I have a romaine mix going right now in hydro, I am curious as to harvesting it in this particular method (cutting the heads off to provide more horizontal growth rather than vertical growth.)

Will it kill the plant if I trim the growing head?

Or do I need to just cut the leaves off from bottom up?

I have 80 lettuce plants under a T5 fluorescent fixture in a DWC system at 560ppm foxfarm and a 15 on 9 off light cycle...