Answer: Actually, alyssum is usually allowed to grow in small clumps because it fills in and grows thicker sooner that way-- each individual plant is quite small on its own. You might leave a clump every six inches or so, say at least several dozen seedlings or so to a clump, or lesser clumps closer together. If the plants are sparser than that, simply leave them be.
To move some, be sure they are well watered ahead of time, then lift them up complete with a nice little clump of soil on the end of your trowel and replant immediatelyinto prepared soil at the same depth they grew before. The idea is to take enough soil that all of their roots are included without being disturbed. Water well. Usually they will not even notice they have been moved.
Other, larger plants such as marigolds would be thinned to individual plants as soon as they are big enough to grasp, or at least by the time they are an inch or so tall so as not to crowd the others. They would also be spaced more carefully, roughly as far apart as their mature width, so that for example, two plants each expected to reach a foot wide would be planted a foot apart.
Enjoy your plants!
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