Answer: Based on your description, I would suspect your plants need more light. Many gardeners find they need to provide supplemental light, for example, a combination of warm and cool fluorescent bulbs kept within a few inches of the plants. To produce stocky transplants, provide ample light and take care not to overfertilize, as this can produce soft lush growth that is very tender. Some plants will also be stockier if pinched back once.
When it is close to time to plant them outdoors, take care to condition them or harden them off to acclimate them to the weather and sunshine outside. Do this over a week or so by placing them in a sheltered spot out of the wind with a bit of early morning sun and slowly moving them into longer periods of sun. Allow them to go just slightly dry between waterings, too. This process "toughens them up" so they can withstand the rigors of the garden.
When you transplant, try to do it on a cloudy day and be sure to water them in well. Keep an eye on them the first few days to make sure they don't dry out until they become established.
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