Pick and prepare the petals: Gently separate the bright yellow petals from the green base and discard any that are brown. Rinse lightly to remove bugs or dirt and pat dry.

Steep the petals: Place petals in a large saucepan and cover with 4 cups of water. Add lemon slices if using. Bring to a gentle simmer, then cover and remove from heat. Let steep for 24 hours in the refrigerator or 2 hours at room temperature.

Strain the infusion: Pour the petal-infused water through a fine sieve lined with cheesecloth into a clean pot, pressing gently to extract all the fragrant liquid. Discard the petals.

Measure 4 cups of the infused liquid; add water if needed. Pour into a heavy-bottomed saucepan, stir in the powdered pectin, and bring to a rolling boil over high heat.

Stir in 5 cups of sugar all at once. Return to a rolling boil and boil hard for 1 minute, stirring constantly. Remove from heat and stir in lemon juice and vanilla extract if using.

Skim any foam with a spoon. Ladle immediately into hot sterilized jars, leaving 1/4-inch headspace. Wipe rims, seal with lids, and process in a boiling water bath for 10 minutes.

Remove jars and set on a towel to cool. As they cool, you will hear the reassuring pop of lids sealing. Let the jelly rest for at least 24 hours before opening.