Hi Thomas,
So, basically you are wondering when to plant your zinnia seeds before a statistical
safe last frost date this Spring. The sooner you plant them, the larger they will be when it is "safe" to set them in the ground indoors. There is a
relatively low probability that the
safe last frost date will come early in the Spring, and
increasingly higher probabilities that it will come later in the Spring.
Apparently my climate is cooler than yours, which means my safe last frost date will come after yours. But "safe" is relative, and there is a good chance that one or both of us might set out zinnias on a "safe" date, and then have a killing frost come a week or so later. Anything weather dependent tends to be a gamble.
I planted some zinnias today that I plan to set out in my garden after about April 15th, which is about 60% safe here. April 30th is not absolutely safe here--there is maybe a 5% chance of a killing frost on May 1st or later for here. I live in a rural area, so there isn't any weather/climate data for right here, but I can interpolate data from two or three nearby places that do have weather data.
With a little adjustment in my growing conditions, I can grow a zinnia longer than I planned without re-potting it.
Those were way too tall for those 3.25-inch pots, so I did re-pot them to 5-inch pots.
The point of all this is that you do have quite a bit of leeway in when you plant your zinnias for a Spring plant-out.
ZM
