Rudbeckia or black-eyed Susan seeds so themselves so liberally that they can become a nuisance. Collect black-eyed Susan seeds so that you can plant them precisely where you want them. Each seed head contains hundreds of seeds. They will not all germinate, so collect more than you will need. (Feed your extras to the birds.)
It is illegal to propagate patented plants, so if the plant tag or the seed packet warns against propagation, just cut the stems when you notice petals falling.
Test When You See Petals Falling
Begin collecting black-eyed Susan seeds 3 or 4 weeks after the plant has bloomed. The seed head should be grey-brown. To test whether or not the seeds are ready, just hold the seed head in your hand and run your thumb across it. If the seeds are dry, you will see some in your hand.
Collect Seeds in Seed Traps
When the seeds are dry, cut the stalks as long as you can without damaging the base of the plant. Group the stalks into bundles and tie the bundles together with string or twine. Seed traps will gather the seed for you. The simplest trap to make is a paper bag. Simply put the stalks into the bag head down and tie the bag to the stalks. Now hang the bundles in a cool, dry place, and wait for the seed to fall.
Store Seeds in Refrigerator
Once the seeds have fallen, put them in a labeled envelope and seal it. Store them in your refrigerator. Black-eyed Susan seeds need at least 3 months of cold to germinate.