To protect plants from excessive sun and heat inside a polytunnel / hoop house you can use a variety of methods such as installing shade cloth, applying whitewash, improving ventilation, mulching, and using drip irrigation. The goal is to balance the amount of sunlight entering the tunnel while preventing overheating, ensuring your plants thrive under optimal conditions. By using these techniques, you can create a controlled environment where plants get the sunlight they need without being damaged by excess heat.