My partner has an Euphorbia cactus which is growing “out” rather than “up”. How can he cut or trim it down do that it is not growing wild?
The Euphorbia looks like it needs more light for more robust vertical growth. With it being so floppy and “stretched towards the light” it is hard to be sure which species but I think it is Euphorbia trigona or one of the close allies.
Yes you can prune for shape, but please note that Euphorbia sap which looks like milk, is both a contact irritant and toxic. Do not get it in your eyes or lips! Think cayenne pepper pain and a trip to the emergency room if it gets in your eyes. So wear safety goggles! Gloves and long sleeves. It is generally best to prune off the whole branch rather than just part of it, but you can do cuts where ever you want, it just scares so cuts in the center of branch will always show. Before you cut cover the floor under with newspaper of a disposable drop cloth so you can get rid of the sap drips easily. After cutting spray each cut with standard hydrogen-peroxide to stop the flow of sap. This is sort of messy, so make sure you have your drop cloth or newspaper well deployed.
The branches you cut off can be saved and re-rooted if you want to clone your plant. Let them dry out and heal up from being cut for a week or two and then pot up in dry cactus soil. Do not water for three or four weeks. It the branches look thirsty you can mist them in the evening. Keep them in a warm sunny location and they should have roots by spring.