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I’ve had my cactus for over a year now. I bought it late winter/early spring (February/March?) and from the time I bought it to around a month ago it had very stunted growth and didn’t grow any new spines. I used to water it roughly every 3 weeks, because I I knew that cacti don’t need watering as much as other houseplants. However, in the past 2 months I’ve increased this to every 2 weeks and it has grown new spines and significantly increased in height. I was happy to see this of course, but the new stem looks really thin compared to the lower half. Do I need to increase its water intake even more?

Also if anyone can tell me what species or family my cactus belongs to I’d be really grateful!

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It looks like euphorbia pentagona as the other answer said.

It is not a desert cactus, so it doesn't need full sun 12 hours a day 7 days a week. In fact too much sun can make them turn a lighter shade of green, since the plant doesn't waste resources producing more chlorophyll than it needs.

Etiolation is a thing, but your problem may be the change in watering. In the wild it grows in regions that get regular rain all year round. Under-watering probably wouldn't kill it (not unless it wasn't watered at all for 12 or 24 months!) but as you discovered, it will slow down the growth rate.

You might try watering it even more (say once a week) in summer, then cut back to nothing at all for the darkest 2 or 3 months in winter and start watering again in about March. Making it have an annual dormant period will encourage it to flower.

The stems won't get thicker over time, so your plant will now always have those thin spots - at least until it grows so many new "branches" that you can get rid of the oldest ones if you really don't like the way they look.

  • Awesome! I’ll experiment with with more water and varying sunlight and see what works best. If I kept it dormant during the winter would it flower in the spring or summer? I’m guessing there’s a low chance of seeing any flowers at this time of year. – anna101 Jun 7 at 18:34
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If new growth is thinner than the rest of the plant it is usually a not enough light problem. This phenomenon is called etiolation, and it is not something you can reverse (the thin growth stays thin), so take action right away. You can prevent it by giving the plant more light, try to give it direct sunlight. Try to put it on a south facing window if available.

I think your cactus is not a real cactus, but a Euphorbia pentagona.

  • Wow thank you so much! If I don’t have a south facing window, what would be the next best direction? It’s currently in a west facing window. I live in England which isn’t known for being sunny so maybe that’s the issue? – anna101 Jun 7 at 14:58
  • West or East is better than North. Try to put it close to the window. Furthermore, in winter when the light is weak, give it no water, so it won't grow at all. – benn Jun 7 at 15:32

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