In winter, let’s face it: when it’s pitch-dark at 5 p.m., it’s comforting to turn on all the lights in the house, inside and out.
In summer, when the evening goes on on the terrace, there’s nothing better than street lamps that light up like daylight to avoid tripping or spilling your drink.
But do you know what impact this light has on your plants and on the natural world around you? While we humans need light to see and make the mood cosy, it’s a different story for many species.

Winter-Flowering Plants
If many plants don’t even notice that the living room lights are on, others may sulk – all season long! The best known of these “difficult” plants is undoubtedly the poinsettia. What a light drama queen that one is!
The pretty red, pink or white “flowers” are not really the flowers of this plant: they are its bracts.
The flowers are tiny and white or yellow: they are in the center of red bracts. When flowering begins, the plant produces leaves at the base of the stalk that are different from the rest of the plant, either in shape, size or, as in this case, color. These special leaves, which accompany the flowers, are known as bracts, and it’s these that are most appreciated in the poinsettia.

This famous Christmas plant, seen everywhere during the festive season, is reputed to be easy to care for. In fact, it’s easy to care for. BUT! After flowering (which can last several months), it loses its pretty, colorful bracts and becomes a rather ordinary green plant. The most optimistic keep their plant until the following year, hoping for the return of these beautiful colors, but in vain! It will stay green, year after year.
Poinsettia flowering is triggered by the shortening days. As soon as the photoperiod (the period of the day when the sun is present) falls below 12 hours, flowering is “triggered”. This happens every year around September 22, at the fall equinox. Everywhere on earth, the length of the day will be exactly 12h, after which the length of the day continues to decrease in the northern hemisphere, while it continues to lengthen in the southern hemisphere.
For the next two months, these plants, which flower with short photoperiods, can be disturbed by the slightest light. Some are more sensitive than others: the poinsettia tolerates NO light after sunset. To see it bloom again, from 4 or 5 p.m. onwards, no lamps, nightlights or telephone lights are allowed to shine on it, even for a minute or two!

The Laidback Gardener suggests throwing away your poinsettia and buying a new one the following Christmas… Or, if you’re so inclined, locking it in a cupboard every evening and taking it out again every morning. Let’s not forget that this plant needs sunlight during the day!
In short, the light from your bulbs is not enough to feed the majority of plants and you sometimes have to add grow lights in winter to help them survive when you lack windows… but this low light is still enough to make the most sensitive plants fail to bloom!
Nocturnal Predators
In winter, outdoor lighting doesn’t affect your plants too much (I know, what a surprise!), but in summer, it’s a different story! I honestly don’t know of any outdoor plants that are directly affected by night-time lighting, but many of the players in their environment are greatly disturbed by outdoor lights.
A study suggests that the leaves of plants lit by street lamps become so tough that insects can no longer consume them. Well, whether you can protect a fruit tree from Japanese beetles by illuminating it remains to be seen? But it’s a pretty interesting effect of light on plants!
Little research has been done on the impact of light on ecosystems, but I had the chance to speak with specialists working in the Mont-Mégantic International Dark Sky Reserve. Did you know that we have such a reserve in Quebec? The original aim was to provide a light pollution-free sky for astronomers observing from the summit of Mont-Mégantic, but the effect on wildlife is undeniable.

When you have a light on your house, what happens? Many insects are attracted to the light and come circling around. Predators such as spiders, bats and even tree frogs enjoy the buffet. So far, so good, right?
But here’s the thing: this artificial lighting could upset the normal prey-predator balance around your home. Imagine that tree frogs, instead of eating the slugs in your garden, decide to settle on your house wall and eat the spiders that make their webs near the light. You’ve just lost two predators that might otherwise have fed on the caterpillars in your fruit trees or the biting mosquitoes invading your terrace.

It sounds anecdotal, and as I said, there aren’t really any studies that show this, but nature is opportunistic enough to forsake a hunting ground for an all-you-can-eat buffet! Personally, I’d much rather all-you-can-eat sushi than go out and catch my own fish!
Similarly, fireflies, which are in catastrophic decline, are very useful predators in the garden when in their larval stage. But since their mode of reproduction is based on the exchange of light signals, it is very difficult, if not impossible, for them to reproduce in the presence of artificial light.
I personally have a small army of fireflies at home. There’s no light to disturb them, and they’re very numerous. In comparison, my neighbors on the street have several large spotlights (I insist, SPOTS. These aren’t dainty little single-bulb lights!) and there are hardly any fireflies around.

Les pollinisateurs nocturnes
Some of the insects “captured” by artificial light cannot fulfill their role. This is the case of saturniidae such as the Luna moth, which is the emblem of Mont-Mégantic Park.
The importance of nocturnal pollinators is often overlooked. The star of pollination is the bee, and because it is diurnal, we think that pollen exchange is an activity that takes place in broad daylight. But reproduction is a 24-hour activity!
In fact, many flowers only open for a few hours, and some open precisely in the evening, after the daytime insects have retired for the night. That’s when the work begins for the stars of the night! That is… if they’re not distracted by the lights in your yard…!


arakso

emanuellec
A few pollinating moths.
But… These Insects Are So Silly!
That’s what you might think when you see a pretty butterfly swirling between mosquitoes around a lamppost instead of feeding. In fact, it’s not a lack of intelligence (although I’m not sure you can call an insect intelligent!), but rather a question of biology.
The current hypothesis is that many insects find their bearings in sunlight, starlight or moonlight. They are highly sensitive to light, and are therefore disturbed by such a large and close light source. Researchers believe that insects, once close to the bulb, are blinded and trapped in this overly bright environment, not knowing which way to turn, which explains their chaotic flight.
Reducing Artificial Light?
As humans, we’re still very dependent on light: sight is our most highly-developed sense, and the one we use the most. And our vision is adapted to the day, not the night! So what do we do with our difficult-to-bloom plants and our evenings out on the terrace?
Well, as Larry said a long time ago, after Christmas, the poinsettia goes in the garbage…hmm compost!
Your Christmas cactus hates being moved, but still needs short days to flower… Fortunately, it is (a little) less sensitive than the other one, but it still doesn’t like light in the evening, so… give it a try for a few years by leaving it in a room that isn’t used much in the evening, and if you don’t succeed… too bad, they sell others!
Summer, however, is your time to change your habits! When you’re not outside, turn off the lights. When you are outside, opt for soft, ground-level lighting. That way, you’ll trap far fewer bugs. And let’s be honest: why waste half your light on the sky when you can direct it on what you really need to see? All you need is a lampshade and you’re done: more light for the terrace and less for the bugs!



Yes, but the thieves! You have to leave a light on to intimidate them!
I’m glad you mentioned it! A study has just been done on the subject! When there’s a lot of light, you get a false sense of security: you can see! Except that ill-intentioned people can see too! In the study, darkness was more of a deterrent: the miscreant can’t see anything, so how can he know what there is to steal? No one can pick a lock without seeing! And if he were to switch on a light, the miscreant would be highly visible and therefore much more exposed than in full daylight! That said, to combine dissuasive darkness with optimum security, why not install an automatic light with motion detector? This way, you remain in the shadows most of the time, but a sudden, targeted light will scare off intruders while signalling their presence.
In short, your light may not make much difference in the heart of the city, but in the suburbs or the countryside, do nature a favor and point your lights at the ground, or even better: turn them off!

Branches of London plane street trees that got too close to street lamps in San Jose used to retain their foliage a bit longer than the rest of their canopies. The weather is so mild that the little bit of heat (rather than light) from the street lamps was enough to interfere with defoliation. Street lamps are more efficient, so do not waste so much energy on warmth nowadays.
Thta’s fascinating!
Outdoor lighting always is a contentious issue. In my neighbourhood people light up their properties like it was a mini-Vegas. Why are we so uncomfortable in the dark? There is much to see if we could only train our senses to observe, hear and appreciate the beautiful night sky?
How long do you have to restrict light exposure? Is it until natural daylight days get longer?
https://laidbackgardener.blog/2020/09/21/the-easy-way-of-getting-your-poinsettia-to-rebloom/