Kelp forests are disappearing worldwide due to climate change and human stressors, highlighting the need for active interventions. A new restoration approach, “green gravel”, has been shown to be a potentially effective tool to restore endangered kelp forests. However, green gravel is still a novel technique that needs better experimentation and optimisation at all process stages. Contamination by other algal species is one of the critical factors in early-stage green gravel production because their overgrowth can lead to the loss of the seeded material. In this study, we assessed the effect of light intensity on kelp growth and on the coverage of contaminating algae on green gravel. Our results show that under high lights, kelps displayed faster growth (recruits on average more than three times the size and covering a six-times-larger area in high light intensity than in low light), but there was also a higher percentage of contaminating algae. In contrast, the green gravel cultivated under low lights showed almost no signs of algal contamination, but the area occupied by kelps and the length of the lamina were dramatically lower. Due to the cultivation conditions, opportunistic species can grow fast. This advantage is expected to disappear once the green gravel is deployed. To obtain cleaner cultures and to avoid the risk of losing the cultivated material, we would advise starting rearing under lower light intensity to reduce the risk of contamination but ensure kelp growth and then increasing the light intensity to boost it. Clear and appropriate protocols are absolutely necessary to minimise production costs and times and for the scaling-up of future attempts at marine forest restoration.