E-mail -- Optimal Plant Height and Spacing

Just wanted to know how tall a plant can a 400 watt HPS light grow well ?

To be funny I’d say as high as you can raise the light. The problem is you will only have growth on the top part of the plant if you grow them too tall. From my personal experience I have grown many plants 3 feet high under a 400 W HPS

However, It depends on how close you have the plants to each other and the plants growth habit (branching or single stem).  Also, the container size and how you space the plants will determine how tall they can grow. (Plant spacing LINK)
Basically, if you space your plants further apart, the light will penetrate further and you can grow plants that are taller (but fewer of them) and the opposite is true, if you crowd your plants close together, the lower leaves won’t get enough light and will start to drop off meaning you won't get any flowers/fruit on the lower part of the plants but you will get more plant tops.
In a nutshell, I would not have your plants get much taller than 3 feet with a 400 W unless you are growing corn or some other slender tall plant. If you are growing a plant that has branches, you want leaves all the way to the bottom of the plant. You could put some side lighting like a florescent mounted on the wall a foot above the pots and then you could grow taller plants.
One thing you should do is notice if there is just a bare stem on the bottom of the plants as they flower, if so, next time start to flower the plants when they are a bit shorter.
Having a bare stem is not bad, but space is usually limited indoors and unless you are growing plants for stems, there is no reasons to have the plants make extra stem and less leaves/flowers.
I hope this helps.

Good Growing,

Dr. E.R. Myers

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