It's crazy how often this happens to me. I swear I actually manage relatively complex tasks on a regular basis but the 'easy' garden plants tend to fail for me. I took me three years to figure out peas and I kill at least one rosemary plant every year. Sunflowers are also a problem for me. I've tried planting them several times and the only ones that come up are the wild ones and the ones that stayed dormant for at least a whole year.
I stumbled across one website where they mentioned in passing that sunflowers prefer cooler soils to germinate. They are such an integral part of my idea of summer and fall that I thought I had to wait till the soils were ready for beans and squash (very warm) before I planted them. With that information I'm going to try soaking them in the fridge for a couple of days before moving them to the ground. I want to plant sunflowers in the grass outside my bedroom window. I think a line of them between me and the neighbors would make a nice temporary hedge. If I can get them to sprout, are these vigorous enough to survive being direct seeded in a relatively healthy patch of lawn?
I don't need enough for a huge crop. We always allow the wild sunflowers to grow where ever they're not in the way because we like the flowers and the wildlife it attracts. As much as people plant for butterflies, my favorites are the tiny little finches that will hang from the sunflower heads as they feast.
We've had good sunflower years and then years with no sunflowers. I made the mistake of allowing them to overtake the vegetable garden a couple of times, which was great for the little goldfinches, but not so great for the vegetables. I hope eventually to establish a good stand of native sunflowers in the front yard where they can get as big as they like. I've never had success growing domestic sunflowers here.
My mother has had success. The most sensational being when she was pregnant with my little brother. When she gave birth the sunflowers against the wall where her bed was, all opened in a span of less than an hour. The rest of the row (on both sides) in the same growing conditions opened more than a week later. They've got a special place in the family history. I can't remember her doing anything special to get hers to grow, though.
I've been direct sewing sunflowers about 4 weeks before my last frost date.
But, I'm growing huge heritage sunflowers with big happy seeds (thus they have more energy for surviving cold snaps).
I don't know if they thrive when being transplanted. In the past, I would have said no way. But, now I often see sunflower transplants in the shop (along with peas and beans - also veg I thought were stunted from transplanting) so maybe they don't mind it. The best way to find out what works for you is to try some started in pots and some direct seeded then let us know how it goes.
What I'm growing is Mongolian giant which is supposed to grow 12 feet tall (grows closer to 16 in good soil). It's such a prolific sunflower, I just love it. I grow it next to the hen yard for shade but this year I had to cut a bunch down as they were touching the power lines and they hadn't even gotten to full height.
Our average last frost date is now officially Feb 13th. So I missed that window. My inclination is to still pay attention to the old frost date which was May 15th as we can have 'blue northers' at any time of the year where the temperature can drop 40 degrees or more in just a few hours. This year there is ever indication that we've already had our last frost. My apple tree started blooming a couple weeks ago.
Even if I'm wrong, I'm probably not hurting the plant. I'll pull them after a few days and then check the paper towel daily until the tiniest hint of root pokes out and then plant them all in the ground. It works well with peas and seems to be working with parsnips and pistachios. Seems worth trying with other letters of the alphabet, too.
If this does work, I'll plant any seed that I save in late January next year. I had been planting several weeks after the last average frost date, which means even longer after the last actual frost. If they need cooler temperatures to germinate that would explain what was going wrong. I know in the late summer when it will be time to start plants for fall it is often recommended to prechill the seeds of things like lettuce and broccoli, for just this reason.