RickCorey said:Zinnias should ideally be brown and dried up. The longer a seed can stay on the plant without rotting or being eaten or blown away, the better.
Let them mature as long as practical.
I don't know about corn.
Weedwhacker said:David, to collect seeds from corn the ears have to mature past the eating stage, until the husks on the outside of the cobs are brown and the kernels are hard and dry.
thank you ....is there a limit?what happens if i leave the cob for a month
That's not a problem... pretty much the longer the better. And you can use the corn stalks for autumn decorations