Your goal when growing seedlings is to make them as strong and sturdy as possible, and do it as quickly as possible.
Excellent instructions for growing healthy seedlings are contained in Let’s Grow Tomatoes pages 51-53 and Grow-Bed Gardening pages 86 – 92, and Grow-Bed Gardening has many pictures illustrating the procedure.
Regrettably, both these books are temporarily not available in hard copy, as we have sold out the most recent editions. We hope to soon have them both available for download on the website. Both books are contained in the Gardening Library CD, which I highly recommend.
Meanwhile, here is what Let’s Grow Tomatoes says about pruning leaves to keep the plant stems from becoming spindly, with parenthetical comments added by me:
“When the leaves begin to overlap the leaves of other plants, the experienced grower who insists on thick-stem plants prunes off the leaves which overlap (carefully pinch them off with your finger and thumb). Pruning off the leaves increases the light around the plant stems, and does not stop the growing tip from growing (NEVER take off the growing tip, and always leave 2 or 3 leaves! You can even take off a partial leaf.). Pruning off the leaves temporarily stops the upward growth of the plants, and encourages the stems to thicken, which is what the grower desires.
“During this temporary period of reduced growth in the plant, the grower has two choices:
1. to wait 7 to 10 days for new leaves to grow and overlap again and pinch as before,
2. or, he can shift the plants into larger pots or gallon-size
“In the first choice, the leaves must be pruned off again at the proper time to keep the plants from getting spindly (because the stems quickly become tall, thin, and weak as the plant stretches looking for light), using the same procedure as explained earlier.
“In the second choice, shifting the plants into larger containers provides more space between plants, delays pruning until a later date, and encourages the stems to thicken – due to increased light and circulation around the stems.
“If, therefore, space is available in the seedhouse to accommodate larger containers, it is recommended that the plants be shifted from the smaller pots into 4-inch pots or gallon containers before they are pruned the second time.”
After 2 prunings, it is recommended that the pots be physically separated to provide light to each plant, rather than continuing to prune.