Gently pick dandelion flowers, removing the green sepals so only the yellow petals remain. Rinse briefly to remove dirt and small insects, and pat dry.

Place petals in a large heatproof bowl or pot. Pour 4 cups of just-boiled water over them, cover, and let steep for 12–24 hours.

Strain the petals through a fine mesh or cheesecloth, pressing gently to extract liquid. If using apple for extra pectin, simmer the apple in the infusion for 10 minutes before straining.

While the infusion steeps, wash jars and lids in hot, soapy water. Keep jars warm in a simmering water bath.

Measure 4 cups of dandelion infusion into a heavy-bottomed saucepan. Stir in powdered pectin and lemon juice, and bring to a full rolling boil over high heat while stirring constantly.

Add sugar all at once and return to a full boil for 1–2 minutes while stirring until the syrup thickens.

Remove from heat, skim any foam, and ladle the hot jelly into warm jars, leaving 1/4 inch headspace. Wipe rims, apply lids, and screw bands fingertip-tight.

Place filled jars in a boiling-water canner with at least 1–2 inches of water above jar tops, and process for 10 minutes.

Carefully remove jars and let cool undisturbed for 12–24 hours.

After cooling, press the center of each lid. If sealed, label with date and store in a cool, dark place.

Refrigerate any unsealed jars and use within a few weeks.