Clean the rosehips and cut them in half. Remove the seeds. Alternatively, freeze the halved rose hips overnight in a plastic bag. Before use, smash the plastic bag against the kitchen table and the seeds will be removed from the hip.
In a saucepan, add the halved rosehips and water and bring to a boil. Let simmer for 15 minutes.
In the last 5 minutes of boiling, mash the rosehips as much as possible.
Strain the liquid and the mashed rosehips. Use a spoon to squeeze out all the juice.
Bring back the mashed rosehips to the saucepan and add another 1 ¼ cups of water (3 deciliters). Bring to a boil for another 15 minutes.
Repeat the straining process and discard the mashed rose hips.
In a clean saucepan, add the strained rosehip juice together with the sugar. Bring to a boil and let the sugar dissolve. Turn off the heat and let the rosehips syrup cool down.
Fill sterilized jars/bottles with the syrup and store in a cool place.