Pick fully open dandelion flowers from a clean, untreated area. Rinse them briefly, remove the stems and as much green calyx as possible, and keep only the yellow petals.

Place the 4 cups of petals and 4 cups of water in a saucepan. Bring the mixture to a boil, reduce the heat, and simmer for about 10 minutes.

Strain the tea through a fine-mesh sieve or jelly bag, pressing gently to collect the liquid. Measure exactly 4 cups of strained tea, adding a little water if needed.

Pour the measured tea into a large pot and stir in the bottled lemon juice and powdered pectin. Bring the mixture to a full rolling boil over high heat, stirring frequently.

Add all the sugar at once and stir constantly until the mixture returns to a full rolling boil. Boil hard for 1 minute, then remove the pot from the heat and skim foam if necessary.

Ladle the hot jelly into hot, sterilized half-pint jars, leaving about 1/4 inch headspace. Wipe the rims, apply the lids, and process in a boiling-water canner according to the directions for your specific pectin and jar size.