We don't use the term 'half-hardy' much in this country. It simply means that the seeds can be direct sown outside after freezing temperatures are over, but the soil doesn't need to warm up. The seeds will still germinate at a higher percentage and much faster if heated.

I wouldn't direct sow zinnia in your location this early. Find out when your projected last frost date is and give it a couple of weeks after that. Or start inside.

A half-hardy annual, such as zinnia, can tolerate light frosts. A heavier frost will take them out.

Dave,

T&M tell what the initials mean on their website. They give recommendations for sowing in their individual product details. I don't agree wholeheartedly with everything they say, but their info is a better guide than nothing.

ZM

Waaaay too cold (air, especially night temps, and ground temp) to plant annual flower seeds outdoors in Michigan. You could start some indoors in the next upcoming weeks, though.

Just out of curiosity, what can Dave sow outside now? Anything?

T & M was founded in the UK and in Britain 'half hardy' applies to annuals which need to be started indoors and planted out after risk of frost is past. This would include zinnias, salvias, petunias, impatiens etc. In our climate the plants would never reach a good flowering size if we waited for outdoor temps to be sufficient for germination. Hardy annuals can be direct sown in early spring e.g calendula, candytuft etc. It doesn't seem a very useful term in the US where conditions vary so greatly.

rhizo,

"Just out of curiosity, what can Dave sow outside now? Anything?"

I don't intentially practice Winter Sowing, but the Winter Sowing enthusiasts would probably answer your question by saying "everything". In the case of zinnias, Winter Sowing doesn't give you a very big headstart, maybe a week or two. You can get a month or more of headstart by sowing inside and setting the seedlings out. However, zinnia seeds can surprise you.

When I was gardening in Maine, I had a lot of zinnias come up "volunteer" in the spring. Those zinnia seeds had wintered over and undoubtedly been frozen solid for a period of months, since that part of Maine had a frostline of about 4 feet and the soil was frozen solid for months at a time. Single-digit highs and sub-zero lows were common. A few of those wintered-over specimens were even good enough to become breeders in my amateur zinnia breeding program.

ZM

I think I am going to start some hardy perennials outdoor this week. I will do some indoor starting of zinnia since I have plenty of zinnia seeds.

dave, I should think you could start seeds for the flowers that I grow in the winter here: pansies, ornamental cabbages, snapdragons, larkspur, poppies, calendula, lobelia, even sweet alyssum. This assumes that you may have light frost, but no temps probably below the mid twenties yet. There are probably more. I don't sow things (more than once) that I know won't stand a chance once high temps move in. I tried Baby Blue Eyes this year, but it's been in the mid eighties the last several days here. It will be interesting to see how they do.