Tammy,

"If you only grow one type of nasturtium or zinnia etc, will then the seed they produce be true to parent?(nothing in your neighbors yards)"

It will be true only if the nasturtium or zinnia is a pure strain, that is, not an F1 hybrid.

"If you are growing a few varieties, and you cover one plant with netting before first flowers open, then will the seeds of this plant be true?"

Once again, it will be true only the flower is already a pure open pollinated strain. For example, I have covered this flower with netting and deliberately pollinated it only with its own pollen. (It is in a cage to protect it from stray dogs and high winds.)

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I netted it because I really liked the look of it when it first bloomed out. I particularly liked the "toothy" petals and the pale pink color.

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I will be saving seed from it shortly and planting them for a Fall crop of blooms. However, even though I netted it and selfed it, I don't expect that the progeny will look uniformly like the parent, because the parent is itself the result of hand-pollinated crosses that I have made. In fact, it was a cross between two different F1 hybrids, so I expect a lot of variation in its offspring. Hopefully some of them will have "toothy" petals. I might even get something better looking than the female parent, due to a recombination of genes. That possibility of something new and better is one of the reasons why I enjoy the hobby of amateur zinnia breeding.

ZM