Idreos, I did not list zinnias the way I did for any frost tolerance, once they are in leaf, and agree with Zenman to that extent. But as counterintuitive as it may seem, some gardeners have sown seeds of tender plants like zinnias and tomatoes outdoors while temps still go below freezing using the wintersowing method. It just works better if you time sowing seeds like this later in spring. This technique has worked for me, not only with zinnias and tomatoes, but also with browallia, ageratum, Salvia coccinea and portulaca to name a few.
I learned the technique from www.wintersown.org , and there are winter sowing forums on this website and others, as well as face book. Closed containers that are slit for drainage of air and water both from above and below are involved in the technique. I have used recycled milk jugs, but variations on the technique are endless.
We have a member on this website that wrote an article on daves garden on spring sowing - Critterologist - which is a great introduction to spring sowing that would easily get you started.
Zenman thank you for giving the subject of hardy annuals greater depth. Personally, I've always thought of hardy annuals as annual flowers, herbs, vegetables that germinate and grow best when sown early in the spring, while there are still frosts, but not so late that they can't mature and bloom before summer temps fry them.
And you're right - zinnias are not considered to be "hardy annuals" as are poppies and larkspur - and that is what makes winter sowing so amazing - who would have thought plants whose leaves could not survive frost would still germinate with their seeds exposed to winter temps? But if done right, and if certain precautions are taken, many modern gardeners have found it works for a wider range of plants than previously thought.
- - - - -
edited to add April 4, 2021 9:25 am -
Zenman, I am not as skilled as you are with regard to germinating seeds indoors in warm temps. Your and my circumstances might be different in some respects, also. But personally I find winter - or spring - sowing easier, because I do not encounter as many problems with pathogens with cold temps as I do with warm temps - although I 've used both ways among others. And then not as much indoor space is required as with warm-temp sowing, and while the containers are still frozen, I can generally ignore them, albeit not totally. But isn't it great to have alternatives to try, if one way doesn't work out?
Ironically, until the vandal and his/her drones let up, indoor gardening might be my only hope for the time being, although I plan to keep trying in what's left of my late dh's and my garden. You'd think we'd get a break during this pandemic. Evidently, drones are used in utility break maintenance, so a human vandal posing for a camera is no longer required for skeptics.
