Answer: You started them a little early which may be why they are getting leggy. I like to sow zinnia seed about 4 wks. before the last frost date. Zinnia germinate and grow very quickly - so quickly in fact, that I often sow them directly outdoors after the possibility of frost has passed. The leaves you see now are not the true leaves, the second set of leaves that appear will be the true leaves...they should be appearing any day now. There is really nothing you can do to hasten the process. Where are you growing the seedlings? They may also be getting leggy because they have insufficient lighting. Seedlings will stretch toward the light. If they are in a window sill, you will notice they stretch toward the window (this can be corrected by regular turning). Lighting from above is best, if you are using lighting from above, you may have it too far above the seedlings. Try to situate the lights so they are 1-2" above the seedlings. Also, hold off on fertilizer until you see that second set of leaves appear.
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