Canning Instructions (Skip if not canning your jam)
Fill your canning pot with enough water to submerge your jars and have 1 inch of water on top. Bring to a boil over high heat.
Set out your jar clamps, funnel, ladle, and clean jars with new seals and clean lids on a dish towel nearby (double if you don't have heat-safe counter tops).
While your jam is cooking (step ) place your clean jars into the boiling water gently with your jar clamps and allow to heat for a minute or so. Remove with jar clamps and carefully place upside down on the dish towel to allow water to drain off.
When jam is finished cooking, use your ladle and funnel to carefully fill each jar, leaving only 1/4 inch of space between the top of the jam and the rim of the jar.*
Wipe the rims of the jars (just where the seal sits) clean with a damp, lint free towel and then dry.
Place the seals on the jars and screw on the lids, using your hand protection while holding the jars. Tighten the lids as much as possible by hand.
Using your jar clamps, lower the jars into the boiling water and allow to process for 10 minutes.
Remove from the water and set on towel. Let them cool and seal. They're ready to cupboard storage when the lids seal. You'll hear a 'pop' as they do. You can also check this by pressing down on the top of the seal, if it clicks, it hasn't sealed yet. This can take hours, so be patient. Once sealed they can be stored at room temperature for 18 months.
Puree watermelon chunks in a blender or food processor and strain it through a fine mesh sieve to collect the juice. Juice your lemon.
Combine watermelon juice, strawberries, lemon juice, butter if using, and pectin in a large pot over medium high heat.
Stir gently but frequently until the mixture comes to a rapid boil that does not stop even as you stir.
Add in sugar and continue to stir frequently but gently until the mixture returns to a rapid boil that doesn't stop even as your stir. Begin timing for 1 minute and 10 seconds, stirring the entire time.
Remove your jam from the heat and proceed with canning. Or, if not canning, ladle into jars and allow to cool completely before storing in the fridge for up to one month.*
*It took half of a large watermelon to yield 2 1/2 cups of juice. *This jam yields more than 48 ounces (six 8 ounce jars), but the final jar will not be full enough to can. Store extra in the fridge and enjoy within 30 days. *Regardless of canning or freezing, once you open a sealed jar of jam it should be used within 30 days of opening. If you do not can or freeze your jam, store it all in the fridge and use within 30 days. *To freeze your jam, ladle cooked jam into freezer jars leaving 1 inch of space between the top of the jam and the rim of the jar. Allow to cool then seal and store in the freezer for 1 year.**1 box of pectin is 1.75 ounces, to use regular (not low sugar) bulk pectin, use 6 level tablespoons unless directed otherwise by the package instructions. This is a shallow 1/2 cup.