Palmaria palmata is a valuable Atlantic seaweed; Ireland is a major producer. Current retail value is €80-€120 kg-1 (dry), with 80-100 tonnes (wet weight) wild-harvested annually. Cultivation is also possible; at sea (longlines) and on shore (tanks). In a series of vegetative growth trials, frond tissue (‘tips’) were grown to assess 1) effect of photoperiod (12:12 and 16:8 light:dark; L:D) with nutrient addition (seawater control, 50 % or 100% F/2 culture medium twice wk-1, 100 % F/2 once wk-1); 2) effect of light quality on cultures in 16:8 L:D red, green, blue and white light LEDs compared to older cool white LEDs 3) effect of 12 days in green light, then 12 days in blue light (12:12 L:D in 20 (25%), 40 (50%) and 80 (100%) µmol m-2 s-1 blue light, 100% green light control, 100% blue light control). Specific Growth Rates (SGRs) of tips were highest (13.89 % day-1) in 16:8 (L:D) and 100 % F/2 twice wk-1. However, 12:12 L:D cultures had significantly higher Total Nitrogen (4%) than 16:8 cultures. Light quality affected biomass of cultures; greatest biomass achieved jointly under Green LEDs and old cool white LEDs, followed by red light, with blue light the least effective. SGRs ranged from 12.2-8.9 % day-1. In the green/blue light experiment, green light alone produced significantly the best biomass results, with overall increases of 220%, compared to green-then-blue light cultures, with 50% and 100% blue cultures not significantly different with 105% and 130%, respectively). Blue light controls increased the least (50%), indicating that initial green light enhanced biomass of blue cultures. Protein results of all trials will be discussed.