You can use them to identify the new generation seeds/colored seeds, and plain seeds (but you have to pot them and dump to get it to tag the I.D. on plain seeds).
They come from the Poppy plant (new generation plant, more spread out than the old poppy plant).
You can get the seeds for them off of wisps.
The Poppies dust is a resource from the same plant. they have 8 charges per dust.
When the charges are used up, they make great deco items.
Check your local growers and auction houses if you don't want to grow the plants.
The price per dust is usually around 2k on Legends (check out the PRAH auction house, they get them from time to time.)
There was another thread about this, but over time I have lost it.
Hope it helps though.