How much light is too much light?

[ElieEstephane]

Me and mmcavall are strong advocates of unblocked full sun cultivation. Everything is going fine and we have great results with stronger spination. However, there seems to be some plants that no matter how slowly you acclimate will stop growth completely in full sun, which brings me to my question....how much sun is too much sun?
I grow my cacti mostly in 2 locactions:
1. My garden: not much morning light, direct sunlight from around 10 to around 5-6 with a little shading depending on the sun's position corresponding to the trees and such..
2. The roof: unblocked sun from dusk till maybe an hour before sunset. It is extremely bright even in the shade

Cacti grown in the garden are growing fast and strong while the ones on the roof are slow and some stopped growing so i had to move them to the garden and they resumed growth. Some other cacti stopped any vertical growth and started to grow laterally (extreme pupping with echinopsis for example, flattening of some globular species like notocactus or underground retraction with gymnocslycium). Now it's very obvious cacti of any kind will actually prefer at least a little shade.
So how much light is actually needed as the maximum threshold of healthy growth? Light levels are around 120,000 lux on the roof and i'm guessing maybe half that or less? How many lux do cacti need as an average value? I've read cacti need 25000-40000 lux but that feels a little too low
What are your thoughts on the topic?

[DaveW]

Light actually retards growth, that's why plants etiolate for lack of it, or they lean towards the light since they grow slower on the light side and quicker on the dark side. However they need a certain level of light to photosynthesize and produce characteristic growth.

In nature our plants often grow much slower than in cultivation, both due to the lack of water plus higher light intensity, and also increased UV radiation at higher altitudes tends to produce dwarfer growing plants. In cultivation it is simply striking the happy medium between slow growth for our plants due to high light intensity and soft or etiolated growth in low intensity lighting. Only you can decide what you think is "natural growth" or "bloated" unnatural growth caused by increased fertilisation and shadier growing conditions,

Many plants in habitat, at least initially, grow under the shelter of nurse plants or grasses which provide some shade and respite from full sun.

See:-

http://www.birdsandblooms.com/blog/cact ... se-plants/

http://www.cactusconservation.org/CCI/cp/aimg/aa.html

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2701848/

See also:-

http://www.plantphysiol.org/content/pla ... 7.full.pdf

[ElieEstephane]

I see your point Dave. That's what we initially thought (about being shaded by plants and rocks) but then thought about some plants that grow in the open.
After the results i've seen, very few cacti will get unblocked full sun. So far only oreocereus trollii is enjoying the full sun (maybe because only a fraction of the light reaches the epidermis). But in your opinion Dave, what is a general value of lux below which cacti will need more light?

[7george]

I think that searching for an "average value" of light is a wrong approach as soon as there is no average cactus or species. I use individual tactic for different cacti. Have never worried about my Oreocereus trollii, yes, because of the spines and hair shading it. But regularly get some major sunburns over some Rebutia been taken outside for the summer, even been shaded. Even many Opuntia plants can be burned after moving and not being watered enough. So every species or variety has his own level of optimal light and limits to stay but I never intend to measure those and use in culture.
So keep an eye on your plants give them what they need at the moment.

[Hanazono]

I keep all my cacti under adjusted sunlight strength except matured Echinocactus and Ferocactus.
They need full sun for the forming of strong spines and they are kept in outdoor.

The adjusted sunlight strength, luminous flux is around 1000 ~ 1500 lumens.
I use a special propagation method for Ariocarpus seedlings and they are kept in 500 lumens condition.

Since the sunlight strength is adjusted, you need a measurement.
I use a simple device which is portable.
I am not sure the accuracy but I believe it makes a good guidance for a hobby cactus collector.

The measuring device:

[TimN]

Saguaros sometimes have nurse plants. After a while the plant outgrows the nurse and is exposed to full available light. In my observation that is how nurse plants generally work: nurse provide shade while the plant is small and vulnerable, they plant outgrows it. Other cactus naturally grow in brushy or forested areas and have adapted to growing in partial shade.

My understanding was that one limiting factor is that cactus have a finite capacity to store Oxygen and CO2. So, when the plant runs out of CO2 to power it's metabolism, the metabolism slows/stops. This is because of the mechanism of CAM3 which has to do gas exchange at night and metabolism during the day as opposed to "normal" plants that can exchange gases throughout the day to feed metabolism.

All that said, I grow under 50% shade cloth in Arizona. I have a few that are exposed to the sun without cover, but those are generally native plants. The shade cloth is more of an insurance policy to avoid sunburn on everything else.

The sun puts out 32,000 to 100,000 lux. (Lumen is luminous energy per unit of time, lux is luminous flux emitted from a surface. A lux is one lumen illuminating one square meter) The variance is in lux output is dependent on the angle the sun hits the object in question, atmospheric blocking, etc. This subject is complex and confusing (to me). My approach is 50% shade cloth and keeping things adequately watered when it's HOT and sunny. It was 116 in my shade hut yesterday at 4pm.

If your eyes aren't spinning enough, find more info on Lux https://en.wikipedia.org/wiki/Lux.

[Auxin]

A worthwhile luxmeter can be got pretty cheap from a popular online auction site, around $12 USD- as cheap or cheaper than a much less accurate unit bought from a garden store.
A luxmeter will really help when hardening off plants that easily sunburn as it will allow accurate measurement of your shading materials so the shading can be progressively reduced in a controlled manner.

[abhikjha]

I keep all my cacti under adjusted sunlight strength except matured Echinocactus and Ferocactus.
They need full sun for the forming of strong spines and they are kept in outdoor.

The adjusted sunlight strength, luminous flux is around 1000 ~ 1500 lumens.
I use a special propagation method for Ariocarpus seedlings and they are kept in 500 lumens condition.

Since the sunlight strength is adjusted, you need a measurement.
I use a simple device which is portable.
I am not sure the accuracy but I believe it makes a good guidance for a hobby cactus collector.

The measuring device:

Hi Frank, I have a similar device. Reading is between 0 and 2000. So you use for your mature cactus when reading is around 1000-1500? And for seedlings around 500?

[Download]

If you want to be technical, as CAM plants, cacti collect and store CO2 at night for photosynthesis during the day. Further light after they deplete their store of CO2 won't make the cacti grow any more. How much CO2 they can store would require you to purchase some expensive scientific instruments, but for some more common cacti you can find papers on CO2 enriched cacti growth where they talk about it.

I sort of agree with the full sun thing mentioned in the OP. There are however a few species that don't like too much sun and I won't expose them to that, but for everything else they get full sun.

I've been told that Epiphyllums burn really easily, such as from only fifteen minutes of full sun. We'll, I've never bothered with that. Mine get a good three or four hours of full sun in the morning and I've never burned one. My mum had a horribly abused Epiphyllum that sat in a big tub next to her drive way for about 4 years that grew "okay" despite full sun Australian summer weather. Then, when the dogs decided it was tasty and started munching on it I moved it onto the balcony for her. It now gets about half a day of full sun and put on a ridiculous amount of new growth in the last year.

[anttisepp]

High temps also matter. Small cacti often suffer from high heat level. You probably have too hot days and very warm nights also?

[Hanazono]

G'morning abhikjha,

So you use for your mature cactus when reading is around 1000-1500? And for seedlings around 500?

Yes it is for my cacti.
Echinocactus and Ferocactus need 2000. I have kept them outside of greenhouse.

I use a special technique for young seedlings to increase growing speed.
A softer light strength than normal method is used, which is 500.

Frank

[abhikjha]

G'morning abhikjha,

So you use for your mature cactus when reading is around 1000-1500? And for seedlings around 500?

Yes it is for my cacti.
Echinocactus and Ferocactus need 2000. I have kept them outside of greenhouse.

I use a special technique for young seedlings to increase growing speed.
A softer light strength than normal method is used, which is 500.

Frank

Thanks Frank!! It's helpful..

High temps also matter. Small cacti often suffer from high heat level. You probably have too hot days and very warm nights also?

Yes, that's one disadvantage of being in tropical climate.. in day, the summer temp goes to 33-35 and night around 24-25. Good thing is being near to sea, air circulation is in abundance which helps in preventing sun scorching!

If you want to be technical, as CAM plants, cacti collect and store CO2 at night for photosynthesis during the day. Further light after they deplete their store of CO2 won't make the cacti grow any more. How much CO2 they can store would require you to purchase some expensive scientific instruments, but for some more common cacti you can find papers on CO2 enriched cacti growth where they talk about it.

I sort of agree with the full sun thing mentioned in the OP. There are however a few species that don't like too much sun and I won't expose them to that, but for everything else they get full sun.

I've been told that Epiphyllums burn really easily, such as from only fifteen minutes of full sun. We'll, I've never bothered with that. Mine get a good three or four hours of full sun in the morning and I've never burned one. My mum had a horribly abused Epiphyllum that sat in a big tub next to her drive way for about 4 years that grew "okay" despite full sun Australian summer weather. Then, when the dogs decided it was tasty and started munching on it I moved it onto the balcony for her. It now gets about half a day of full sun and put on a ridiculous amount of new growth in the last year.

Due to position of sun in summers, I don't get enough sunlight on my South facing windows (it there from 2pm till 6pm), so I have kept most of my sun loving plants outside in balcony to get as much sunlight as they can get. However as Frank said, all of them atleast get 1000-1500 reading on the meter..