Cactus - mammaliaria ?amajacencis - root depth and spread? Stability?

Low Altitude

Low Altitude
Sep 22, 2017
Reaction score
Hardiness Zone
7b but houseplants only - apartment
United States
Hi all. We have a small ceramic windowsill planter, about 10 inches long, 3 inches wide and deep, home to cacti:

20221203 cactus growth.jpg

The tall one, maybe a mammilaria, has grown significantly in the last few months. It's now about 7 inches tall. I'm concerned that if it keeps growing at the same kind of rate, it will become so massive, and particularly so tall, that it will be in danger of falling over and ripping its roots. So the natural precaution would be to transplant it to a bigger pot / planter so that its roots could deepen and spread. BUT (1) while information on the 'Net is scarce, it seems to suggest that these cacti, like most, have surprisingly small root mass relative to the mass above ground (2) there's no indication that this one is becoming rootbound, and (3) as i gently tilt the planter, it seems firmly anchored and the whole thing is sufficiently heavy that it's not all going to tip over any time soon.

SO, I'm inclined to let it grow more and see what happens, watching for signs of distress – none yet: it looks perfectly healthy. BUT, by the time it shows signs of distress, it might be too late.

So any experience would be gratefully listened to. Thanks all,


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