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This is a short and sweet answer to a query I sometimes get.
In general, plants turning purple isn’t that big of a deal. It’s often a sign of stress, but not like, bad stress. Well, then plant doesn’t much enjoy it, but we do.
Heat-stressed plants can turn purple
I’m actually colourblind so I could be wrong, but heat-stressed plant usually go a reddish purple. I’t starts more purple and then fades to red. Here is my poor, poor neglected jade plant that lives outside:
(He’s picked up a spider friend, so here’s your spider trigger warning, though he is hiding behind a leaf)
The red is a result of heat stress, but it’s actually a form of sun protection. The plant produces carotenoids and anthocyanidins that protect the plant from UV rays.
(And no, eating a lot of beta carotene won’t give you any meaningful sun protection)
I’ve turned to Google to try to ascertain whether tomatoes/strawberries/bell peppers are red because they grow in bright light but it won’t say. It seems likely though.
Plant leaves turning purple can be a nature response to bright light, and will actually help them grow well – for example a lot of succulents naturally turn purple on summer so they maximise the amount of photosynthesising they’re doing without getting burnt.
In general, a plant that’s turning purple due to heat stress is just doing it’s thing – if the plant was heat stressed to a dangerous degree, it would simply burn to a crisp.
Whilst plant leaves turning purple in the sun is likely a perfectly natural response to higher light, you do need to make sure you’re not stressing them out in other ways, so be sure to keep them well hydrated and fed. The combination of good care and a lot of light will result in a lot of good growth.
Phosphorus deficiency can cause plants to go purple
Phosphorus deficiency also causes purple leaves in plants. It’s more of a blue purple than a reddish one (though again, colourblind, so my eyes lie to me) and I don’t have a photo because…it’s not that common in house plants. It’s more likely in crops that grow quickly and use up a tonne of nutrients.
It actually looks quite pretty, and often doesn’t really look like there’s anything wrong with the plant. If you suspect phosphorus deficiency, then add a fertiliser that has fertiliser in it. I use the general hydroponics one that will work well, but fish emulsion is a great option (so long as you don’t mind you’re plant smelling of fish).
Your plant may be naturally purple
Some plants are…just purple. Rubber plants (Ficus robusta/elastica) come in green and burgundy varieties, and sometimes get greener or more burgundy as they age.
Plants like Calathea and Stromanthe have purple on the undersides of their leaves which apparently helps them absorb light from lower light levels (seems a bit weird that they’d only have purple on the undersides then, but who am I to judge?).
There is also a theory that plants in the undergrowth have purple leaves because they’re less likely to be eaten by predators (herbivorous ones, that is). I’m not sure if it’s a camouflage thing or a ‘I look poisonous thing’ but that makes sense.
There are also theories that plants with higher levels of anthocyanins are better protected from funguses (of which I bet there are many on the rainforest floor because it’s warm and humid) or even from the odd ray of bright light that makes it to the ground.
Purple plants still have chlorophyll, it’s just masked by the amount of purple pigment (such as the anthocyanins I mentioned before)
Bugs may be leaving purple marks on your plants
Specifically aphids. There are also a few funguses that could leave your plants with purple marks on the leaves.
Since a lot of funguses turn up after a pest infestation (because they feed off the honeydew secreted by the bugs) generally if you treat for pests and give the plant good care, it’ll recover. In general, the purple marks won’t go green again, but new growth will be fine.
I like getting rid of aphids by spraying down the plant with insecticidal soap BUT it’s also SOOOO satisfying to move a ladybird (ladybug) onto your plant and watch them eat the aphids (yes, I am vegan, but the only one getting exploited here is the ladybird, and I’m feeding it). They are weirdly loud eaters.
If your plant leaf is purple and you’re still not sure why, I recommend you google ‘purple plant leaves’ and look at the images.
Just bear in mind that infestation and deficiencies don’t necessarily manifest in consistent ways because that would be too easy.
Oh, and if you have a plant with purple undersides, and the purple is disappearing in a patchy fashion, spray for thrips or spider mites. Trust me, I’ve been there.