Answer: Allow the best flowers on the best plants to mature fully and dry on the plant. When the seedhead is quite dry nearly ready to open on its own, harvest it. Allow it to continue drying and when fully dry store in a cool dry location with a steady temperature. Inside the refrigerator is a good spot -- place the dry seeds in a closed jar or other airtight container and be sure to label them.
If you plant them in spring, note that they may need a cold stratification period of a month or so (place in a plastic bag with damp soilless mix or sand or peat moss and chill in the fridge) in order to germinate, although some seeds will germinate without this treatment.
Alternatively, they can be planted in the garden now and will germinate this summer and/or next spring. Another (easier) method is to allow the plants to self sow and then transplant the small seedlings.
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