why do my mums in my garden separate in the middle and lay down?

Answer 1

They get too leggy and can't support themselves when a strong rain storm comes. Push a thin stake into the middle and loosely tie the flopped-over stalks to that central stake to keep them up.

Answer 2

Sometimes they get too leggy if you've got too many nitrates in your soil/fertiliser and the stems aren't then strong enough to support the heads.

Give them support stakes this year and next year reduce the amount of fertiliser.