Answer: Cosmos are so fast growing that they are difficult to raise from seed at home (without a greenhouse) and in my experience they are better direct sown as soon as the soil has warmed in spring. If they are outgrowing their starter pan or cell packs then you will need to transplant them quickly so they do not become stunted. You can transplant a hair deeper than they are growing now, but this may not be enough to help them support themselves. Once they have been transplanted, one way to slow them a bit is to keep them cooler, but be prepared to raise the lights quite high between now and when they can be set outside on sunny days for the sun they crave. Keep in mind, too, that they do not tolerate frost so you will need to be conservative in planting them out.
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