Some plants thrive on abuse and neglect! Marigold definitely don't like fuss and like to get a little dryer now and then. Don't over water. Try buynig a heated seed mat for under your tray.
The marigolds I am so over. They just don't wanna grow. The zinnias did come up after I resowed. But the problem with just sowing direct is that my ground is covered with mulch. Like a lot of mulch. so I cant direct seed.
these are 2 of my favorite annuals. i put some seeds in my cold-frame and i think i started to early, i got nothing. my planting guide says marigolds can be planted in the ground in april and zinnias in march. so maybe i started to early, i will try again in about a week. the seeds may have drown with all our rain here. oh and my guide is for zone 9.
They all should start real easy once the ground is warm.
If ya plant them, then it suddenly cools off, lay some plastic over the seed bed, lightly, rocks at corners and edges, maybe a few twigs across the middle.
Just be careful to remove it if it suddenly warms up again.
Remember though, there are some varieties of each of these things that are harder to sprout than other varieties, but most of the above should be good and easy, and give high percentage germination.