Q.Zinnia Seedlings Not Doing Well, Any Way I Can Help Them?
This is my FIRST year to sow zinnia seed on tilled ground. Prior to this I raised all zinnias in containers then transferred them one by one to a garden plot. Our zone 6 soil is clay-ey which requires digging up the plot manually. Very labor intensive work, although the end results were usually spectacular! But I am nearing 70 with a very arthritic back, also hands and arms that are losing strength. I have a hired guy for lawn mowing who also gardens. I asked if he could till some ground for me (about 10×20’ or so) and he did, and it was well done, good and deep. Afterward I threw the flower seed over it then liberally sprayed liquid miracle gro over the area. With that plus several good rains I assumed my zinnias would emerge quickly and soon be a tall bonanza of color. It has been almost a month, the seedlings are up but only about 2” high. I fear it will be a loss. For one thing, I did not add compost to the soil before it was tilled. I didn’t have time and the man came on a certain day before I could arrange for the addition of compost. NEXT time you can bet I will have it raked over the soil before tilling is even discussed. So my question is—Would there be any thing I can do to help along these poor struggling seedlings right now? Hose watering even with added fertilizer isn’t the answer, I can see. What is more, our weather has turned windy and dry which will dehydrate the soil.

Certified GKH Gardening Expert
I do think compost would be the best option, rather than chemical fertilizers. They enjoy rich, loose soil full of organic matter, so that would be more beneficial.
Unfortunately, there isn't much that you can do at this time. for now, you could try and introduce earthworms to the area. In abundance, they will create rich soil as they help tunnel through. At the beginning of winter you can put cardboard over the area and cover with compost. It will kill weeds that are already established which will make it easier to plant. Once it is closer to planting time you will want to till this all together. Planting will be much more successful after that.
Here is an article that will help once you get your area prepared for the next planting season.
https://www.gardeningknowhow.com/ornamental/flowers/zinnia/growing-zinnia-flowers.htm