As greedy as I am for the plants to produce seed, I still cut some stems for vases in the house - the flowers last about a week. They grow about 3 feet tall before they start going to seed.
|Texas A & M photo (link)|
I have never seen Larkspur seedlings in any garden center, have you? Maybe the wholesale greenhouses sell the liner trays to flower growers.
In the northern U.S., the plants can be bothered with blight and mildew but ours bloom so early that they have always been disease and problem free in our yard. See the Maryland Extension Service Fact Sheet for commercial growers tips. They recommend Consolida orientalis for commercial growers.
Rocket Larkspur - American Meadows
Giant Imperial, Choice,Consolida ajacis etc. 1,000 seeds $2 - Swallowtail Garden
Tall Larkspur - Prairie Moon (catalog still calls them Delphinium)
Sublime Formula Mix plus individual colors - Johnny's Seeds
French Florist, Earl Grey, and more - Renee's Garden
Include these beauties in your seed order so you can plant them this fall!