Harvest dandelion heads in the morning after the dew dries. Pick only the yellow petals, discarding the green bases. You need about 4 packed cups of petals.
Place the dandelion petals and water in a non-reactive pot. Bring the mixture to a simmer over medium heat.
Remove from heat, cover, and let the mixture steep for 12 to 24 hours.
Strain the liquid through a fine-mesh sieve lined with cheesecloth into a clean pot. Squeeze out as much liquid as possible from the petals. Discard the solids.
Add the sugar and lemon slices to the strained liquid.
Bring the mixture to a boil over medium-high heat, stirring until the sugar dissolves completely.
Reduce the heat and simmer gently, uncovered, for 45 to 60 minutes, or until the syrup thickens slightly and coats the back of a spoon. Skim off any foam that rises to the top.
Remove from heat. Remove the lemon slices.
Pour the hot syrup into sterilized jars, leaving a small headspace. Seal the jars tightly.
Notes
Use only dandelions picked from areas free of pesticides or pollution.
The syrup will thicken more as it cools.
This syrup works well in tea, over pancakes, or in mixed drinks.