How long do Zinnia seeds remain viable in the soil or mulch?
This is the 3rd year that I have been growing these Burpee zinnias from seed that, although advertised to grow to 30-36", usually grow to at least 5-6 feet tall every year I have grown them, regardless of how much sun they receive! They grow flowers of at least 4" diameter or a little larger, and the packs seem to include flowers of at least 7 different colors and shades of similar colors.
They got powdery mildew last year for the 1st time, in August after about 2 months of blooming, but although their leaves became whitish gray, they still continued blooming. That brings up question 1: How to prevent powdery mildew on zinnias? They are fairly tightly packed because they get larger than the packages claim, and wider as well. They also grow LOTS of large lateral branches that spread out all over! They have large, deciduous hibiscus in front of them, but no large plants on the other 3 sides.
Anyway, I mixed in 2 of these large Burpee cactus flowered zinnias with the other variety, so there are about 8-9 plants in front of the house, and they have been blooming for nearly a month now, and are already between 4-5 feet tall!
Back on topic: Last year's crop dropped countless seeds onto the bed they are planted in, and the bed was re-mulched this year in early May right after planting the zinnia seeds there. But I dont yet see but 1 or 2 plants that apparently have grown from the seeds that fell last year, and no other seedlings anywhere! I do have zillions of these white-flowered Daturas that are popping up all over the place, from seeds that dropped from their flowers last year, but no zinnias.... So, if they arent coming up this year, how long will they last in or on the soil or mulch? Will the seeds dropped last year come up next year or in future years?