Trim the garlic scapes by cutting off any dried ends then chop into lengths to fit into the mason jars you're using.
Arrange scapes in the jars leaving about ¼" headspace at the top and add peppercorns and mustard seeds.
Combine apple cider vinegar, water, salt and sugar in a medium sauce pot. Bring to a boil and stir until salt and sugar dissolve.
Pour the brine into the jars making sure the garlic scapes are completely covered with the liquid. Loosely fit the lids onto the jars and leave out on the counter until room temperature.
Once cooled, tighten lids and place in the refrigerator to pickle for at least 2 weeks before using.