I have used crowns to buy the deadly helephant ears and the magma peas. I did not receive a seed from them at any time.
Pink Dandelions, King Parsley, and Silver Trumpet Vines often drop seeds but I am not sure if this is every time they are elder. Also, those are often found as drops, so if you are patient, you can get the seeds without spending crowns.
Hannah Lifebringer Level 60 Life Grandmaster Gardener
Certain special crown plants, like the Giving Tree, which was only available for a short time, don't give seeds. I don't think the Deadly Helephant Ears or Evil Magma Peas give seeds, but most other plants will give their seeds MOST OF THE TIME. You can encounter a plant which doesn't give you a seed during a harvest.
But, a lot of these same seeds are available as drops from enemies in the Spiral, so you'll get more through questing.