Maybe? Depends on your exact growing conditions. 24" is probably a little too far with 21W 36" lamps. Watch the growth habit of your cacti to see yourself if it is enough. This is so specific to your plants, how big they are, and how they are doing, no manufacturer can really answer. If it's not enough light, put them closer or buy additional fixtures.

if you have spare space.. try growing other faster growing plants .. and see how they react ... if they etoliate.. etc ... and then.. using that info.. you might have some idea how to adjust the lights ...

i think you are failing because you want info on highly specific plants.. and unless you run across the one person who grows them.. there just isnt info out there ... and it wouldnt surprise me that the manufacturer hast tested every plant known to man or woman ...

im thinking something in the very fast veg groups... something that matures in 30 days ... e.g.

ken

Too small. Would be cheaper to go with 5000k shop lamps (4 ft. long) ratted at 45 watt each. Using about 4 of them. About $20 ea. Cost in long run would be cheapest going LED. That would give you 20,000 lumens of light using under 200 watts.

Cactus require the most light of any of the house plants that I grow.

You could retrofit a drop in fluorescent fixture then cut the ballast out and replace with higher 6500k LED lamps. That would be cheapest and best. Could be found on line in a google search.

All my plants do excellent under the shop lights purchased at Walmart.

I switched from fluorescent T-8s to 4 ft linkable LED shop lights, 42W, 5,200lm, color temperature 6500K daylight white, with pull chain ON/Off. I run a good number of fixtures (28) over 2 - 4x8 growing tables, so I noticed quite a savings on my electric bill. After 2 years with no problems, I changed the 9 fixtures in my garage from t-8s to the same lights. They were just a touch under the $20.00 mark.

I also added to the garage a 60w LED corn light (250w metal halide equivalent), 8,115 lumens in 5,500K over the area where I work most. I love that thing!

Al