Following from the previous post.

I was going to move on to another topic, but thought of something else related to light: ultraviolet radiation. This doesn’t play much part in keeping houseplants as such; it becomes more of a concern when the weather warms up and you start thinking of acclimatising indoor winter plants to outdoor conditions.

UV is a type of electromagnetic radiation, just like light. In fact, it’s the set of wavelengths just after the violet wavelengths of the visible spectrum (hence the name ultraviolet). In one sense, there are broadly three categories of UV: A, B, and C. UVC is screened out by the ozone layer of the atmosphere, so we only have to deal with UVA and UVB. A lot of UVB also gets filtered by the ozone layer, but enough makes it to the surface that it matters. However, glass (such as in windows) screens out the great majority of the remaining UVB – so houseplants are exposed to far less UV than plants outside.

This difference in UV gets overlooked because humans normally cannot see UV; generally we consider only light and temperature when we think of hardening off indoor plants. But it’s worth bearing it in mind. While UV intensity outside generally corresponds to visible light intensity, if you use glass-covered cold frames, Wardian cases, or cloches as part of your hardening-off routine, your plants can still get burned by UV even though they’ve acclimated to bright sunlight and cold spring nights.

UV has other effects as well: it causes tanning in humans, it causes fading of many pigments (so keep your precious pictures out of the sun), and it degrades many types of plastic. If you intend to keep or use a plastic item outdoors, make sure it’s made of an outdoor-rated type of plastic first, or it will start to go brittle and maybe even crumble after a few weeks in the sun. This has happened to me with plastic flowerpots that were apparently only intended for houseplants (admittedly cheap ones from the dollar store, made in China). UV is present in all unfiltered sunlight, weak or strong, so believe it or not, you can tan in the shade. It just takes much much longer than in direct sun and will never be very pronounced.

Next topic: dunno. Whatever I feel like when I next sit down to type.