Wash the beach plums thoroughly to remove any dirt or debris.

Place the plums in a large saucepan, add 1 cup of water, and bring to a boil.

Reduce heat and simmer until the plums are soft and bursting, about 15 minutes. Stir occasionally to prevent sticking.

Strain the mixture through a jelly bag or fine mesh strainer into a large bowl. Let it drip overnight for a clearer juice, or press gently for a more robust flavor.

Measure 4 cups of beach plum juice and pour it into a large saucepan.

Add the sugar, stirring until dissolved. Heat the mixture until it reaches a rolling boil.

Stir in the liquid pectin quickly and return to a full boil. Boil hard for 1 minute, stirring constantly.

Remove from heat and skim off any foam with a spoon.

Carefully ladle the hot jelly into sterilized jelly jars, leaving 1/4 inch of headspace.

Before putting the lids on, wipe the rims and tighten the bands with your finger.

To seal the jars, process them in a boiling water bath for ten minutes.