Gently pluck the yellow petals from the dandelion heads, discarding green parts and stems. Rinse petals briefly in cool water and drain.

In a medium saucepan, combine the petals and 4 cups of water. Bring to a simmer, cover, and let steep for 20 minutes.

Line a fine-mesh sieve with cheesecloth and strain the petals, pressing gently to extract all the liquid. Discard the spent petals.

Return the dandelion liquid to the cleaned saucepan. Stir in the lemon juice and pectin according to the package directions.

Add the sugar all at once and stir over medium-high heat. Bring to a full, rolling boil.

Boil hard for 1 to 2 minutes. Remove from heat and skim off any foam with a spoon.

Ladle the hot jelly into sterilized jars, leaving 1/4 inch headspace. Wipe rims, apply lids, and process in a boiling water bath for 10 minutes.

Allow jars to cool undisturbed for 12 to 24 hours. Check seals, label with date, and store in a cool, dark place for up to a year.