As noted above in response and in guidance, cosmos are not demanding. Your generic potting soil is probably fine; 鈥渓ousy鈥 was your word re your in-ground cosmos and we meant only that potting soil does not need to be rich/composted/fertilized. Another thought is that you use the in-ground soil in the pot. If indeed the pot is appropriate. You may have identified the problem as a too-small pot; the cosmos may need more leg room, so to speak. Does this help?
As noted above in response and in guidance, cosmos are not demanding. Your generic potting soil is probably fine; 鈥渓ousy鈥 was your word re your in-ground cosmos and we meant only that potting soil does not need to be rich/composted/fertilized. Another thought is that you use the in-ground soil in the pot. If indeed the pot is appropriate. You may have identified the problem as a too-small pot; the cosmos may need more leg room, so to speak. Does this help?