Questions about planting seeds

This is probably a stupid question, but I'm just starting to experiment with growing from seeds. All the seed packets I have provide specific depths to plant the seeds under the soil, so I assume I shouldn't mulch over them if I plant directly in the ground (as opposed to containers or flats). I do want to mulch around the plants eventually, so when is the most appropriate time to do that? Does it depend upon how tall the seedlings are, what type of plant, etc.? For example, I just planted sunflowers, zinnias and cosmos. All three of the packets say to thin and separate when the seedlings stand about 2 inches tall. Would it be OK to mulch at that point, or should I wait until they get taller? Thanks in advance for any assistance.
Cheers, Rhonda Alexandria, VA Zone 7
********* It's a rave, darling, a mad rave!
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Natty_Dread wrote:

It should be OK to mulch at that point. As long as the mulch doesn't cover the true leaves of the plant. The purpose of the mulch is to reduce weeding requirements, so before applying the mulch, kill all the weeds around the plants. This can be done by simply stirring the soil with a small trowel. Just stir the top inch or two of soil away from the plant. Near the plant stir the top half inch or less, trying not to disturb the plant's established roots.
One of the definitions of a weed is a plant that you don't want that is really persistent. If such a weed seed has germinated (not necessarily emerged from the surface), it is likely to grow through your mulch, so stirring the soil will damage the roots of the weed and kill it. The mulch will keep the ungerminated weed seeds deep so they won't germinate. Most weed seeds are small and there's a rule of thumb that seeds won't germinate if they are buried more than seven times their maximum dimension (maybe it's their average dimension, I'm not really sure, but at any rate it's just a rule of thumb [which means that there are lots of exceptions to the rule]). If you stir the soil too deep, you will bring up weed seeds that wouldn't otherwise germinate, so shallow cultivation is best. When cultivating, if you see little white threads, those are the roots of weeds that have germinated, but haven't emerged yet. Exposing them to air so that they dry out kills them. This doesn't work well on cloudy days after a lot of rain.
I plant sunflowers and zinnias 9 inches apart, and it works well, so that's a good way to thin them when they're large enough. When thinning, start with the healthiest plant at the end of the row, then find the healthiest plant at your desired thinning distance away, within maybe 50%. You don't need exact spacing. The healthiest plant is not necessarily the tallest, but rather the one with some combination of the thickest stem and the best looking leaves. Thinning is the hardest part of gardening for a new gardener: one never likes to kill the plants they planted in the first place.
Cosmos seems to come in two varieties: early and late. Every time I plant cosmos about half of them bloom. The others just form a large green plant. Later in the summer the blooming plants start dying and the green plants take over the blooming. So don't expect them all to bloom at once.
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