rhubarb as well, I’m definitely going to try this recipe too. I think I’ll try normal pectin however, since that is what I have on hand. I know that blackberries are a high pectin fruit, but I would feel a bit more confident in terms of the ‘setting’ if I use pectin. Many years ago, I didn’t use pectin when making jams, but I must have had more time to stir the pot! ha ha.

  • I made this jam today and strained out the blackberry seeds — it made a fabulous, velvety smooth, tart and fruity jam! I made your seedy spiced blackberry jam last week too. And the blackberries just keep coming! Thank you for all of your ideas!

  • Question: if I want to strain out the seeds in the blackberries, do I need to account for that lost weight?

    1. Typically, removing the seeds doesn’t represent a great deal of product loss, but you could always an additional 6 or 8 ounces to compensate.

  • Sorry for sounding “unknowledgeable” but how do you strain out the blackberry seeds. Thanks! BTW I make your Rhubarb/Strawberry Jam all the time and it’s fantastic! My Irish mother-in-law eats it straight from the jar!!!

  • I use a Foley food mill to get the seeds out. Much easier than pushing it through a strainer with a spoon.

    1. You must have a very fine screen for your food mill, as I find that mind isn’t fine enough to catch most of the blackberry seeds.