Hello Amy,

I can't give you good short answers to your questions. And my experience is limited to indoor zinnias, and they involve a lot of factors. Setting the timers on your lights determines the photoperiod your plants see, and photoperiod can have a big effect on how plants grow. But that is just one of many factors affecting your plants. I suggest studying a good book on the entire subject of growing plants under lights, like for example, Gardening Under Lights. (a clickable link)

Hopefully, others can provide some helpful responses to your questions. They are good questions.

ZM (not associated with any product or vendor mentioned or linked)

What plants are you growing?

What medium are they growing in?

What is the air temperature?

I do not think grow lights are causing the stunting here. If you can provide specifics or growing conditions -which plants, what lights, distance from plants, how long are the lights on, temperature, humidity, potting mix used - then we may be able to help figure out where the problem lies.