Wait until the flower head has faded and dried out on the stalk, ideally in late summer, though you can harvest zinnia seeds throughout the growing season. Pick a dry, sunny day and try to cut in the early morning before the afternoon sun stresses the plant.
Use clean, sharp flower snips or pruners to cut the dried head from the stem. Look for heads that are mostly brown and papery, though a little leftover color in the petals is fine as long as the head itself is dry.
Lay the cut heads out somewhere with good airflow, away from moisture, for about 7 to 14 days depending on humidity. Skipping this step or storing seeds too soon is the most common reason saved seeds mold instead of sprouting the next season.
Gently rub the center of the dried flower head between your fingers. The seeds are small, narrow, and pointy-tipped, and they'll pull away from the papery chaff fairly easily once fully dry.
Label each envelope or paper bag with the zinnia variety and the date, especially if you're saving seeds from more than one flower type. Keep different varieties separated so you know exactly what you're planting next season.
Paper alone won't stop mice, so place your labeled envelopes inside a metal tin, a glass jar with a tight lid, or a hard plastic storage box, and keep it off the floor if you're storing in a shed or garage. Check on your seeds every few weeks through fall and winter.
