Prepare a boiling water bath and four half pint jars. Wash lids and rings in warm, soapy water.
Place the chopped rhubarb and strawberries in a large, nonreactive pot.
Whisk the sugar and pectin powder together. Add it to the fruit and stir to combine. Let it sit for 15 to 20 minutes, until the sugar begins to dissolve.
Place the pot on the stove over high heat and bring to a boil. Reduce the heat to medium high.
Add the lemon juice and zest to the pot and let it cook at a low boil for approximately 20 minutes. As the jam cooks, use the back of a wooden spoon or a potato masher to help break down any large pieces of fruit.
Once the jam has reduced by about 1/3, looks thick, and very shiny, it is done. Double check for set using your favorite method. When you've determined it is done, remove the pot from the heat.
Funnel the jam into your prepared jars, leaving 1/2 inch headspace. Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 10 minutes.
When the time is up, turn off the heat and remove the lid from the pot. Let the jars stand in the cooling water for five minutes. Then remove the jars from the hot water, placing them on a folded kitchen towel or wooden board to protect them from heat shock.
Let the jars cool undisturbed until they are no longer warm to the touch. Remove the rings and check that the seals are good. Sealed jars are shelf stable for up to 18 months. Any unsealed jars should be refrigerated and used promptly.