From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1817-961273
Question: What is the proper time to begin thinning seedlings? What
is the best way to go about this? Clipping at soil level? If your
answer includes something about two *true* leaves, what does that
mean? I see leaves, but they are the first ones to appear sodoes that
mean these are not the first true leaves? Should I wait before
thinning the seedlings? Thanks for your patience. Sara McDonald
Atlanta, GA
Answer: The "true leaves" are generally the second set of leaves that
a new plant produces and look like the adult leaves. After the
second set of leaves are produced is the time to start thinning. You
can usually identify a plant by it's second set of leaves. This helps
you to distinguish between the weeds and what you have planted.
Clipping at ground level is the preferred way to thin.