My zinnia's have pretty much gone through their cycle. A lot of them are turning brown and dying. I was thinking of just letting them die and let them reseed. Is there anything I need to do or just let nature take its course? Or would it be better to pull them up and plant new ones? They look bad like this. Also if I cut/dead head the flowers will they produce more? And if so how far down should I cut? I'm in zone 9 btw.