I've tried the bio domes and they work quite good. The plastic domes are made of PET (polyethylene terephthalate). PET is crystal clear with full spectrum transmission within the necessary wavelengths (PAR), but it does "absorb" below 380nm (UV). All plastics absorb UV, so greenhouse film adds a component to reflect it so it can last 5-6 years in direct sunlight. PET absorbs harmful UV, blocking it from young seedlings, but it also causes it to turn cloudy and brittle over time when exposed to direct sunlight. Be very careful where you store the domes. Keep them out of direct sunlight when they're not covering plants. It's long-term exposure to direct sunlight that's a problem, not setting it out in the sunshine full of seedlings.
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