Prepare water bath canner by filling it halfway with water, and setting it on the stove. Turn the burner to high. Once it reaches a boil, reduce it to simmer. You want to keep the water hot so that everything is ready when the jam is.
Wash and clean your jars. Fill with hot water or keep them warm in a clean dishwasher or oven.
Wash your lids and set aside in clean place.
Making the Jam
Rinse the strawberries. Remove the tops, cut in half or quarters, and place in a large bowl.
2.5 cups strawberries
Gently mash with a potato masher or pastry cutter until it yields 2.5 cups of mashed berries.
Add the clean chopped rhubarb to a heavy-bottomed saucepan with 1/2 cup water.
4 cups chopped rhubarb, 1/2 cup water
Cook on medium heat, stirring often until the rhubarb softens.
Stir in the strawberries and continue to cook the mixture until the fruits are well-combined.
In a small bowl, combine 1/2 tsp calcium powder from the Poman's Pectin box with 1/2 cup cool water to create calcium water.
1/2 cup cool water
In a separate bowl, combine 1.5 cups of sugar and Pomona's pectin. Set aside.
1.5 cups sugar, 3 tsp Pomona's Pectin
Add 2 tbsp lemon juice and calcium water to the pot with the rhubarb and strawberries, stir.
2 tbsp bottled lemon juice, 3 tsp calcium water
Add the sugar/pectin mixture, and bring to a gentle rolling boil.
Stir constantly for 3 minutes, turning down the burner to medium if the jam starts popping.
Remove from heat and continue stirring for 5 more minutes. At the end of the 5 minutes, remove any foam that remains.
At this point, turn the burner under the canner back up to high and get that water boiling again.
Canning the Jam
Using the funnel and a ladle, add the jam to your prepared jars, leaving 1/4 inch of headspace at the top. Headspace is the amount of room between the top of the food, and the rim of the jar.
Using a wet clean rag, wipe the rim to make sure there isn't any sticky jam on there.
Place a clean lid on the jar and then a ring. Tighten the ring to fingertip tight.
Using canning tongs, add jars to the boiling water, and put the canner lid on. Pro tip: you must have at least 1 inch of water over the tops of the jars for safe canning.
Process (boil) for 10 minutes.
Remove the canner from heat, and let sit for 5 minutes.
Remove the jars and place them on towels on the counter undisturbed for 12-24 hours.
After everything has cooled, check the seal by pushing down on the middle of the lid. If it doesn't give way, it's sealed. If the lid bows a little bit, put that in the fridge and use it within 3 weeks.
Label the sealed jars, and store in a cool dark place for up to a year.
Notes
Makes 5, 8 oz jars.
Will last for 1 month in the fridge or 12 months sealed and at room temperature.