Elderflower cordial is highly concentrated, so don't be afraid to experiment and dilute it in any beverage to which you would like to add a refreshing, lemony zip. Pick heads of blooming elderflowers on bright, sunny days from early May through early July. Choose large flower heads bursting with fully open blooms, which will display copious amounts of pollen when mature.
Cut the large stems from the elderflower heads. Rinse the heads well under cool running water in a colander. Hand-pick out any debris or unopened blooms. Drain for a couple of hours. Use scissors to cut the elderflowers from the head, trimming the blooms right beneath their bases. Discard the trimmed greenery.
Cut two medium-sized lemons and two medium-sized oranges into quarter-inch slices. Spread them over the bottom of a large, deep bowl or casserole dish. Make as many layers as necessary to use all of the slices. Cover with the elderflowers.
Put 4½ cups of white granulated sugar in a large heat-proof bowl. Bring 1½ quarts of water to a rolling boil, and pour it onto the sugar. Stir well to dissolve the sugar completely and allow the syrup to cool at room temperature for about 30 minutes. Stir in two oz. of powdered citric acid and a ½ cup of lemon juice, and blend the syrup well.
Pour the sugar syrup over the flowers and sliced fruit. Stir and mix everything very well. Cover with a kitchen towel and set to steep in a cool spot for 24 hours. Stir the elderflower cordial mixture occasionally.
Squeeze the mixture through coffee filters or cheesecloth to strain all solids from it. Store the cordial concentrate in sterilized containers with tight-fitting lids, and refrigerate for a week or freeze for up to a year.
Use your elderflower cordial concentrate as a lemony liqueur in your favorite beverages.