Artificial UVA Light to Stimulate Plant Stress

[Ferocactus]

I have a ≈2" F. latispinus/recurvus (greenwoodii?) seedling that I'm keeping indoors in a dry, bright environment under a grow light, but I'd like to help it receive some UVA to encourage dense spination before I move it outside.

I'm wondering what watt UVA (reptile) bulb would produce enough UVA radiation to be comparable to what a plant would receive in a day's time in "full sun". Maybe I could use a larger bulb for less amount of time, or might I just be better off moving it outside with some 30% shade cloth over the top of it?

*Also, would it be appropriate to keep a F. latispinus plant in a 75% lava rock/pumice 25% cactus potting mix combination? Or would that be too high an organic content? I can't find any info as to the specific composition of its native soil type (xerophyllus scrub/grassland/quercus forest are all I can find).

[Download]

It might be worth looking at data for reptiles.

[Ferocactus]

It might be worth looking at data for reptiles.

Thank you! Could you help me know what I should be looking out for (how reptile light needs correlate to cactus needs)?

Maybe I could try getting a big bulb and keep it on for 30 minutes and increase it by 15 minutes each day until the seedling shows signs of sunburn/bleaching?

[Download]

It might be worth looking at data for reptiles.

Thank you! Could you help me know what I should be looking out for (how reptile light needs correlate to cactus needs)?

Maybe I could try getting a big bulb and keep it on for 30 minutes and increase it by 15 minutes each day until the seedling shows signs of sunburn/bleaching?

I'd suggest finding a reptile species from the same area as the cactus and finding out how much they need. They'll probably be in the same ballpark.

[Ferocactus]

It might be worth looking at data for reptiles.

Thank you! Could you help me know what I should be looking out for (how reptile light needs correlate to cactus needs)?

Maybe I could try getting a big bulb and keep it on for 30 minutes and increase it by 15 minutes each day until the seedling shows signs of sunburn/bleaching?

I'd suggest finding a reptile species from the same area as the cactus and finding out how much they need. They'll probably be in the same ballpark.

Ah, that makes sense, thank you! I guess the only info I'd need then would be how much UVA radiation is produced by certain bulb wattages; maybe I could pick up some UV test cards and compare their color under noon-day sun to a 100 watt UVA bulb? I've read that some UVA bulbs on Amazon don't even provoke a response on these cards, so that's kind of worrying.

I know that reptiles in the wild will seek shelter if it gets too intense (mid-day), but cacti can't uproot themselves and run to hide under a rock, which would be kind of funny to see, I guess.

For the moment, I put my seedling outside in the mini greenhouse, but I'm checking it everyday to see how it's doing. It's insanely humid and damp right now, so I might need to bring it back inside (unless humidity shouldn't be harmful so long as the soil is dry?).