Thickening Roman Chamomile Planting

I have a question regarding Roman chamomile. Last June I planted a 45 X 7 foot (15 X 2.3meter) garden of chamomile in Hamilton, Ontario, Canada, beside my driveway. It has lots of sun and I also included a row of roses and a row of bearded irises. Everything grew well but I would like to thicken up the chamomile.
I had planted around 80 plants in this garden can I use seeds to thicken it up? Do I need to start them indoors, do I sow them directly?

To get a fairly thick lawn-like planting of chamomile the first year, we recommend planting on 15 centimeter (6 inch) centers. I don't know where you planted the roses and bearded irises in respect to to the chamomile, but unless the tall plants are very thick they should not cut out enough sunlight from the chamomile to make it unthrifty. Also Hamilton Ontario has a zone 6 to 7 climate and the chamomile should overwinter well there, even as a seedling. I would therefore recommend direct sowing in the spaces left bare. In a colder climate I would recommend an indoor start to have larger plants go into a harder winter.
But as long as you control weeds well and have patience, you will find that the chamomile will take off the second year and spread more than in the first year. You may not have to do anything in most spots, but wait for the plants to grow into them.