No, you cannot get vitamin D through a greenhouse in any real amount. Your skin needs UVB rays to make vitamin D3 on its own, and the glass or plastic of a greenhouse blocks nearly all UVB before it hits you.
UVB greenhouse glass facts trip up a lot of new growers and sunbathers alike. The room feels warm and bright, so the body assumes the light must be the same as outdoor sun, but the wavelengths tell a different story.
I noticed this myself one March when I spent a full hour in my sunny greenhouse with no shirt. My skin felt warm and the light looked strong, but my skin showed no tan or pink hue at all the next day.
That moment made me look up the science behind vitamin D through glass myself. The answer surprised me, since I had always thought sun was sun no matter where you got it from.
Vitamin D3 builds in your skin only when UVB wavelengths hit it from the sun. The exact range runs from 290 to 315 nm on the light spectrum, which sits just past visible blue light.
Standard window glass blocks more than 95% of UVB before it can reach your skin below. The rays bounce off or get absorbed by the glass long before they make it through to your body.
Most greenhouse glazing follows the same rule with poor results for vitamin D. Plastic panels block UVB just like glass does. They still let in plenty of visible light and warmth for your plants to grow.
Specialty UV-transmitting acrylics do let some UVB through for science labs and reptile cages. These panels cost five to ten times more than standard glass and almost no garden greenhouse uses them.
Heat and brightness fool your senses but not your skin cells that need UVB to work. The sun on your face inside the greenhouse warms you but skips the one wavelength your body needs for vitamin D.
I tested this with a cheap UV meter from an online shop for 15 dollars total. The meter read zero UVB inside my greenhouse on a sunny noon day, while outside the same meter jumped to 3.5 on the index.
Sunlight through greenhouse glass still helps your mood and lifts your spirits in winter though. Bright light fights seasonal blues even if it skips the vitamin D side of the deal for your body.
For real vitamin D gains, step outside for 10 to 30 minutes of direct sun on bare skin. Aim for face, arms, and legs a few times per week when the sun sits high in the sky.
In winter months at high latitudes, the sun angle is too low for UVB even outside. A vitamin D3 supplement of 1,000 to 2,000 IU per day fills the gap until spring sun returns to your area.
When I first learned this, I felt cheated by my own greenhouse for the missed vitamin D. Now I treat the space as a plant haven only, and I step outside for my own dose of real sun each day.
Read the full article: Cold Frame Gardening: Complete Guide
