Rinse flowers, shaking them to remove any debris or bugs that might be hiding in them. Trim stems, getting as close to the base of the florets as possible.
Place flowers in a bowl along with sliced lemon.
In a saucepan, heat sugar and water together until simmering and sugar is completely dissolved. Stir in citric acid or lemon juice.
Pour warm syrup over flowers. Cover and refrigerate for 3 to 5 days, until the flowers start to turn brown and the syrup takes on a pale yellow hue. Strain syrup through a fine mesh sieve, discarding leftover flowers.
At this point the syrup can be refrigerated in a jar or other airtight container for up to 1 month.
For longer preservation, prepare a water bath canner and wash and sterilize canning jars. Heat the syrup in a saucepan until boiling, then pour into hot jars, leaving 1/4 inch of headspace. Process for 10 minutes.
Properly sealed, the syrup will keep for up to 1 year (refrigerate after opening).
