Hold the seeds for the spring. I plant sunflowers around the edge of the garden... a couple in the middle of the hills of pumpkins or cukes or melons.... They are usually very hardy. Don't like to be disturbed so plant where you want them to be....I have never worried about type of soil since there is not much changing what is there.... amending with compost and any and all organic matter I can find. Plant when you are pretty sure of no frost... a little won't hurt them but I usually get my garden in about last frost... here it is mid-May . I am not in a rush and everything gets put in within a week except sweet potatoes. They are much less cold tolerant. I don't get in a rush to get potatoes in early, or cold weather crops since I am lucky to get it tilled when it suits someone.... I like broccoli and stuff for fall crops since I don't get them in early enough for early harvesting...
If the garden needs water, the sunflowers can use it. MULCH and you will use alot less water....