When I started feeding thistle to the birds, I heard somewhere that it will not germinate. I felt safe that I was not adding any more of the invasive plant to our environment.
A month ago, a friend gave me an opened bag of thistle, saying she got it from someone who started feeding this to the birds, but noticed thistle sprouting around her place where she had none before. This friend did not want to risk having any in her yard, so was passing it on to me. Now I hear others saying they have stopped feeding thistle to the birds for that reason. This aroused my curiosity: Maybe I've been wrong all this time.
Here is what I learned after searching:
1. It is not a thistle at all, but rather Niger Plant (Guizotia abyssinica). It is believed "thistle" was used in early marketing to take advantage of the Goldfinches' preference for thistle seeds. The plant has no prickles and it is related to sunflowers. The Wild Bird Feeding Industry has trademarked the name Nyjer in an attempt to eliminate further confusion and avoid possible mispronunciation.
2. This seed should not germinate here. As of 1982, the USDA required the imported seed to be sterilized through a heat-treatment process. In 2001, they increased the temperature because the original requirement didn't quite do the job. Some known invasives would get into the Nyjer and grow here.
3. Cultivars have been developed in the US for agricultural production, and this is not considered to be a Federal noxious weed.
If you find thistles growing in your yard, it might be from seed blowing in from a nearby location. Continue to fill your socks and finch feeders with this seed and enjoy the birds!
For interest's sake, I found some Canada Thistle seed and placed it alongside Nyjer.
Birds pictured: One Pine Siskin and two American Goldfinches, a heap of Common Redpolls (We fed about 70 of these birds for a month in 2013. They ate anything, but by far, they preferred Nyjer. )