This is a completely novice question, so forgive me. I sowed some mixed cosmos seed at the beginning of April, at the same time as some marigolds. They have been in the warm with lots of light on a windowsill. The marigolds have grown into lovely little sturdy plants, but the cosmos have shot up (about 2 1/2 to 3 inches tall) to become thin and leggy, and although they each now have 2 pairs of leaves they are bending over and can't stand up under their own weight. They can't cope with a fine-rose watering can and a single raindrop would flatten them! What have I done wrong? And can I retrieve the situation?
Any advice greatly appreciated! Many thanks.