Collect fresh, fully open dandelion heads from a pesticide-free area on a sunny day. Carefully remove all green parts (sepals and base) from the petals, as they can impart bitterness. You should aim for about 4 cups of firmly packed pure yellow petals. This step is the most time-consuming.

Place the dandelion petals in a large heatproof bowl or pot. In a separate saucepan, bring 4 cups of filtered water to a rolling boil. Carefully pour the boiling water over the dandelion petals. Stir gently to ensure all petals are submerged. Cover the bowl and let the mixture steep for at least 4 hours, or preferably overnight (up to 12 hours) to extract maximum flavor and color. Allow it to cool to room temperature if steeping overnight.

Strain the dandelion infusion through a fine-mesh sieve lined with several layers of cheesecloth into a clean large stockpot. Press firmly on the petals in the cheesecloth to extract as much liquid as possible; you should have approximately 3 to 3.5 cups of dandelion 'tea.' Discard the used petals.

Wash your 6 half-pint canning jars, lids, and bands in hot, soapy water, then rinse thoroughly. Place the jars (but not the lids or bands) in a large canning pot with a rack. Fill the pot with water until the jars are covered by at least 1 inch of water. Bring to a boil and simmer for at least 10 minutes to sterilize. Keep the jars hot in the simmering water until you are ready to fill them. Set new lids and bands aside.

To the strained dandelion infusion in the large stockpot, add the fresh lemon juice and lemon zest. Whisk in the powdered pectin until it is completely dissolved. Place the pot over high heat and bring the mixture to a full, rolling boil that cannot be stirred down, stirring constantly.

Once at a full rolling boil, add all 5 cups of granulated sugar at once. Stir continuously to dissolve the sugar. Return the mixture to a full, rolling boil that cannot be stirred down. Boil hard for exactly 1 minute, stirring constantly to prevent scorching.

Remove the pot from the heat. (Optional: To check for set, place a small amount of jelly on a chilled plate. After a minute in the refrigerator, push it with your finger; if it wrinkles, it's ready. If not, return to heat and boil for another 30 seconds to 1 minute, then retest.)

Carefully remove one hot sterilized jar from the canning pot, emptying any water back into the pot. Using a canning funnel and ladle, immediately fill the hot jar with the hot jelly, leaving 1/4 inch headspace from the rim. Repeat for the remaining jars.

Wipe the rims of the filled jars clean with a damp cloth to ensure a proper seal. Place a new lid on each jar, then screw on the bands just until fingertip tight.

Carefully place the filled jars onto the rack in the canning pot, ensuring they are covered by at least 1 inch of water. Bring the water to a rolling boil. Once boiling, process the jars for 10 minutes (adjust time for altitude if necessary).

After 10 minutes, turn off the heat and let the jars sit in the hot water for 5 minutes before carefully removing them with a jar lifter. Place the hot jars on a wire rack or a towel-lined surface to cool completely, undisturbed, for 12-24 hours. You should hear popping sounds as the lids seal. Do not tighten bands while cooling.

After 24 hours, check seals: lids should be concave and not flex when pressed. Remove bands, wipe jars, and store properly sealed jars in a cool, dark place for up to 1 year. Unsealed jars should be refrigerated and consumed within 3 weeks.