Seedlings started indoors tend to be weaker than those that sprout outside in the garden. Indoor seedlings tend to be taller and more tender, even if grown in strong light. The difference is the absence of wind—as wind shakes outdoor seedlings, their stems become stronger and their growth remains stocky.
(Related: Should you pinch back your seedlings?)
Indoors, you can re-create the effect of the blowing wind by "brushing" your seedlings. Brush a folded piece of paper across the tops of small seedlings, quickly but gently bending them to horizontal and letting them pop up again. Do this twice a day for about a minute. As seedlings get larger, you can use your hand instead of the paper.
In studies, seedlings that were brushed had smaller, darker leaves that grew closer together than those of seedlings that were not brushed.
Another way to strengthen indoor seedlings is to set up a small electric fan near their growing station. The fan should be positioned and set so that the air flow just makes the seedlings vibrate or wiggle, not bend. Keep in mind that this method can make the growing medium dry out more quickly.
