Bring 4 cups of water to a boil. Remove from heat and let cool for 1-2 minutes. Add the thistle blossoms and push them down until fully submerged. Cover and steep for at least 1 hour or overnight in the refrigerator.

Strain the tea through a fine mesh strainer or jelly bag, pressing gently on the blossoms. Measure 4 cups of tea, adding water if needed to make up the difference.

Place the thistle tea in a large pot and add the lemon juice. Whisk in the powdered pectin until completely dissolved. Bring to a full rolling boil over high heat, stirring constantly.

Add all the sugar at once and continue stirring. Return to a full rolling boil and boil hard for exactly 1 minute. Remove from heat and skim off any foam.

Ladle hot jelly into prepared jars, leaving 1/4 inch headspace. Wipe rims clean, center lids, and apply bands fingertip-tight.

Process in a boiling water bath for 10 minutes, adjusting for altitude. Turn off heat and let jars rest 5 minutes before removing. Cool undisturbed for 12-24 hours before checking seals.